Respiratory System and Tissues

Question 1

Name three major functions of the respiratory system.

Answer 1

1. Gas exchange
2. Filtration of particulate matter
3. Thermoregulation

Question 2

Explain the relationship between gas exchange and cellular metabolism.

Answer 2

Aerobic cellular respiration requires oxygen and releases carbon dioxide as a waste product. Gas exchange removes this unwanted CO₂ from the cells and replaces it with O₂ from the environment.

Specifically, O₂ is required for the electron transport chain, while CO₂ is produced mainly during the Krebs cycle.

Question 3

List the structures of the respiratory tract in order, beginning at the entrance for incoming air and ending at the site of gas exchange.

Answer 3

Question 4

Define:

lung

Answer 4

It is the major organ in the respiratory system, acting to provide a site for gas exchange. Humans have two lungs that are connected to the outside environment via the respiratory tract.

The prefix “pulmo-“ and the root word “pleura” are often used to refer to the lungs and their associated structures.

Question 5

Define:

larynx

Answer 5

A cartilagenous structure that contains the vocal cords. It sits directly below the pharynx and above the trachea.

Question 6

Define:

pulmonary alveoli

Answer 6

In the lungs, alveoli are small sacs at the ends of bronchioles. Each alveolus is enclosed by an epithelial lining and surrounded by pulmonary capillaries.

Alveoli are the site of gas exchange between the environment and the bloodstream. Oxygen moves from the alveoli to the capillaries, while carbon dioxide travels in the opposite direction.

Question 7

In which pulmonary structure(s) is the activity of surface tension most relevant?

Answer 7

alveoli

Surface tension causes liquids to form spherical droplets, maximizing their contact with other liquid particles. Since the alveoli contain fluid, this property would tend to make alveoli collapse and is countered by the action of surfactant.

Question 8

What substance serves to decrease surface tension in the alveoli?

Answer 8

Surfactant

It prevents the alveoli from collapsing.

Pulmonary surfactant is amphipathic, meaning that its molecules possess both hydrophobic and hydrophilic groups.

Question 9

Premature infants often suffer from respiratory distress syndrome, a condition that involves insufficient surfactant production. What effect will this condition likely have on the alveoli?

Answer 9

It will cause the alveoli to collapse.

Surfactant serves to lower surface tension in the fluid associated with the alveolar lining. Without surfactant, this fluid will tend to minimize its own surface area, collapsing into the hollow interior of the alveolus and making inhalation very difficult.

Question 10

The image below depicts a number of alveoli surrounded by pulmonary capillaries. What type of transport is used by gases to travel between these two structures?

Answer 10

diffusion

Since gases are small and nonpolar, they are easily able to diffuse through cell membranes. Gas exchange is a passive process, meaning that oxygen and carbon dioxide move down their concentration gradients without using ATP.

Question 11

Compare the partial pressure of carbon dioxide in the pulmonary capillaries to that in the alveoli.

Answer 11

The pulmonary capillaries have a higher partial pressure of CO₂.

The pulmonary capillaries bring deoxygenated blood, which is CO₂-rich, in close contact with the alveoli. If CO₂ is to diffuse from the bloodstream to the alveolar interior, it must travel down its concentration gradient. The alveoli, then, must contain comparatively less carbon dioxide than the capillaries.

Question 12

Place the following terms in order according to their oxygen partial pressures, from the lowest P_O2 to the highest.

• Alveolar air
• Atmospheric air
• Blood from the pulmonary artery

Answer 12

blood from the pulmonary artery < alveolar air < atmospheric air

The pulmonary artery carries deoxygenated blood from the heart to the lungs, so it contains a comparatively low amount of O₂. It’s easy to assume that the alveoli contain only atmospheric air, but they also hold residual carbon dioxide from the previous respiration, resulting in a slightly lower oxygen partial pressure than that of atmospheric air.

Question 13

What are the steps involved in inspiration?

Answer 13

1. The diaphragm and external intercostal muscles contract, expanding the thoracic cavity.
2. The lungs expand along with the surrounding cavity, lowering their internal pressure.
3. Air flows from the comparatively high-pressure environment to the low-pressure lungs.

