Lecture exam #2

Question 1

Compare and contrast the five major classes of infectious agents in their:
a. cellular characteristics
b. selected diseases caused by the agent (exclude multicellular parasites)

Answer 1

Bacteria are microscopic, single-celled organisms composed of prokaryotic cells.

Selected Diseases Caused by the Agent: Streptococcal infections (e.g., strep throat), staphylococcal infections, tuberculosis, syphilis, diphtheria, tetanus, Lyme disease, salmonella, and anthrax

Viruses are not cells; DNA or RNA within a capsid protein
Selected Diseases Caused by the Agent: Common cold, influenza, polio, mumps, measles, hepatitis, rubella, chicken pox, ebola, herpes, and HIV (which leads to AIDS)

Fungi are eukaryotic
Selected Diseases Caused by the Agent: Ringworm, diaper rash, jock itch, athlete’s foot, yeast infections, and histoplasmosis

Protozoans are eukaryotic
Selected Diseases Caused by the Agent: Malaria, toxoplasmosis, giardiasis, amoebiasis, leishmaniasis, trichomoniasis, and African sleeping sickness

Multicellular Parasites are eukaryotic
Selected Diseases Caused by the Agent: Parasitic infection from tapeworms, lung flukes, liver flukes, blood flukes, hookworms, Trichinella, Ascaris, whipworms, and pinworms

Question 2

List the types of leukocytes of the immune system and describe where they may be found.

Answer 2

Leukocytes circulate in blood: Basophil, Eosinphil, Neutrophil, Monocyte, Lymphocyte

Secondary lymphatic structures (e.g., lymph nodes, spleen, tonsils, MALT): T-lymphocyte, B-lymphocyte, Macrophage, Dendritic cell, NK cell

Select organs (e.g., lungs): Macrophages

Skin and mucosal membranes: Dendritic cell

Connective tissue throughout the body: Mast cells

Question 3

Define cytokines, describe their similarities to hormones, and list the general categories

Answer 3

Cytokines are small, soluble proteins produced by cells of both the innate and adaptive immune system to regulate and facilitate immune system activity. These soluble proteins (1) serve as a means of communication between the cells; (2) control the development and behavior of immune cells; (3) regulate the inflammatory response of the innate immune system; and (4) function as weapons to destroy cells.

A cytokine is released from one cell and binds to a specific receptor of a target cell, where its action is similar to that of a hormone. Cytokines can act on the cell that released it (autocrine stimulation), local neighboring cells (paracrine stimulation), or circulate in the blood to cause systemic effects.

Four categories: interleukin (IL), tumor necrosis factor (TNF), colony-stimulating factor (CSF), and interfer- on (IFN).

Question 4

Compare and contrast the primary features of innate and adaptive immunity

Answer 4

Innate Immune System: Provides Innate Immunity–> Multiple components that protect against a wide array of substances

Innate immunity includes Skin and mucosal membranes (prevent entry) and Nonspecific internal defenses

Nonspecific internal defenses includes Cells (e.g., macrophages, NK cells), Chemicals (e.g., interferon, complement), Physiologic responses (e.g., inflammation, fever)

Adaptive Immune System: Provides Adaptive Immunity–> Lymphocytes that are activated to replicate and respond when stimulated by a specific antigen

Adaptive Immunity includes T-lymphocytes (cell-mediated immunity) and B-lymphocytes (humoral immunity)

B lymphocytes include Plasma cells that synthesize and release antibodies

Question 5

Recognize the components in the first line of defense and its general function

Answer 5

The skin forms a physical, chemical, and biological barrier that plays a significant role in preventing entry of pathogens at the body’s surface if the skin is intact.
The mucous membranes also form a physical, chemical, and biological barrier, but these membranes function to prevent entry at the openings of the body.
The respiratory tract has cilia that sweep mucus with trapped microbes upward from the lungs to be expectorated (spit out) or swallowed and the coughing and sneezing reflexes, which remove microbes with blasts of exhaled air.
Commensal microflora (or normal microflora) are microorganism that reside on body surfaces (e.g., the skin, GI tract). These non-pathogenic microorganisms interfere with the attachment and growth of other, potentially more virulent types.

Lacrimal fluid (which contains lysozyme and IgA) washes microbes from the surface of the eye and cerumen (i.e., earwax) is thought to impede microbial growth within the external acoustic meatus

Question 6

List the four main components of the nonspecific internal defenses, the second line of defense

Answer 6

(1) selected immune cells; (2) chemicals such as interferon and complement and antimicrobial proteins and (3) physiologic processes that include the inflammatory response and (4) development of a fever

Question 7

Describe the cells that function as part of the nonspecific internal defenses in providing innate immunity

Answer 7

Phagocytic cells include neutrophils, macrophages, and dendritic cells, which engulf unwanted substances such as infectious agents and cellular debris through phagocytosis.

Chemical-secreting cells that enhance inflammation include both basophils and mast cells. Substances secreted by basophils and mast cells increase fluid movement from the blood to an injured tissue. They also serve as chemotaxic chemicals, which are molecules that attract immune cells as part of the inflammatory response. Basophils and mast cells release granules during the inflammatory response. These granules contain various substances, including histamine, which increases both vasodilation and capillary permeability, and heparin, an anticoagulant. They also release eicosanoids from their plasma membrane, which increase inflammation.

NK (natural killer) cells, which are located within secondary lymphatic structures, destroy a wide variety of unhealthy or unwanted cells through apoptosis. The types of cells eliminated by NK cells include virus-infected cells, bacteria-infected cells, tumor cells, and cells of transplanted tissue. NK cells patrol the body in an effort to detect unhealthy cells, a pro- cess referred to as immune surveillance.

Eosinophils target multicellular parasites, attacking the organisms’ surfaces. Mechanisms of destruction include degranulation and release of enzymes and other substances (e.g., reactive oxygen-containing compounds, neurotoxins) from the eosinophils that are lethal to the parasite. Like NK cells, eosinophils can release proteins that form a transmembrane pore to destroy cells of the multicellular organism.

Question 8

List antimicrobial proteins

Answer 8

Interferons (IFNs) are a category of cytokines

Complement is a diverse array of proteins (at least 30) produced by
our liver and released into the blood.

Question 9

Define inflammation, and discuss the basic steps involved

Answer 9

Inflammation, or the inflammatory response, is an immediate, local, nonspecific event that occurs in vascularized tissue against a great variety of injury-causing stimuli. This is the major effector response of the innate immune system and is successful in helping to eliminate most infectious agents and other unwanted substances from the body!

1 Release of inflammatory and chemotactic factors
2 Vascular changes include: Vasodilation of arterioles, Increase in capillary permeability, Display of CAMs
3 Recruitment of immune cells
* Margination: the process by which CAMs on leukocytes adhere to CAMs on the endothelial cells of capillaries within the injured tissue.
* Diapedesis: the process by which cells exit the blood by “squeezing out” between vessel wall cells, usually in the postcapillary venules, and then migrate to the site of infection
* Chemotaxis: migration of cells along a chemical gradient
4 Delivery of plasma proteins

Question 10

List the cardinal signs of inflammation and explain why each occurs

Answer 10

∙ Redness, due to increased blood flow
∙ Heat, due to increased blood flow and increased metabolic
activity within the area
∙ Swelling, resulting from increase in fluid loss from capillaries into the interstitial space
∙ Pain, which is caused by stimulation of pain receptors from compression due to accumulation of interstitial fluid, and chemical irritation by kinins, prostaglandins, and substances released by microbes
∙ Loss of function (which may occur in more severe cases of inflammation due to pain and swelling)

Question 11

Define fever and describe how it occurs

Answer 11

A fever is defined as an abnormal elevation of body temperature of at least 1°C (1.8°F) from the typically accepted core body temperature of 37°C (98.6°F). It results from release of fever- inducing molecules called pyrogens that are released from either infectious agents (e.g., bacteria) or immune cells in response to infection, trauma, drug reactions, and brain tumors. A fever is a physiologic process of the innate immune system and may accompany the inflammatory response.

onset, stadium, and defervescence

During the onset of a fever, the hypothalamus stimulates blood vessels in the dermis of the skin to vasoconstrict to decrease heat loss through the skin, and a person shivers to increase heat production through muscle contraction. Consequently, body temperature rises.

The period of time when the elevated temperature is maintained is referred to as stadium. The metabolic rate increases to promote physiologic processes of the innate and adaptive immune systems that are involved in eliminating the harmful substance.

Defervescence occurs when the temperature returns to its normal set point. This happens when the hypothalamus is no longer stimulated by pyrogens, prostaglandin release decreases, and the temperature set point reverts to its normal value. The hypothalamus then stimulates the mechanisms to release heat from the body, including vasodilation of blood vessels in the skin and sweating.

Question 12

List the benefits and risks of a fever

Answer 12

A fever inhibits replication of bacteria and viruses, promotes interferon activity, increases activity of lymphocytes, and accelerates tissue repair. Most recently, it has been demonstrated that a fever also increases CAMs on the endothelium of capillaries in the lymph nodes, resulting in additional immune cells migrating out of the blood and into the lymphatic tissue.

High fevers (103F in children, and slightly lower in an adult) are potentially dangerous because of the changes in metabolic pathways and denaturation of body proteins. Seizures may occur at sustained body temperature above 102F, irreversible brain damage may occur at body temperatures that are sustained at greater than 106F, and death is likely when body temperature reaches 109F.

Question 13

Describe the features of an antigen and explain what is meant by antigenic determinant

Answer 13

An antigen is a substance that binds to a component of the adaptive immune system (T-lymphocyte or an antibody). Antigens are unique to each infectious agent and are usually proteins or large polysaccharide molecules. Examples of antigens include parts of infectious agents such as the protein capsid of viruses, cell wall of bacteria or fungi, and bacterial toxins.

The specific site on the antigen molecule that is recognized by lymphocytes (and antibodies) is referred to as the antigenic determinant, or epitope. Each type of antigenic determinant has a different shape, and a pathogenic organism can have numerous different antigenic determinants.

Question 14

Explain immunogenicity, and list attributes that affect it

Answer 14

An antigen that induces an immune response is more specifically called an immunogen, and its ability to cause an immune response is termed its immunogenicity. Important attributes that affect an antigen’s immunogenicity include degree of foreignness, size, complexity, and quantity of the antigen. An increase in one or more of these attributes increases the antigen’s ability to elicit an immune response, and thus its immunogenicity.

Question 15

Describe receptors of both T-lymphocytes and B-lymphocytes

Answer 15

T-lymphocytes and B-lymphocytes differ from other immune cells because each lymphocyte has a unique receptor complex, which are composed of several different and separate proteins. A receptor complex will bind one specific antigen. The antigen receptor (which is a portion of a receptor complex) of a T-lymphocyte is referred to as the TCR (or T-cell receptor), and the antigen receptor of a B-lymphocyte is called a BCR (or B-cell receptor).

T-lymphocytes must first have the antigen processed and presented in the plasma membrane of another type of cell. T-lymphocytes simply are not able to recognize the antigen without this preliminary step. In contrast, B-lymphocytes can make direct contact with an antigen.

Question 16

Define antigen presentation to T-lymphocytes by antigen-presenting cells and list the cells that serve this
function

Answer 16

Antigen presentation is the display of antigen on a cell’s plasma membrane surface. This is a necessary process performed by other cells so that T-lymphocytes can recognize an antigen. Generally, two categories of cells present antigen to T-lymphocytes: all nucleated cells of the body (i.e., all cells except erythrocytes) and a category of cells called antigen-presenting cells. The term antigen-presenting cell (APC) is used to describe any immune cell that functions specifically to communicate the presence of antigen to both helper T-lymphocytes and cytotoxic T-lymphocytes. Dendritic cells and macrophages, as well as B-lymphocytes, function as APCs.

Helper T with APC
Body cells with cytotoxic T

Question 17

Diagram the interactions of T-lymphocytes with antigen-presenting cells

Answer 17

We are all born with MHC I. Antigen presenting cells like dendritic, macrophages, and b-lymphocytes have both MHC I and II.

CD8 cells bind to MHC I to kill cells
CD4 cells bind to MHC II to generate immune response
APC will only bind to helper T cells

Question 18

Describe the three significant events that occur in the lifetime of a lymphocyte

Answer 18

Formation of lymphocytes: Both cells originate in red bone marrow. B-cells mature in red bone marrow and T-cells mature in thymus. Here T-lymphocytes and B-lymphocytes become able to recognize only one specific foreign antigen.

∙ Activation of lymphocytes. Following their formation, lymphocytes then migrate to secondary lymphatic structures (e.g., lymph nodes, the spleen, tonsils, MALT) where they are housed. Typically, these locations are where lymphocytes have their first exposure to the antigen that they bind, and thus become activated. In response to activation, lymphocytes replicate to form many identical lymphocytes.

∙ Effector response. The effector response is the specific action of the T-lymphocytes and B-lymphocytes to help eliminate the antigen at the site of infection. T-lymphocytes leave the secondary lymphatic structures, migrating to the site of infection. B-lymphocytes primarily remain within the secondary lymphatic structures, synthesizing and releasing large quantities of antibodies against the antigen. The antibodies enter the blood and lymph and are transported to the site of infection.

Question 19

Explain the formation of T-lymphocytes

Answer 19

Millions of pre-T-lymphocytes, which are called thymocytes, migrate from the red bone marrow to the thymus; they possess a unique TCR receptor and initially neither the CD4 nor the CD8 proteins. Within the thymus, these cells will synthesize and display both CD4 and CD8 proteins (referred to as “double positive”).

1 Positive selection: Survival dependent upon ability to bind to MHC molecule
Thymic epithelial cell presents MHC molecule to pre-T lymph
Binds to MHC molecule? YES - survives NO - destroyed by apoptosis

2 Negative selection: Survival dependent upon not recognizing self-antigen
Dendritic cell presents self-antigen to pre-T-lymphocyte
Recognizes self-antigen? YES-destroyed by apoptosis NO-survives

3 The final step in T-lymphocyte selection is the differentiation of each thymocyte into either a helper T-lymphocyte (CD4 cell) by the selective loss of the CD8 protein, or a cytotoxic T-lymphocyte (CD8 cell) by the selective loss of CD4 protein. Consequently, two primary types of T-lymphocytes leave the thymus: helper T-lymphocytes (that are CD4+) and cytotoxic T-lymphocytes (that are CD8+)

Question 20

Explain why T-lymphocytes leaving the thymus are called both immunocompetent and naive

Answer 20

The T-lymphocytes that leave the thymus are immunocompetent cells (able to bind antigen and respond to it). However, each of these T-lymphocytes is also classified as a naive T-lymphocyte. The term naive refers to T-lymphocytes that lack experience because they have not yet encountered the antigen that they recognize. Naive immunocompetent helper T-lymphocytes and naive immunocompetent cytotoxic T-lymphocytes migrate from the thymus to secondary lymphatic structures, where they are housed.

Question 21

Describe the formation and function of T-lymphocytes (Tregs) in peripheral tolerance

Answer 21

Tregs are formed from T-lymphocytes that bind self-antigens to a moderate extent compared to other CD4+ cells. Tregs migrate to the periphery (body structures outside the primary lymphatic structures), where they release inhibitory chemicals that turn off both the cell-mediated immune response and the humoral immune response. Tregs function in self-tolerance outside the primary lymphatic structures—a process that is more specifically called peripheral tolerance.

Question 22

Describe how both the helper T-lymphocytes and cytotoxic T-lymphocytes are activated, including the specific role of IL-2 in both activations.

Answer 22

Cytotoxic T-lymphocyte
1 First signal: CD8 binds
with MHC class I molecule of infected cell; TCR interacts with antigen within MHC class I molecule

2 Second signal: IL-2 released from activated helper T-lymphocyte activates the cytotoxic T-lymphocyte.

Activated cytotoxic T-lymphocyte proliferates and differentiates to form a clone of activated and memory cytotoxic T-lymphocytes.

Helper T-lymphocyte
1 First signal: CD4 binds
with MHC class II molecule of APC; TCR interacts with antigen within MHC class II molecule.

2 Second signal: Other receptors interact (not shown) and the helper T-lymphocyte releases IL-2, which binds with the helper T-lymphocyte.

Activated helper T-lymphocyte proliferates and differentiates to form a clone of activated and memory helper T-lymphocytes.

Question 23

Compare the activation of B-lymphocytes with that of T-lymphocytes

Answer 23

Immunocompetent but naive B-lymphocytes are also activated by a specific antigen in secondary lymphatic structures. As with T-lymphocytes, two signals are required. However, B-lymphocytes do not require antigen to be presented by other nonlymphocyte cells. B-lymphocytes recognize and respond to antigens outside of cells.

1. First Signal- intact antigen binds to the BCR, and the antigen cross-links BCRs. The stimulated B-lymphocyte engulfs, processes, and presents the antigen to the helper T-lymphocyte that recognizes that antigen
2. Second Signal- activated helper T-lymphocyte releases IL-4 to stimulate the B-lymphocyte.
   Activation of B-lymphocytes causes the B-lymphocytes to proliferate and differentiate. Most of the activated B-lymphocytes differentiate into plasma cells that produce antibodies, and the remainder become memory B-lymphocytes

B-lymphocytes can be stimulated by antigen without direct contact between a B-lymphocyte and helper T-lymphocyte under certain conditions. However, the production of memory B-lymphocytes and the various forms of antibodies requires helper T-lymphocyte participation during B-lymphocyte activation.

Question 24

Describe lymphocyte recirculation and explain its general function

Answer 24

One of the hurdles facing adaptive immunity is the requirement of direct physical contact between antigen and the specific lymphocyte with the unique receptor that recognizes the antigen. The odds for contact are increased because lymphocytes reside only temporarily in any given secondary lymphatic structure, and after a period of time they exit and then circulate through blood and lymph every several days. This process is referred to as lymphocyte recirculation, and provides a means of delivering different lymphocytes to secondary lymphatic structures, making it more likely that a lymphocyte will encounter its antigen, if present.

Question 25

Summarize the effector response of helper T-lymphocytes and cytotoxic T-lymphocytes

Answer 25

Activated and memory helper T-lymphocytes leave the secondary lymphatic structure after several days of exposure to antigen. They migrate to the site of infection, where they continue to release the cytokines to regulate other immune cells.
Although helper T-lymphocytes were named based on their function in helping activate B-lymphocytes, their contributions are much more encompassing. Helper T-lymphocytes activate cytotoxic T-lymphocytes

Activated and memory cytotoxic T-lymphocytes also leave the secondary lymphatic structure after several days and migrate to the site of infection in the body’s tissue. Cytotoxic T-lymphocytes destroy unhealthy or infected cells that display the antigen. The effector response of cytotoxic T-lymphocytes is initiated when physical contact is made between a cytotoxic T-lymphocyte and the specific foreign antigen displayed on an unhealthy or a foreign cell.
If the cytotoxic T-lymphocyte recognizes the antigen presented by the infected cell (with MHC class I molecules), it destroys the cell by releasing granules containing the cytotoxic chemicals perforin and granzymes

Question 26

Explain why the processes of T-lymphocytes are collectively called the cell-mediated branch of adaptive
immunity

Answer 26

Immune response of T-lymphocytes is effective against antigens associated with cells that it is referred to as cell-mediated immunity.

Question 27

Describe the function of plasma cells in the effector response of B-lymphocytes

Answer 27

Plasma cells release and synthesize antibodies in the lymph nodes.