Question 14

What is the significance of the term “negative pressure breathing?”

Answer 14

It describes the mechanism of inspiration. The pressure inside the lungs becomes lower than atmospheric pressure, causing air to flow inward.

Negative pressure also relates to MCAT physics. Remember that absolute pressure can never have a negative value. Instead, it is the gauge pressure that is negative, meaning simply that it is lower than ambient pressure.

Question 15

Name the main muscles involved in inspiration.

Answer 15

1. The diaphragm
2. The external intercostals

Both muscles contract to enlarge the thoracic cavity. The diaphragm flattens downward, while the external intercostals push outward.

Question 16

The diaphragm is composed of which of the three types of muscle?

Answer 16

skeletal muscle

For the MCAT, skeletal muscle can usually be considered to be voluntary. The diaphragm is a notable exception, since it can be consciously contracted but is usually under involuntary control.

Question 17

Name the main muscles involved in expiration.

Answer 17

Normal expiration is passive, requiring no muscular involvement. Expiration that is consciously forced involves the internal intercostals.

Other muscles aid in both expiration and inspiration, but they are not necessary to know for the MCAT.

Question 18

During exhalation, the diaphragm must be in which state?

Answer 18

relaxed

When relaxed, the diaphragm becomes rounded, pushing upward and decreasing the volume of the thoracic cavity. Lower volume results in increased pressure, forcing air outward.

Question 19

What measurement describes the volume of air inhaled per breath during normal breathing?

Answer 19

Tidal volume

The tidal volume of an average human is approximately 500 mL.

Question 20

What is the difference between inspiratory and expiratory reserve volume?

Answer 20

• Inspiratory reserve volume (IRV) refers to the maximum volume of air that can be inhaled in addition to a normal inhalation.
• Expiratory reserve volume (ERV) refers to the maximum volume that can be exhaled in addition to a normal exhalation.

Question 21

The sum of IRV, ERV, and tidal volume can be described by which lung measurement?

Answer 21

vital capacity

Vital capacity is the largest volume that can possibly be inhaled or exhaled.

Question 22

What measurement describes the volume of air that always remains in the lungs, regardless of the force of exhalation?

Answer 22

Residual volume

Since the residual volume never leaves the lungs, it cannot be directly measured, but must be calculated using the other lung volumes.

Question 23

What is the difference between total lung capacity and vital capacity?

Answer 23

• Total lung capacity (TLC) includes the entire volume of air that can be held in the lungs.
• Vital capacity (VC) only includes the volume that can be inhaled or exhaled.

TLC is equal to the sum of vital capacity and residual volume.

Question 24

If inspiratory reserve volume and tidal volume are known, what additional measurements are needed to calculate residual volume?

Answer 24

Total lung capacity and expiratory reserve volume.

ERV, IRV, and tidal volume can be added to find vital capacity. From there, residual volume can be found by subtracting vital capacity from total lung capacity.

Question 25

Which substance, secreted by membranes in the nasal passages, helps protect the lungs from foreign particles?

Answer 25

mucus

In the respiratory tract, mucus functions as part of the innate immune system. Mucous membranes also exist in the stomach.

Question 26

Which organelle associated with respiratory epithelium would be seriously impaired by the inhibition of microtubule synthesis?

Answer 26

Cilia

They are small hairlike organelles that line much of the respiratory tract, are composed of microtubules. Cilia function to protect the lungs by moving mucus and particles toward the throat.

The MCAT often tests other microtubule-based structures, including the cytoskeleton and spindle apparatus.

Question 27

Which type of immune cell is highly active in the alveoli of the lungs?

Answer 27

Macrophages

While macrophages exist throughout the body, specialized cells known as alveolar macrophages are present only in the lungs. If exposed to a pathogen, these cells can perform phagocytosis and release factors that activate other immune cells.

Question 28

Define:

hypoxia

Answer 28

This refers to the condition of oxygen deprivation, usually due to low oxygen levels in the blood.