Question 28

Define antibody titer

Answer 28

Antibody titer (concentration of antibody) in blood serum is one mea- sure of immunologic memory. The degree of protection is indicated by levels of circulating IgG.

Question 29

Describe the actions of the antibodies

Answer 29

immunoglobulin proteins produced against a particular antigen.

Antibodies tag a specific antigen for destruction and immobilize them.

Binding at antigen-binding site of antibody results in neutralization, agglutination, and precipitation

EXPOSED FC PORTION FOLLOWING ANTIGEN BINDING BY ANTIBODY PROMOTES
Complement fixation, Opsonization, and Activation of NK cells
Humoral immunity

Question 30

Define immunologic memory and explain how it occurs

Answer 30

For first exposure, there is a slower response in regards to the antigen and the physical contact with lymphocytes required to develop an immune response.

After first exposure, the formation of memory cells occurs in response to the activation of T-lymphocytes and B-lymphocytes. They will recognize these specific antigens that attacked previously more rapidly.

Virus is eliminated by activated memory T-lymphocytes, memory B-lymphocytes, and antibodies before it causes harm.

Question 31

Discuss the difference between the primary response and the secondary response to antigen exposure

Answer 31

Primary Response
Lag phase: no detectable antibody in the blood.
Antigen detection, activation, proliferation, and differentiation of lymphocytes occur in lag phase (3-6 days)

Production of antibody: IgM and IgG (1-2 weeks)

Secondary response
Shorter latent phase: due to memory lymphocytes
Production of antibody: More quicker and abundant antibody formation (specifically IgG)

Question 32

Define active immunity & passive immunity, and describe how both active and passive immunity can be
acquired naturally and artificially

Answer 32

Active immunity: Production of memory cells due to contact with antigen
Naturally acquired: Direct exposure to antigen following entry of the pathogen into the body
Artificially acquired: Antigen exposure from vaccine

Passive immunity: No production of memory cells; antibodies from another person or an animal
Naturally acquired: Transfer is mother to child (the placenta or in breast milk)
Artificially acquired: Transfer of serum containing antibody from another person or animal

Question 33

Describe lymph and its contents

Answer 33

Lymph originates as interstitial fluid surrounding tissue cells; it moves passively into the lymphatic capillaries due to a hydrostatic pressure gradient. Lymphatic capillaries merge to form larger lymph vessels.

Once inside the lymph vessels, the interstitial fluid is called lymph. The components of lymph include water, dissolved solutes (e.g., ions), a small amount of protein, sometimes foreign material that includes both cell debris and pathogens, and perhaps metastasized cancer cells

Question 34

Discuss the location and anatomic structure of lymphatic capillaries and the process of fluid entry

Answer 34

Lymphatic capillaries begin as closed-end vessels within connective tissue among most blood capillary networks and absorb excess interstitial fluid left during capillary exchange. A lymphatic capillary takes up excess interstitial fluid through overlapping endothelial cells. The fluid is then called lymph.

Lymphatic capillaries are typically larger in diameter than blood capillaries, lack a basement membrane, and have overlapping endothelial cells. These overlapping endothelial cells act as one-way flaps to allow fluid to enter the lymphatic capillary but prevent its loss. Anchoring filaments help hold these endothelial cells to the nearby structures.

The driving force to move fluid into the lymphatic capillaries is an increase in hydrostatic pressure within the interstitial space. Interstitial hydrostatic pressure rises as additional fluid is filtered from the blood capillaries. This pressure exerted by interstitial fluid at the margins of the lymphatic capillary endothelial cells “pushes” interstitial fluid into the lymphatic capillary lumen when the interstitial fluid hydrostatic pressure becomes greater than the lymph hydrostatic pressure.

Question 35

Explain the mechanisms that move lymph through lymphatic vessels, trunks, and ducts

Answer 35

The lymphatic system lacks a pump and, thus, relies on other mechanisms to move lymph through its vessels. These include (1) contraction of nearby skeletal muscles in the limbs (skeletal muscle pump) and the respiratory pump in the torso, which is similar to how blood movement is assisted through the venous circulation, (2) rhythmic contraction of smooth muscle within the walls of larger lymph vessels (trunks and ducts), which narrows the lumen and squeezes the lymph within the lymph vessel, and (3) pulsatile movement of blood in nearby arteries. All of these mechanisms are dependent upon valves within lymph vessels, which prevent the backflow of lymph, causing the lymph to move in one direction to be returned to venous blood circulation.

Question 36

Describe the flow of lymph from lymphatic capillaries through the various types of lymph vessels until it is returned to the cardiovascular system

Answer 36

Lymphatic vessels are fed by lymphatic capillaries (Located adjacent to arteries and veins)
* Have valves to prevent pooling and backflow of lymph
Lymphatic trunks are fed by lymphatic vessels
Lymphatic ducts are fed by lymphatic trunks
* Largest lymphatic vessels
* Bring lymph to venous blood circulation

The lymph flows one way, from the lymphatic capillary system to the subclavian veins, where it joins the venous circulation to return to the heart. Fluid begins in the interstitial fluid between the cells. Most, but not all, of the fluid is returned to the heart via the veins of the cardiovascular system. Fluid that is not returned through the veins of the cardiovascular system enters lymphatic capillaries and flows into a lymphatic vessel. Lymphatic capillaries are closely connected to the capillaries of the cardiovascular system. The lymph capillaries take up plasma fluid, which, under great pressure, has been forced out of the capillaries of the circulatory system and has not been reabsorbed. This fluid bathes the cells assisting the capillaries in delivering glucose, oxygen, salts, amino acids, and other nutrients. Excess tissue fluid entering the lymphatic capillaries is now called lymph. Lymph flows from the lymphatic capillaries into larger lymphatic vessels until it eventually empties into venous blood of the cardiovascular system.

Question 37

Name the two categories of lymphoid structures, and identify components of the body that belong to each category

Answer 37

Primary lymphatic structures are involved in the formation and maturation of lymphocytes. Both the red bone marrow and thymus are considered primary lymphatic structures.
∙ Secondary lymphatic structures are not involved in lymphocyte formation; instead, they house both lymphocytes and other immune cells following their formation. Secondary lymphatic structures are the sites where an immune response is initiated. The major secondary lymphatic structures include the lymph nodes spleen, tonsils, lymphatic nodules, and MALT.

Question 38

Describe the location and general function of the red bone marrow

Answer 38

Red bone marrow is located within trabeculae in portions of spongy bone within the skeleton. In adults, these include the flat bones of the skull, the vertebrae, the ribs, the sternum, the ossa coxae, and proximal epiphyses of each humerus and femur.
Red bone marrow is responsible for hemopoiesis (or hematopoiesis), which is the production of formed elements.

Question 39

Identify the unique step required by T-lymphocytes in their maturation

Answer 39

Unlike the other formed elements, T-lymphocytes must migrate to the thymus to complete their maturation

Question 40

Describe the structure and general function of the thymus

Answer 40

The thymus is a bilobed organ that is located in the superior mediastinum and functions in T-lymphocyte maturation. The thymus in a child consists of two fused thymic lobes, each surrounded by a connective tissue capsule. Fibrous extensions of the capsule, called trabeculae, subdivide the thymic lobes into lobules. Each lobule is arranged into an outer cortex and inner medulla. Both parts are composed primarily of epithelial tissue infiltrated with T-lymphocytes in varying stages of maturation. The cortex contains immature T-lymphocytes, and the medulla contains mature T-lymphocytes. The epithelial cells secrete thymic hormones that participate in the maturation of T-lymphocytes.
Because the thymus contains both lymphatic cells and epithelial tissue, it is described as a lymphoepithelial organ.

Question 41

Describe both the structure and function of lymph nodes

Answer 41

Lymph nodes are small, round or oval, encapsulated structures located along the pathways of lymphatic vessels, where they serve as the main lymphatic organ.
Lymph is continuously monitored for the presence of foreign or pathogenic material as it passes through nodes. Macrophages residing in the lymph node remove foreign debris from the lymph by phagocytosis.
Outer area of node is cortex which consists of Cortical sinus, Germinal center, Mantle zone
Inside cortex is medulla which consists of Medullary sinus and Medullary cord

Question 42

Describe the location, structure, and function of the spleen

Answer 42

The spleen is the largest lymphatic organ in the human body. It is located in the left upper quadrant of the abdomen, inferior to the diaphragm and adjacent to ribs 9–11. This deep red organ lies lateral to the left kidney and posterolateral to the stomach.
The spleen’s posterolateral aspect (called the diaphragmatic surface) is convex and rounded; the concave anteromedial border (the visceral surface) contains the hilum, where blood vessels and nerves connect to the spleen.
The spleen is surrounded by a connective tissue capsule from which trabeculae extend into the organ. The spleen lacks a cortex and medulla. Rather, the trabeculae subdivide the spleen into regions of white pulp and red pulp.
The spleen serves several functions, including (1) phagocytosis of bacteria and other foreign materials in the blood as part of the body’s defense (red and white pulp); (2) phagocytosis of old, defective erythrocytes and platelets from circulating blood (red pulp); and (3) a role as a blood reservoir and storage site for both erythrocytes and platelets (red pulp).

Question 43

Distinguish between white pulp and red pulp

Answer 43

beculae subdivide the spleen into regions of white pulp and red pulp. White pulp consists of spherical clusters of T-lymphocytes, B-lymphocytes, and macrophages, which surround a central artery.
The remaining splenic tissue, called red pulp, contains erythrocytes, platelets, macrophages, and B-lymphocytes. The cells in red pulp are housed in reticular connective tissue and form structures called splenic cords. Splenic sinusoids are associated with red pulp. Red pulp of the spleen serves as a blood reservoir, including a storage site for both erythrocytes and platelets

Question 44

Identify the main groups of tonsils and their location and function

Answer 44

Tonsils are secondary lymphatic structures that are not completely surrounded by a connective tissue capsule. They are found in the pharynx (throat) and oral cavity. The pharyngeal tonsil is found in the posterior wall of the nasopharynx; when this tonsil becomes enlarged, it is called adenoids. Palatine tonsils are in the posterolateral region of the oral cavity, and lingual tonsils are along the posterior one-third of the tongue. Tonsils help protect against foreign substances that may be either inhaled or ingested.

Question 45

Describe the composition of individual lymphoid nodules

Answer 45

Lymphatic nodules, or lymphatic follicles, are small, oval clusters of lymphatic cells (e.g., B-lymphocytes, T-lymphocytes, macrophages) with some extracellular matrix that are not completely surrounded by a connective tissue capsule. Scattered lymphatic nodules are referred to as diffuse lymphatic tissue. This tissue can be found in every body organ and within the wall of the appendix, where it helps to defend against infections in these structures. However, in some areas of the body, many lymphatic nodules group together to form larger structures, such as MALT.

Question 46

Compare the locations of MALT and Peyer patches

Answer 46

MALT (mucosa-associated lymphatic tissue) is located in the lamina propria of the mucosa of the gastrointestinal, respiratory, urinary, and reproductive tracts. The lymphatic cells in the MALT help defend against foreign substances that come in contact with mucosal membranes.
MALT is very prominent in the mucosa of the small intestine, primarily in the ileum. There, collections of lymphatic nodules called Peyer patches can become quite large and bulge into the gastrointestinal tract lumen.

Question 47

Six actions of antibodies

Answer 47

Precipitation
antibody cross-links circulating particles forming an insoluble antigen-anybody complex

complement fixation
Fc region of an antibody binds complement proteins; complement is activated

Opsonization
Fc region of antibody binds to receptors of phagocytic cells, triggering phagocytosis

Activation of NK cells
Fc region of antibody binds to an NK cell, triggering release of cytotoxic chemicals

Neutralization
antibody covers biologically active portion of microbe or toxin

Aggultination
antibody cross-links cells forming a “clump”

Question 48

Lymphatic trunks and ducts

Answer 48

Receive lymph from lymphatic vessels. Lymphatic trunks drain into
lymphatic ducts. Lymphatic ducts drain lymph into the venous circulation

Question 49

Define respiration and list 4 continuous and simultaneously occurring processes in ventilation

Answer 49

Respiration is a general term for the exchange of respiratory gases (oxygen and carbon dioxide) between the atmosphere and the sys­temic cells of the body.
∙ Pulmonary ventilation—movement of respiratory gases between the atmosphere and the alveoli of the lungs
∙ Alveolar gas exchange (or external respiration)—exchange of respiratory gases between the alveoli and the blood in the pulmonary capillaries
∙ Gas transport—transport of respiratory gases within the blood between the lungs and systemic cells of the body
∙ Systemic gas exchange (or internal respiration)—exchange of respiratory gases between the blood in the systemic capillaries and systemic cells of the body

Question 50

Define the general concept of ventilation and the specific physiologic process of pulmonary ventilation

Answer 50

The principles involved in breathing, whether quiet or forced, use the same physiologic processes. Autonomic nuclei in the brainstem regulate the skeletal muscles involved with breathing to rhythmically contract and relax, resulting in thoracic cavity volume changes. Dimensional changes within the thoracic cavity during breathing result in pressure changes, establishing a changing pressure gradient between the lungs and the atmosphere. Air moves down the pressure gradient either to enter the lungs during inspiration or to exit the lungs during expiration.

Question 51

Explain how pressure gradients are established by skeletal muscles of breathing and result in pulmonary ventilation

Answer 51

Muscles of quiet breathing: Increase dimensions of thoracic cavity
Diaphragm External intercostal

Muscles of forced inspiration: Pull upward and outward
Sternocleidomastoid Scalenes
Serratus posterior superior
Pectoralis minor
Erector spinae

Muscles of forced expiration: Pull downward and inward
Transversus thoracis
Serratus posterior inferior
Internal intercostal
External oblique
Transversus abdominis

Question 52

Explain the relationship between pressure and volume as described by Boyle’s law

Answer 52

Volume changes in the thoracic cavity cause gas pressure changes in the thoracic cavity. Boyle’s law states that at a constant tempera­ture, the pressure (P) of a gas decreases if the volume (V) of the container increases, and vice versa. The law may be expressed by the following formula:
P1V1 = P2V2
Boyle’s law states that volume and pressure are inversely related.
The figure shows that air does not move if the pressure is equal between two areas. It also shows how air moves from an area of high pressure to an area of low pressure when pressure gradients are established by changes in volume.

Question 53

Compare and contrast quiet and forced breathing in term of skeletal muscle involvement

Answer 53

During quiet breathing, the diaphragm and external intercostals must contract. During expiration, The diaphragm and external intercostals relax, decreasing the dimensions of the thoracic cavity. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm. This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help to compress the rib cage, which also reduces the volume of the thoracic cavity

Question 54

Describe the anatomic structure involved in regulating breathing

Answer 54

The autonomic nuclei within the central nervous system (CNS) that control breathing are collectively called the respiratory center. One portion of this center, which is located within the medulla oblongata, is called the medullary respiratory center. Two groups of nuclei compose the medullary respiratory center. These are the ventral respiratory group (VRG), located within the anterior region of the medulla (which contains both inspiratory neurons and expiratory neurons), and the dorsal respiratory group (DRG), located posterior to the VRG. The other portion of the respiratory center is within the pons and is called the pontine respiratory center.
Skeletal muscles of breathing include the diaphragm, the external intercostal muscles, and other accessory muscles of breathing. Upper motor neurons from the VRG synapse with lower motor neurons that extend from the spinal cord to the skeletal muscles of breathing. These lower motor neurons are within either the phrenic nerves that innervate the diaphragm or the intercostal nerves that innervate the intercostal muscles. Accessory muscles of respiration are innervated by other individually named somatic nerves

Question 55

Explain the physiologic events associated with controlling quiet breathing

Answer 55

Quiet inspiration is initiated when the inspiratory neurons of the VRG within the medullary respiratory center sponta­neously depolarize, or are “turned on.” Nerve signals initiated within the inspiratory center of the VRG are sent through the somatic nerve pathways to the skeletal muscles of quiet breathing for approximately 2 seconds. The intensity of the nerve signals increases over these 2 seconds. This stimulates both the diaphragm and external intercos­tal muscles to contract, resulting in an increase in thoracic cavity volume. Thus, a pressure gradient is established, and air moves from the atmosphere into the alveoli.
Quiet expiration occurs when the VRG is inhibited, or “turned off.” This inhibition results because, during quiet inspiration, nerve signals from the VRG inspiratory neurons are also relayed to the VRG expiratory neurons. The VRG expiratory neurons in response will then send inhibitory signals back to the VRG inspiratory neurons. This inhibition causes the VRG inspiratory neurons to “turn off.” Consequently, nerve signals are no longer sent through the nerve pathways to the skeletal muscles of quiet breathing; this lasts typically for approximately 3 seconds. Lack of somatic nerve stimulation causes both the diaphragm and external intercostal muscles to relax, resulting in a decrease in thoracic cavity volume. Thus, a pressure gradient is established and air moves from the alveoli into the atmosphere.

The specific role of the pontine respiratory center in breathing is to relay nerve signals to the medullary respiratory center to facilitate a smooth transition between inspiration and expiration.

Question 56

Explain the reflexes that alter breathing rate and depth

Answer 56

Proprioceptors
Baroreceptors (inhalation reflex or Hering-Breuer
Reflex)
Irritant Receptors
Actions of Higher Brain Center

-Breathing rate and depth are primarily altered by reflexes that respond to sensory input from chemoreceptors.
-When the DRG is activated, nerve signals are subsequently relayed to the VRG, resulting in a change in the rate and depth of breathing
-Change in the rate of breathing is accomplished by altering the amount of time spent in both inspiration and expiration, whereas altering the depth of breathing is accomplished through stimulation of accessory muscles, results in greater thoracic volume changes.
-Breathing rate and depth can be reflexively increased if either the central chemoreceptors detect an increase in H concentration in the CSF or the peripheral chemoreceptors detect an increase in blood H concentration

Question 57

Compare and contrast autonomic nervous system innervation of structures of the respiratory system and the somatic nervous
system innervation of the skeletal muscles involved in breathing

Answer 57

The control of the breathing muscles comes from both autonomic nuclei in the brainstem and somatic nuclei in the cerebral cortex. The autonomic nuclei forming the respiratory center within the brainstem regulate normal breathing with their rhythmic output along the lower motor neurons of the phrenic and intercostal nerves, and this center alters breathing rate and depth in response to various sensory input, as described. The cerebral cortex consciously regulates breathing by directly stimulat­ ing lower motor neurons that extend to the skeletal muscles of breath­ ing. This diverse motor output from the nervous system allows breathing to be controlled both reflexively and consciously.

Question 58

Explain how pressure gradients and resistance determines airflow

Answer 58

flow is directly related to the pressure gradient between atmosphere and lungs and inversely related to resistance. If the pressure gradient increases, air­ flow into the lungs increases, but if the pressure gradient decreases, airflow into the lungs lessens. In contrast, if resistance increases, airflow lessens, whereas if resis­tance decreases, airflow increases (assuming the pressure gradient remains the same).
The pressure gradient (ΔP) is the difference between atmospheric pressure and intrapulmonary pressure (Patm – Palv). It can be changed by altering the volume of the thoracic cavity. The contraction of both the diaphragm and the external intercostals during quiet breathing causes small volume changes that allow approximately 500 mL of air to enter the lungs. The thoracic cavity volume is further increased if accessory muscles of forced inspiration are stimulated, causing a larger decrease in intrapulmonary pressure. Airflow into the lungs increases because a steeper pressure gradient is established between atmospheric pressure and intrapulmonary pressure.
Airflow is always opposed by resistance. Resistance includes all the factors that make it more difficult to move air from the atmosphere through the respiratory passageway into the alveoli. Resistance may be altered in three ways: (1) a decrease in elasticity of the chest wall and lungs, (2) a change in the bronchiole diameter or the size of the passageway through which air moves, and (3) the collapse of alveoli.