On the MCAT, prefixes and suffixes can often help in discerning a word’s significance. If this term were unfamiliar, the prefix “hypo-,” meaning “low” or “under,” could give away its meaning: low oxygen.

Question 29

What homeostatic system relates to the equation below?

H₂O + CO₂ ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

Answer 29

The bicarbonate buffer functions according to this equation and is the main homeostatic method of maintaining constant pH in the blood.

The equation for the bicarbonate buffer is reversible, meaning that it can progress in either direction to reach an equilibrium. Most MCAT questions on this topic involve the use of Le Châtelier’s principle.

Question 30

In the first step of the bicarbonate buffer, dissolved carbon dioxide reacts with water in the plasma. Which acid is produced by this reaction?

Answer 30

carbonic acid

(H₂CO₃)

Carbonic acid is often confused with bicarbonate (HCO₃^-), since the “bi” prefix can be misunderstood to mean “two” protons. It’s important to remember that bicarbonate is actually the conjugate base of carbonic acid. The MCAT often tests this distinction.

Question 31

How does the respiratory system respond to low blood pH levels?

Answer 31

Low pH, or acidity, in the blood results in an increased respiratory rate.

Consider the equilibria involved in the blood buffer:

H₂O + CO₂ ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

Acidic blood has a high proton concentration, which will shift the reaction to the left to regain equilibrium. This will result in the production of water and carbon dioxide. To excrete this additional CO₂, breathing rate must increase.

Question 32

Rapid or excessive breathing, commonly known as hyperventilation, results in an abnormally high rate of CO₂ exhalation. What condition can be caused by prolonged hyperventilation?

Answer 32

High blood pH, or alkalosis

More than any other factor, blood pH level depends on CO₂ concentration. If respiratory rate is increased above normal levels, more CO₂ will be exhaled than usual, and the bicarbonate buffer equilibrium will shift away from the production of hydrogen ions. With fewer protons, blood will become basic or alkaline.

Question 33

If a normal individual suddenly began inhaling air with a significantly higher partial pressure of O₂, what immediate effect would this have on her plasma pH?

Assume that her rates of respiration, carbon dioxide exhalation, and carbon dioxide production remain unchanged.

Answer 33

Plasma pH would be unaffected.

While blood pH depends directly on CO₂ concentration, its relationship to O₂ levels is much more indirect. Oxygen is not part of the bicarbonate buffer system, so if all else remained the same, it would have no immediate effect on pH.

Question 34

Oxygen concentration in blood vessels is monitored by two clusters of peripheral receptors, shown below. What are the names of these two structures?

Answer 34

• aortic bodies
• carotid bodies

They function as chemoreceptors and signal the brain when O₂ levels are low.

Question 35

How will the aortic body respond if it senses low plasma O₂ levels?

Answer 35

As part of the peripheral nervous system, the aortic body must first send an impulse to the brain, which can then increase respiratory rate.

Specifically, both the aortic and carotid bodies signal the medulla oblongata, which controls involuntary actions such as breathing.

Question 36

What physiological value has the strongest effect on the brain’s control of respiratory rate?

Answer 36

Plasma CO₂ levels

(which determine plasma pH)

The brain contains its own chemoreceptors, but they monitor only blood pH and CO₂ levels. These central receptors stimulate a stronger response than the peripheral bodies. For the MCAT, then, consider respiration to be more directly affected by CO₂ than O₂.

Question 37

Human tissue is broadly categorized into which four basic types?

Answer 37

1. Epithelial tissue
2. Connective tissue
3. Nervous tissue
4. Muscle tissue

Question 38

What are the major functions of epithelial tissue?

Answer 38

Epithelial tissue forms the external and internal linings of the body. Its main functions include protection and the facilitation of movement of materials between organs.

Specifically, epithelium can be specialized for absorption (as in the small intestine), secretion (as in the kidney) or diffusion (as in the alveoli), or it can serve multiple roles.

Question 39

Epithelial cells can be classified according to their shape or their arrangement in layers. With regard to shape, what are the three categories of epithelial cells?