Question 59

Compare and contrast minute ventilation and alveolar ventilation and explain the significance of each

Answer 59

The amount of air that reaches the alveoli and is available for gas exchange per minute is termed alveolar ventilation. The term pulmonary ventilation may also refer to the amount of air that is inhaled in 1 minute.

Tidal volume (amount of air per breath) x Respiration rate (number of breaths per minute) = Pulmonary ventilation

(Tidal volume - anatomic dead space) × Respiration rate = Alveolar ventilation

Question 60

Describe the relationship between anatomic dead space and physiologic dead space

Answer 60

When air is moved from the atmosphere into the respi­ratory tract, a portion of it remains in the conducting zone. This collec­tive space, where there is no exchange of respiratory gases, is referred to as the anatomic dead space, and it has an average volume of approximately 150 mL. The amount of air that reaches the alveoli and is available for gas exchange per minute is termed alveolar ventilation. This volume is less than pulmonary ventilation because the volume of air in the anatomic dead space must be subtracted from the volume of air inhaled with each breath.

Deeper breathing is more effective for maximizing alveolar venti­lation than faster, shallower breathing. Assuming you take one deep breath, you have to overcome the dead air space only one time. All the additional air inhaled in that breath is available for gas exchange.
Some respiratory disorders result in a decreased number of alveoli participating in gas exchange. This decrease can be due either to damage to the alveoli or to a change in the respiratory membrane, such as when fluid accumulates in the lungs with pneumonia. The difference in volume of air available for gas exchange is accounted for by the more inclusive term physiologic dead space, which is the normal anatomic dead space plus any loss of alveoli. The anatomic dead space is equivalent to the physiologic dead space in a healthy individual, because the loss of alveoli should be minimal.

Question 61

Define partial pressure and explain how it is altered with changes in total pressure and the percentage of a gas

Answer 61

Partial pressure is the pressure exerted by each gas within a mixture of gases and is measured in mm Hg; it is written with a P followed by the symbol for the gas. For example, the partial pressure for oxygen is written as Po2.
Total pressure × % of gas = Partial pressure of that gas
The relationship of partial pressure to total pressure is summarized by Dalton’s law, which states that the total pressure in a mixture of gases is equal to the sum of all of the individual partial pressures.

Question 62

Describe partial pressure gradient and the movement of a gas relative to its partial pressure gradient

Answer 62

A partial pressure gradient exists when the partial pressure for a specific gas is higher in one region than in another. If a partial pres­sure gradient exists between two regions for a given gas, then the gas moves from the region of its higher partial pressure to the region of its lower partial pressure, and it may continue to move until the par­tial pressures in the two regions become equal. The exchange of respiratory gases in both alveolar gas exchange and systemic gas exchange is dependent upon partial pressure gradients.

Question 63

Explain the laws that govern gas solubility

Answer 63

Specific additional chemical principles govern the exchange of gas between air (a gas) and blood (a liquid). These principles are sum­marized by Henry’s law, which states that at a given temperature, the solubility of a gas in a liquid (i.e., how much gas can either enter or leave the liquid) is dependent upon (1) the partial pressure of the gas in the air and (2) the solubility coefficient of the gas in the liquid.
The solubility coefficient is the volume of gas that dissolves in a specified volume of liquid at a given temperature and pressure. This coefficient is a constant that depends upon the interactions between molecules of both the gas and the liquid. The more favorable these molecular interactions, the greater the amount of gas that dissolves in the liquid.
Because the amount of a gas that dissolves in a liquid is depen­dent upon both its partial pressure and its solubility coefficient, gases with low solubility coefficients

Question 64

Describe pulmonary gas exchange and the partial pressure gradients responsible for this exchange

Answer 64

Oxygen diffuses across the respiratory membrane from the alveoli into the capillaries because of the Po2 partial pressure gradient, until the Po2 in the blood is equal to that of the alveoli at 104 mm Hg. Thus, blood Po2 has increased from 40 to 104 mm Hg as blood moves through the pulmonary capillar­ies. However, the Po2 in the alveoli remains constant because oxygen is continuously entering the alveoli through the respiratory passageways.
Simultaneously, CO2 is diffusing in the opposite direction; the Pco2 in the alveoli is 40 mm Hg and that of the blood entering the pulmonary capillaries is 45 mm Hg. Carbon dioxide diffuses down its partial pressure gradient from the blood into the alveoli until the Pco2 in the blood is equal to that of the alveoli at 40 mm Hg. Thus, blood Pco2 has decreased from 45 to 40 mm Hg as blood moves through the pulmonary capillaries. As with oxygen, the Pco2 in the alveoli also remains constant because carbon dioxide is continuously leaving the alveoli through the respiratory passageways.

Question 65

Name the two anatomic features of the respiratory membrane that contribute to efficient pulmonary gas exchange

Answer 65

The efficiency of both O2 and CO2 diffusion during alveolar gas exchange is dependent upon anatomic features of the respiratory mem­brane: its large surface and its minimal thickness.

Question 66

Explain ventilation-perfusion coupling and how it maximizes pulmonary gas exchange

Answer 66

The smooth muscle of both the bronchioles that lead into the alveoli and the arterioles that carry blood to pulmonary capillaries can contract and relax to maximize gas exchange. This inherent ability of bronchioles to regulate airflow and arterioles to regulate blood flow simultaneously is called ventilation-perfusion coupling.

Ventilation is altered by changes in bronchodilation and bronchoconstriction. Bronchioles dilate in response to an increase in Pco2, whereas they constrict in response to a decrease in Pco2.
Perfusion is altered by changes in pulmonary arteriole dilation and constriction. These arterioles dilate in response to either an increase in Po2 or a decrease in Pco2, whereas they constrict in response to either a decrease in Po2 or an increase in Pco2. Note these changes in bronchioles and pulmonary arterioles occur independently of one another.

Question 67

Explain tissue gas exchange

Answer 67

Oxygen diffuses out of sys­temic capillaries into the interstitial fluid around systemic cells, prior to crossing the plasma membrane to enter a cell. At the same time, carbon dioxide exits the cell moving in the opposite direction to enter the blood in systemic capillaries. The driving force is the same as in alveolar gas exchange—the partial pressure gradients that exist for both O2 and CO2.

Notice that the Po2 in the systemic cells is 40 mm Hg, and the blood as it enters the surrounding systemic capillaries has a Po2 of 95 mm Hg. Therefore, oxygen diffuses out of the systemic capillaries down its partial pressure gradient into the cells until the blood Po2 is equal to the partial pressure in the cells at 40 mm Hg. Thus, blood Po2 has decreased from 95 to 40 mm Hg as blood moves through the systemic capillaries.
Simultaneously, carbon dioxide is diffusing in the opposite direction. The Pco2 in systemic cells is 45 mm Hg, and the blood entering the systemic capillaries is 40 mm Hg. Carbon dioxide diffuses down its partial pressure gradient from the cells into the blood until blood Pco2 is 45 mm Hg. Thus, blood Pco2 has increased from 40 to 45 mm Hg as blood moves through the systemic capillaries.

Question 68

Compare and contrast pulmonary and tissue gas exchange

Answer 68

During alveolar gas exchange, blood Pco2 decreases from 45 to 40 mm Hg; during systemic gas exchange, blood Pco2 increases from 40 to 45 mm Hg. Notice that the Pco2 values reverse as the blood makes its way through the two cardiovascular circuits: 45 to 40 mm Hg in the pulmonary capillaries, 40 to 45 mm Hg in the systemic capillaries.
Blood Po increases from 40 to 104 mm Hg during alveolar gas 2
exchange, and blood Po2 decreases from 95 to 40 mm Hg during sys­temic gas exchange. The bronchial veins drain small amounts of deoxygenated blood into the pulmonary veins prior to the blood returning to the heart, where it is subsequently pumped by the left ventricle through the systemic circulation. This input of deoxygenated blood accounts for the decrease in Po2 from 104 to 95 mm Hg

Question 69

Explain why hemoglobin is essential to oxygen transport

Answer 69

Oxygen then diffuses from the blood within systemic capillaries into systemic cells. The ability of blood to transport oxygen is dependent upon two factors: the solubility coefficient of oxygen in blood plasma and the presence of hemoglobin (Hb).

The solubility coefficient of oxygen is very low (0.024). This means that only small amounts of oxygen (less than 2%) are dis­solved in the plasma. Consequently, about 98% of the oxygen in the blood must be transported within erythrocytes, where it attaches to the iron within hemoglobin molecules.

Question 70

Describe the three ways carbon dioxide is transported in the blood

Answer 70

Whereas hemoglobin is the major means of transporting oxygen, carbon dioxide has three means of being transported in the blood from the systemic cells to the alveoli: (1) CO2 dissolved within plasma, (2) CO2 attached to the globin portion of hemoglobin, and (3) as bicar­bonate (HCO3-) dissolved within plasma.

The remaining 70% of the CO2 diffuses into erythrocytes and combines with water to form bicarbonate (HCO3-) and H+ (a chemi­ cal reaction catalyzed by carbonic anhydrase):
CO2 + H2O ⇄ H2CO3 ⇄ HCO3- +H+
HCO3- then diffuses into the plasma. Thus, the largest percentage of CO2 is carried from the systemic cells to the lungs in plasma as dis­ solved HCO3-. Carbon dioxide is regenerated when blood moves through pulmonary capillaries and this process is reversed.

Question 71

Explain oxygen binding to hemoglobin and the oxygen-hemoglobin saturation curve

Answer 71

Hemoglobin saturation is determined by several variables. The most important variable is the Po2. Predictably, as the Po2 increases, hemoglobin saturation increases. The binding of each O2 molecule causes a conformational change in hemoglobin that makes it progressively easier for each additional O2 molecule to bind to an available iron. This increase in the ease of oxygen binding is termed the cooperative binding effect of O2 loading.
Notice that the relationship between Po2 and hemoglobin saturation is not linear (a straight line). The plotted points on the graph pro­duce an S-shaped, or sigmoidal, curve.

Question 72

Describe hemoglobin as a transport molecule and what occurs during pulmonary and tissue gas exchange

Answer 72

If we ascend a high mountain, the air thins and environmental Po2 decreases; this is accompanied by a decrease in alveolar Po2.
Ascending to a very high altitude, with its accompanying large decrease in alveolar Po2, results in large decreases in hemoglobin saturation.

Under resting conditions only a small per­centage of the oxygen (approximately 20–25%) transported by the hemoglobin is released as it passes through systemic capillaries. If systemic cell Po2 decreases to 20 mm Hg, as occurs during vigorous exercise, then what is the hemoglobin satura­tion? Vigorous exercise produces a large decrease in the hemoglobin saturation, meaning that more oxygen is unloaded.

The most important variable that influences oxygen release from hemoglobin during systemic gas exchange is blood Po2. Other variables cause a conformational change in hemoglobin that increases the release of oxygen, including (b) an increase in temperature; (c) an increase in H+; (d) the binding of 2,3­BPG; and (e) the binding of carbon dioxide. The influence of temperature and H+ levels (i.e., pH) on the oxygen­hemoglobin saturation curve is shown in figure (b) and figure (c), respectively.

Question 73

Explain how hyperventilation and hypoventilation influence the chemical composition of blood

Answer 73

Hyperventilation: a breathing rate or depth that is increased above the body’s demand
-Po2 levels increase and Pco2 levels decrease in the alveoli.
-Additional carbon dioxide leaves the blood to enter the alveoli due to a steeper Pco2 gradient
-Hypocapnia (decrease in blood carbon dioxide (CO2) levels below the normal level) occurs
-Low blood Pco2 may also cause decrease in blood [H+] (increased blood pH), leading to respiratory alkalosis

Hypoventilation: breathing that is either too slow or too shallow to properly meet metabolic needs.
-Oxygen levels decrease and carbon dioxide levels increase in the alveoli
-Lower amounts of oxygen diffuse from the alveoli into the blood, and blood Po2 decreases (hypoxemia)
-Lower amounts of carbon dioxide diffuse from the blood into the alveoli (hypercapnia)
-Low blood oxygen levels = decrease in aerobic cellular respiration
-High blood Pco2 may result in a decrease in pH which leads to respiratory acidosis

Question 74

Describe how breathing rate and depth affect venous return of blood and lymph

Answer 74

Respiratory pump action increases during hyperventilation (increased breathing depth primarily), increasing venous return of blood and lymph, whereas during hypoventilation (decreased breathing depth primarily), decreased action of the respiratory pump decreases venous return of blood and lymph.

Question 75

Explain the changes in breathing that accompany exercise

Answer 75

A person’s breathing depth increases but the breathing rate remains the same (hyperpnea).
Oxygen consumption and carbon dioxide production increase in response to elevated rates of cellular respiration during exercise but blood Po2 and Pco2 levels remain the same.

Question 76

Briefly explain the process of absorption

Answer 76

Absorption involves membrane transport of digested molecules, electrolytes, vitamins, and water across the epithelial lining of the GI tract into the blood or lymph. Absorption occurs primarily within the small intestine.

Absorption occurs when substances are moved through the simple columnar epithelial cells that line the GI tract wall and are absorbed into blood or lymphatic capillaries located within the lamina propria

Question 77

Distinguish the action of the muscularis mucosae of the mucosal tunic from that of the muscularis tunic.

Answer 77

The muscularis mucosae is the deepest layer of the mucosa and is composed of a thin layer of smooth muscle. Contractions of this smooth muscle layer cause slight movements in the mucosa to gently “shake things up,” which (1) facilitates the release of secretions from the mucosa into the lumen and (2) increases contact of materials in the lumen with the epithelial layer of the mucosa for more efficient absorption.

The muscularis is located deep to the submucosa and is composed of smooth muscle. Fine branches of nerves and their associated autonomic ganglia are located between these two layers of smooth muscle; these nerve branches control contractions of the muscularis and are collectively referred to as the myenteric nerve plexus, or Auerbach plexus.
The collective contractions of these smooth muscle layers are associated with two primary types of motility: mixing and propulsion:
∙ Mixing is a “backward-and-forward” motion that blends secretions with ingested material within the GI tract, but does not result in directional movement of the lumen contents. Mixing includes mixing waves (by the stomach) and segmentation (by the small intestine).
∙ Propulsion, in comparison, is the directional movement of materials through the GI tract, and it occurs by the muscularis of the GI tract by peristalsis. Peristalsis is the sequential contraction of the muscularis within the GI tract wall that moves like a wave within the different regions of the GI tract (the esophagus, stomach, small intestine, and large intestine). Persistalsis results in one-way movement.

Question 78

Describe the general function of the enteric nervous system and autonomic nervous system in the regulation of the digestive system

Answer 78

The enteric nervous system (ENS) is an array of both sensory neurons and motor neurons, which extends from the esophagus to the anus. This network of neurons is located within the submucosal plexus and the myenteric plexus of the gastrointestinal (GI) tract wall. It innervates the smooth muscle and glands of the GI tract and mediates the complex coordinated reflexes for the mixing and propulsion of materials through the GI tract.

Autonomic Nervous System (ANS)
The GI tract wall is also innervated by both the parasympathetic and sympathetic divisions of the autonomic nervous system (ANS). The parasympathetic and sympathetic axons synapse with smooth muscle and glands of the GI tract wall (to control these structures directly) and with neurons within the ENS (to regulate these structures indirectly). In general, parasympathetic innervation promotes GI tract activity: It stimulates GI motility and relaxes GI tract sphincters. In contrast, sympathetic innervation opposes GI tract activity: It inhibits GI tract motility, contracts GI tract sphincters, and vasoconstricts blood vessels within the GI tract wall. Thus, any conditions that activate the sympathetic division (e.g., exercise, anger, stress) may slow or interfere with digestion.

Question 79

Compare short reflexes and long reflexes that regulate the digestive system

Answer 79

A short reflex is a local reflex that only involves the ENS (and does not involve the central nervous system). Sensory input detected by either baroreceptors or chemoreceptors is relayed to neurons within the ENS to alter smooth muscle contraction and gland secre- tion. These reflexes function in coordinating small segments of the GI tract to changes in stimuli.

A long reflex involves sensory input relayed to the central nervous system (CNS), which serves as the integration center. Autonomic motor output is then relayed to alter smooth muscle contraction and gland secretion of the GI tract wall. Note that autonomic motor output is often relayed to other structures, including the accessory digestive organs. The results are coordinated smooth muscle contractions and secretory activity of potentially many different components of the digestive system.

Question 80

Describe the general function of hormones that regulate the processes of digestion

Answer 80

Several primary hormones participate in the regulation of the processes of digestion: gastrin released from the stomach and secretin, cholecystokinin, and motilin released from the small intestine.
Gastrin: Gastrin is a peptide hormone that stimulates gastric mucosal development, motility, and the release of hydrochloric acid (HCl) into the stomach. The presence of specific nutrients in the stomach lumen, particularly peptides, certain amino acids, and calcium, is the major trigger for gastrin release.

Motilin: Motilin is a hormone produced by entero-endocrine cells (Mo cells) in the upper small intestine that is released cyclically during fasting. It promotes gall bladder emptying, increased pancreatic insulin release, GI tract motility, and increased appetite.

Cholecystokinin: CCK is a hormone released by I-cells in the upper small intestine and it aids in pancreatic secretion and gallbladder contraction, as well as regulating gastrointestinal motility and inducing satiety.

Question 81

Describe the function of saliva and the regulation of its release

Answer 81

Salivary glands, which produce saliva, are located both within the oral cavity (intrinsic salivary glands) and outside the oral cavity (extrinsic salivary glands). Intrinsic salivary glands are unicellular glands that continuously release relatively small amounts of secretions independent of the presence of food. Only the secretions from the intrinsic salivary glands contain lingual lipase, an enzyme that begins the digestion of triglycerides. Most saliva, however, is produced from multicellular exocrine glands outside the oral cavity called extrinsic salivary glands.

These components permit saliva to participate in various functions:
∙ Moistens ingested food as it is formed into a bolus, a globular, wet mass of partially digested material that is more easily swallowed
∙ Initiates the chemical breakdown of starch in the oral cavity because of the salivary amylase it contains
∙ Acts as a watery medium into which food molecules are dissolved so taste receptors may be stimulated
∙ Cleanses the oral cavity structures
∙ Helps inhibit bacterial growth in the oral cavity because it contains antibacterial substances, including lysozyme and IgA antibodies (IgA is formed by plasma cells in the lamina propria and transported across the epithelial cells

The salivary nuclei within the brainstem regulate salivation. A basal level of salivation in response to parasympathetic stimulation ensures that the oral cavity remains moist. Input to the salivary nuclei is received from chemoreceptors or baroreceptors in the upper GI tract. These receptors detect various types of stimuli, including the introduction of substances into the oral cavity, especially those that are acidic, such as a lemon; and arrival of foods into the stomach lumen, especially foods that are spicy or acidic. If one eats spoiled food, bacterial toxins within the stomach stimulate receptors that initiate sensory nerve signals to the salivary nuclei. Input is also received by the salivary nuclei from the higher brain centers in response to the thought, smell, or sight of food. Stimulation of the salivary nuclei by either sensory receptors or higher brain centers results in increased nerve signals relayed along parasympathetic neurons within both the facial nerve (CN VII), which innervates the submandibular and sublingual salivary glands, and the glossopharyngeal nerve (CN IX), which innervates the parotid salivary glands, and additional saliva is released.
Sympathetic stimulation, which occurs during exercise or when an individual is excited or anxious, results in a more viscous saliva by decreasing the water content of saliva.