Answer 39

• squamous
• cuboidal
• columnar

Squamous epithelial cells are flat and thin, while cuboidal cells, as their name implies, appear cubelike. Columnar cells are significantly taller and rectangular.

Question 40

Epithelial cells can be classified according to their shape or their arrangement in layers. With regard to layered arrangement, what are the two categories of epithelial tissue?

Answer 40

• simple
• stratified

Simple epithelium consists of a layer that is a single cell thick, which is ideal for diffusion and absorption. Stratified epithelium consists of multiple tightly-packed layers and often serves protective purposes.

Question 41

The cells that form capillary walls are part of which major tissue type?

Answer 41

Epithelial tissue, specifically simple squamous epithelium.

This tissue is only one cell thick, which helps facilitate diffusion.

On the MCAT, you’ll often see the term endothelium used to describe the epithelium that lines blood vessels. This term refers more narrowly to cells that are part of internal linings.

Question 42

The lining of an alveolus is mainly composed of simple squamous epithelium. If alveoli were instead formed from stratified epithelium, what would result?

Answer 42

Gas exchange would not occur or would be severely limited.

Stratified epithelium is composed of many layers of cells, and is more impenetrable than simple epithelium. O₂ and CO₂ would need to diffuse through several cell layers to move in and out, and would be unable to do so efficiently.

Question 43

Blood, cartilage, and bone are part of which basic tissue type?

Answer 43

connective tissue

Connective tissue usually fulfills a structural role and is surrounded by an extracellular matrix. Other examples include cartilage, ligaments, fibroblasts, and lymph.

Question 44

Name the largest organ in the integumentary system.

Answer 44

skin

The skin is the largest organ not only in the integumentary system, but in the entire human body.

Overall, the integumentary system is tested less than other organ systems on the MCAT, but its functions are still important to know.

Question 45

What are three main functions of the skin?

Answer 45

• Physical protection of internal organs.
• Regulation of body temperature.
• Prevention of water loss.

Question 46

What is the difference between sebaceous glands and sweat glands?

Answer 46

• Sebaceous glands secrete oil, or sebum, to prevent the skin from drying out. These glands are often found near the base of hair follicles.
• Sweat glands secrete sweat, a fluid that contains water and electrolytes. The evaporation of sweat serves to decrease body temperature.

Question 47

Which layer of the skin lies between the epidermis and the subcutaneous layer?

Answer 47

dermis

The dermis is also the thickest layer, and contains connective tissue, hair follicles, and glands.

Question 48

A tissue sample taken from the skin contains adipose tissue and collagen, but lacks sebaceous glands. From which layer did this sample most likely originate?

Answer 48

Subcutaneous layer, sometimes called the hypodermis.

The hypodermis is the deepest layer of the skin and is formed mainly from adipose (fat) cells.

Question 49

In the skin, what is the role of keratin?

Answer 49

A protective protein that forms a watertight layer in the epidermis.

Most epidermal cells are keratinocytes, meaning that they do or did produce keratin. Only the innermost cells of the epidermis are alive.

Question 50

In the skin, what is the role of melanin?

Answer 50

It protects the skin from the harmful effects of UV radiation. As a pigment, it also provides skin color.

Question 51

Name two ways in which the skin is involved in thermoregulation.

Answer 51

• In cold conditions, insulation is provided by a layer of fat cells in the subcutaneous layer, or hypodermis.
• In hot conditions, secretions from sweat glands can lower body temperature.

Question 52

Define:

piloerection

Answer 52

This involves the raising of hairs on the skin in response to cold or fear. The hair forms an insulating layer, assisting in thermoregulation.

The common name for piloerection is simply “goosebumps.”

Question 53

When temperatures are cold, blood vessels near the skin change in diameter. What is the term for this change, and what does it accomplish?

Answer 53

Vasoconstriction, or a decrease in diameter. This reduces flow and keeps blood away from the cold exterior environment, where heat can easily be lost.

The opposite process, vasodilation, occurs in hot temperatures. Vessels near the skin widen, allowing more blood near the surface of the body so heat can more readily dissipate.