Question 82

Describe the functions of the pharynx and esophagus and their complementary activities in the process of swallowing.

Answer 82

The pharynx is a funnel- shaped, muscular passageway with distensible (stretchable) lateral walls that serves as the passageway for both air and food.
Swallowing has three phases: the voluntary phase, the pharyngeal phase, and the esophageal phase.

Voluntary phase
Bolus of food is pushed
by tongue against hard palate and then moves toward oropharynx.

Pharyngeal phase (involuntary)
As bolus moves into
oropharynx, the soft palate
and uvula close off the nasopharynx, and the larynx elevates so the epiglottis closes over laryngeal opening.

Esophageal phase (involuntary)
Soft palate, uvula, and epiglottis return to preswallowing position.
Superior esophageal sphincter closes.
Bolus passes through esophagus and enters the stomach.
Inferior esophageal sphincter opens.
Peristaltic contractions of esophageal muscle push bolus toward stomach.

The superior and inferior esophageal sphincters are normally closed at rest. When the bolus is swallowed, these sphincters relax to allow it to pass through the esophagus. The inferior esophageal sphincter contracts after passage of the bolus, helping to prevent reflux of materials and fluids from the stomach into the esophagus.

Question 83

Explain the two general functional activities of the stomach

Answer 83

Chemical digestion of both protein and fat begins in the stomach, but absorption from it is limited to small, nonpolar substances that are in contact with the mucosa of the stomach. Both alcohol and aspirin are examples of substances that are absorbed in the stomach. One significant function of the stomach is to serve as a “holding bag” for controlled release of partially digested materials into the small intestine, where most chemical digestion and absorption occur. One of the most vital functions performed by the stomach is the release of intrinsic factor (a substance required for the absorption of vitamin B12, which occurs within the small intestine).

Question 84

List the secretions of the stomach and their functions

Answer 84

Five types of secretory cells of the gastric epithelium are integral contributors to the process of digestion. Four of these cell types produce the approximately 3 liters per day of gastric juice that are released into the stomach lumen. The fifth type of cell (G-cell) secretes a hormone into the blood.

Surface mucous cell (secretes alkaline fluid containing mucin)
Mucous neck cell (secretes acidic fluid containing mucin)
Parietal cell (secretes intrinsic factor and hydrochloric acid)
Chief cell (secretes pepsinogen and gastric lipase)
G-cell (enteroendocrine cells that secrete gastrin into the blood)

Question 85

Describe the phases that regulate motility in the stomach

Answer 85

Motility
1 Contractions of smooth muscle in stomach wall mix bolus with gastric secretions to form chyme.
2 Peristaltic waves result in pressure gradients that move stomach contents toward the pyloric region.
3 Pressure gradient increases force in pylorus against pyloric sphincter.
4 Pyloric sphincter opens,
and a small volume of chyme enters the duodenum.
5 Pyloric sphincter closes, and retropulsion occurs.

Question 86

Describe the phases that regulate secretion in the stomach

Answer 86

The cephalic reflex:
Initiated by thought, smell, sight, or taste of food (or even sounds of food preparation)
1. Receptors: Special senses (e.g., nose, eyes)
2. Sensory input: Increased nerve signals relayed from the cerebral cortex and hypothalamus to the medulla oblongata
3. Medulla oblongata integrates input from higher brain centers
4. Motor output: Increased nerve signals relayed along the vagus nerve from medulla oblongata to stomach
5. Effector: Stomach stimulated to increase both its force of contraction and release of secretions

The gastric reflex:
Initiated by presence of food in stomach
1. Receptors: Baroreceptors in stomach wall detect stretch; chemoreceptors detect protein or high pH in stomach contents
2. Sensory input: Increased nerve signals relayed to medulla oblongata
3. Medulla oblongata integrates sensory input
4. Motor output: Increased nerve signals relayed along the vagus nerve from medulla oblongata to stomach
5. Effector: Stomach stimulated to increase both its force of contraction and release of secretions
In addition, the presence of food in the stomach causes release of gastrin, which targets the stomach to increase both its the force of contraction and the release of secretions (especially HCl). Gastrin also stimulates contraction of the pyloric sphincter.

Intestinal reflex
Initiated by presence of acidic chyme in duodenum
1. Receptors: Chemoreceptors in intestinal wall detect acidic chyme or low pH in stomach contents
2. Sensory input: Decreased nerve signals relayed to medulla oblongata
3. Medulla oblongata integrates sensory input
4. Motor output: Decreased nerve signals relayed along the vagus nerve from medulla oblongata
to stomach
5. Effector: Stomach inhibited to decrease both its
force of contraction and release of secretions
In addition, the presence of fatty chyme
in the duodenum causes release of cholecystokinin (CCK), which decreases the force of contraction in the stomach. The presence of acidic chyme causes release of secretin,
which inhibits release of stomach secretions.

Question 87

List the glands found in the small intestine and their secretions

Answer 87

Four types of secretory cells of the intestinal epithelium contribute to the process of digestion. Three of these cell types produce intestinal juice. The fourth type of cell secretes hormones into the blood.
∙ Goblet cells produce mucin that when hydrated form mucus, which lubricates and protects the intestinal lining. These cells increase in number from the duodenum to the ileum, because more lubrication is needed as digested materials (and water) are absorbed and undigested materials (and less water) remain in the lumen.
∙ Unicellular gland cells synthesize enteropeptidase.
∙ The enteroendocrine cells release hormones such as CCK and secretin into the blood.

Another type of gland housed within the submucosal layer and found only in the proximal duodenum is called a duodenal submucosal gland (or Brunner gland). This gland produces a viscous, alkaline mucus secretion that protects the duodenum from the acidic chyme entering the duodenum from the stomach.

Question 88

Explain motility within the small intestine

Answer 88

Smooth muscle activity of the muscularis within the small intestine wall has three primary functions: (1) mixing chyme with accessory gland secretions, (2) moving the chyme continually against the brush border, and (3) propelling the contents through the small intestine toward the large intestine.
All these functions facilitate chemical digestion and absorption, employing the processes of segmentation and peristalsis. When chyme first enters the small intestine, segmentation is more prevalent than peristalsis. Segmentation mixes chyme with accessory gland secretions through a “backward-and-forward” motion.
Peristalsis then propels material within the GI lumen by alternating contraction of the circular and longitudinal muscle layers in small regions. The rhythm of muscular contractions is more frequent in the duodenum than in the ileum; thus, the net movement of intestinal contents is toward the large intestine.

Question 89

Describe the pancreas and explain its general function in digestion

Answer 89

The pancreas has both endocrine and exocrine functions. Endocrine cells produce and secrete hormones such as insulin and glucagon. Exocrine cells (called acinar cells) produce pancreatic juice to assist with digestive activities. The pancreas is the “work- horse” for providing digestive enzymes into the small intestine for chemical digestion. Disorders that affect either (1) the pancreatic ducts that lead from the pancreas into the duodenum or (2) the pancreas have serious and potentially fatal effects on the ability to digest and absorb nutrients.

Question 90

Explain the regulation of the accessory digestive glands associated with the small intestine

Answer 90

Recall that regulation of the stomach is organized into three phases: cephalic phase, gastric phase, and intestinal phase. The increase in vagal stimulation in the cephalic phase and gastric phase, in addition to stimulating stomach motility and secretion, also activates the pancreas to release pancreatic juice. Recall that in the intestinal phase, both cholecystokinin (CCK) and secretin are released. Cholecystokinin is a hormone released from the small intestine primarily in response to free fatty acids in chyme.

The functions of CCK include:
Initiating smooth muscle within the gallbladder wall to strongly contract, causing the release of concentrated bile (this primary function of stimulating the gallbladder, also called the cholecyst, is how the name cholecystokinin is derived)
∙ Stimulating the pancreas to release enzyme-rich pancreatic juice
∙ Relaxing the smooth muscle within the hepatopancreatic ampulla, allowing entry of bile and pancreatic juice into the small intestine

Secretin is released from the small intestine primarily in response to an increase in chyme acidity. Secretin primarily causes the release of an alkaline solution from both the liver and ducts of the pancreas. Upon entering the small intestine, this alkaline fluid helps neutralize the acidic chyme.

Question 91

Describe the bacterial action that takes place in the large intestine

Answer 91

Numerous normal bacterial flora inhabit the large intestine; they are termed the indigenous microbiota. These bacteria are responsible for the chemical breakdown of complex carbohydrates, proteins, and lipids that remain in the chyme after it has passed through the small intestine. Bacterial actions produce gases (carbon dioxide, hydrogen, hydrogen sulfide, and methane) from digestion of carbohydrates, and indoles and skatoles from digestion of protein. Additionally, B vitamins and vitamin K are produced by the bacterial flora, which are then absorbed from the large intestine into the blood. (these vitamins are also absorbed in the small intestine from the foods that we eat.) Feces is the final product formed and then eliminated from the GI tract.

Question 92

Describe several types of movement in the large intestine

Answer 92

Several types of movements are noted in the large intestine:
∙ Peristalsis of the large intestine is usually weak and sluggish, but otherwise it resembles the peristalsis that occurs in the wall of the small intestine.
∙ Haustral churning occurs after a relaxed haustrum fills with digested or fecal material until its distension stimulates reflex contractions in the muscularis. These contractions increase churning and move the material to more distal haustra.
∙ Mass movements are powerful, peristaltic-like contractions involving the teniae coli, which propel fecal material toward the rectum. A wave of contraction begins in the middle of the transverse colon, forcing a large amount of fecal matter into the descending colon, sigmoid colon, and rectum. Generally, mass movements occur two or three times a day, often during or immediately after a meal.

Question 93

Describe the pathways in the defecation reflex

Answer 93

1 Rectum contents stimulate baroreceptors in rectal wall.
2 Sensory input initiated by baroreceptors in rectum
is relayed to the spinal cord.
3 Nerve signals relayed along parasympathetic axons are increased.
4 Increased nerve signals to smooth muscle of the sigmoid colon and rectum, which contract, squeezing the contents. Increased nerve signals to the internal anal sphincter cause sphincter relaxation.

Question 94

Name the three classes of carbohydrates

Answer 94

Carbohydrates are organized based upon the number of repeating units of simple sugars. Carbohydrates may be classified as monosaccharides (e.g., glucose, fructose, and galactose), disaccharides (e.g., sucrose, maltose, and lactose), and polysaccharides (e.g., starch and cellulose).

Question 95

Explain the processing in the oral cavity that initiates carbohydrate digestion and how it is completed in the small intestine

Answer 95

Digestion of starch begins in the oral cavity. It is catalyzed by salivary amylase that is synthesized and released from the salivary glands. Salivary amylase breaks the chemical bonds between glucose molecules, within the starch molecule, to partially digest the starch molecule. The extent of starch digestion is dependent upon the length of time the salivary amylase is allowed to act on the starch.

AFTER
1 Pancreatic amylase is produced by the pancreas and secreted into the small intestine.
2 Pancreatic amylase continues digestion of starch that began in the oral cavity by salivary amylase.
3 Brush border enzymes
complete the breakdown of starch to individual glucose molecules, and are responsible for the digestion of disaccharides.

Question 96

Explain why the proteolytic enzymes of the stomach and pancreas are synthesized in inactive forms

Answer 96

Proteins are broken down into amino acids by enzymes that target peptide bonds between either specific adjacent amino acids within the protein or any amino acid from the end of a protein. All enzymes that digest protein are released from both the stomach and pancreas as inactive enzymes. These enzymes must be activated (e.g., pepsinogen is activated to pepsin within the low pH of the stomach). This is because the proteolytic enzymes would destroy the proteins within the cells that produce them or, in the case of protein-digesting enzymes produced in the pancreas, would destroy the cells lining the main and accessory pancreatic ducts as they passed through those ducts.

Question 97

Describe the activation and role of pepsin in the initiation of protein digestion in the stomach

Answer 97

Protein digestion begins within the stomach lumen with the enzyme pepsin. Pepsin is formed from pepsinogen, an inactive precursor released by chief cells. Hydrochloric acid that is released from parietal cells causes a low pH within the stomach that both activates pepsinogen to pepsin and denatures proteins to facilitate their chemical breakdown

Question 98

Describe the activation of proteolytic enzymes and the chemical digestion of proteins that occurs in the small intestine

Answer 98

1. Proteolytic enzymes are released from pancreas.
2. Enteropeptidase activates trypsinogen to trypsin; trypsin then activates other proteolytic enzymes.
3. Activated pancreatic proteolytic enzymes break proteins into peptides and amino acids.
4. Brush border peptidases break peptides into single amino acids to be absorbed through epithelial cell into blood.

Question 99

Explain the role of bile salts in mechanical digestion of lipids and the role of pancreatic lipase in the chemical digestion of triglycerides

Answer 99

1. Bile salts released from the liver and gallbladder emulsify lipid droplets to form micelles.
2. Pancreatic lipase functions within micelles to digest each triglyceride into a monoglyceride and two free fatty acids.
3. Monoglycerides and free fatty acids enter an epithelial cell, while bile salts remain in the intestinal lumen to be reabsorbed and recycled.
4. Triglyceride molecules are reassembled within epithelial cells. Lipids are then wrapped with protein to form a chylomicron. Chylomicrons are packaged within secretory vesicles and then exocytosed from the cells and absorbed into lacteals.

Pancreatic lipase digests each triglyceride into a monoglyceride and two free fatty acids. However, because triglycerides are lipids and do not dissolve in the luminal fluids of the digestive system, triglycerides form relatively large lipid masses. Thus, the large lipid droplets must first be mechanically separated into smaller droplets before pancreatic lipase effectively digests the fat. This process is called emulsification. Emulsification occurs by the action of bile salts, which are part of bile. Bile is produced by the liver and stored, concentrated, and released from the gallbladder. Bile salts are amphipathic molecules composed of a polar head and a nonpolar tail. The nonpolar tails position themselves around the fat with the polar heads next to the aqueous fluid in the lumen. This structure is called a micelle. Thus, the function of bile salts is to emulsify fats so that pancreatic lipase has greater “access” to the triglyceride molecules and may more effectively chemically digest the fat molecules. Cholesterol is also within the micelle, but it is not chemically digested. No brush border enzymes are required in the breakdown of triglycerides.

Question 100

Discuss the process by which lipids are absorbed

Answer 100

Digested triglycerides (monoglycerides and free fatty acids), cholesterol, other lipids, and fat-soluble vitamins are contained within micelles. Micelles transport lipids to the simple columnar epithelial lining of the small intestine. Here, the lipids enter the epithelial cells, whereas the bile salts remain in the small intestine lumen to be recycled and reused. Once inside the epithelial cells, the fatty acids are reattached to the monoglyceride to re-form triglycerides. Triglycerides, cholesterol, and other lipid molecules are then “wrapped” with protein to form a chylomicron. The Golgi apparatus packages chylomicrons into secretory vesicles. Vesicles containing chylomicrons merge with the plasma membrane of epithelial cells to release chylomicrons by exocytosis. Chylomicrons are too large to pass through blood capillary walls but instead enter the lacteals, the lymphatic capillaries of the small intestine. The overlapping endothelial cells of the lacteals act as one-way valves to permit entry of chylomicrons.
All lymph enters the blood, via either the right lymphatic duct or the thoracic duct, at the junction of the internal jugular vein and the subclavian vein. Chylomicrons enter the blood and deliver lipids to the liver and other tissues.

Question 101

Describe the digestion of nucleic acids

Answer 101

Nucleic acid digestion occurs in the small intestine. The nucleases (deoxyribonuclease and ribonuclease), synthesized and released by the pancreas, begin the digestion of nucleic acids. Each breaks the phosphodiester bond between the individual nucleotides of DNA and RNA, respectively. Nucleotides are the products: deoxyribonucleotides from DNA and ribonucleotides from RNA.
The breakdown of the nucleotides is accomplished by brush border enzymes embedded in the epithelial lining of the small intestine. These enzymes include (1) phosphatase, which breaks the bond holding the phosphate to the rest of the nucleotide (without the phosphate, this molecule is called a nucleoside), and (2) nucleosidase, which breaks the bond between the sugar and the nitrogenous base of the nucleoside, releasing the sugar and nitrogenous base.
All nucleic acid component building blocks are absorbed across the epithelium of the small intestine into the blood. These include phosphate, the sugar, and the nitrogenous bases.

Question 102

Describe the absorption of water, electrolytes, and vitamins

Answer 102

Our small intestine functions to absorb almost all of the water (about 8 L) that enters the small intestine. Thus, the daily water content of chyme entering the large intestine is only about 1 liter. The large intestine will then absorb about 0.8 liter, which leaves, on average, only about 0.2 liter (or 200 mL) of water lost daily in the feces. Water is absorbed across the epithelial lining of the small and large intestines into the blood capillaries by osmosis. Blood is transported throughout the body; as it moves through blood capillaries, water leaves the blood to enter the interstitial cells and systemic cells to help maintain fluid balance.

Our small intestine functions to absorb almost all of the electrolytes that enter the small intestine. Most electrolyte absorption is unregulated and is, instead, dependent upon the amount in the diet. The greater the amount ingested, the greater the amount absorbed. Diarrhea (e.g., caused by food poisoning, gastroenteritis, laxatives, medications) leads to an excessive loss of both water and K+, with the associated risk of hypokalemia. Diarrhea (with the loss of HCO3-) can also result in metabolic acidosis.
Iron is unusual in that its absorption is controlled. The hormone hepcidin is released from the liver in response to iron levels. Hepcidin inhibits the transport protein (ferroportin) located in the epithelial (basolateral) membrane of the GI tract. Thus, when iron levels are low, hepcidin release is decreased, which removes this inhibition, allowing for greater iron absorption.

Vitamins are organic molecules that are categorized as either (1) fat- soluble vitamins or (2) water-soluble vitamins. Fat-soluble vitamins (A, D, E, and K) are absorbed from the small intestine lumen into epithelial cells with lipids within micelles. Note that fat-soluble vitamins require lipid for their absorption—without it, the fat-soluble vitamins are not absorbed, continue through the GI tract, and are lost in the feces.

Question 103

Explain the details of vitamin B12 absorption

Answer 103

Water-soluble vitamins (B and C) are absorbed through various membrane transport mechanisms, including simple diffusion and active transport. Vitamin B, because of its large molecular size, must be transported by receptor-mediated endocytosis. The process requires intrinsic factor, which is released from parietal cells of the stomach. Intrinsic factor is a glycoprotein that, following its formation by parietal cells, continues within the GI tract lumen, ultimately reaching the distal portion of the ileum. The intrinsic factor, during its transport from the stomach to the ileum, binds vitamin B12 that is within the chyme to form a B12–intrinsic factor complex. It is within the distal ileum that these complexes bind to receptors on the epithelial cell lining and are taken up by receptor-mediated endocytosis. The lack of intrinsic factor prevents the binding and absorption of the vitamin B12 that is required for erythrocyte formation, resulting in the development of pernicious anemia.