Circulatory System Flashcards
With the new MCAT’s increased focus on biology, understanding the circulatory system has become especially important. Use these cards to master MCAT favorites like the blood buffer and the Bohr effect, as they appear on the Biological & Biochemical Foundations section.
What are the primary components of blood?
- plasma
- red blood cells
- white blood cells
- platelets
About 45% of total blood volume is due to cellular components, while the remaining 55% comes from plasma.
What is the normal pH of blood?
7.4
Normal blood is slightly basic, and even minute deviations can be very harmful. Thus, pH is tightly regulated by mechanisms such as the bicarbonate buffering system.
What is the difference between acidosis and alkalosis?
- Acidosis is an increase in blood acidity, occurring when blood pH drops below 7.35.
- Alkalosis is an increase in blood basicity, occurring when blood pH rises above 7.45.
If a person were to hyperventilate continuously, would they eventually experience acidosis or alkalosis?
alkalosis
Hyperventilation, or quickened breathing, results in an increased loss of CO2. The reaction proceeds according to this equilibrium:
H2O + CO2 ⇔ H2CO3 ⇔ H+ + HCO3-
When CO2 is excreted, the equilibrium shifts to the left to regenerate it. This requires protons to combine with bicarbonate ions, decreasing the plasma H+ concentration. Lowered H+, or decreased acidity, means that pH will rise.
Define:
plasma
It is the liquid component of blood in which red and white blood cells are suspended.
Though primarily composed of water, plasma also contains proteins, glucose, hormones, electrolytes, and gases.
Which homeostatic function is most relevant to albumin?
osmoregulation
Albumin, the most common plasma protein, is produced in the liver and primarily maintains osmotic pressure. It also helps in buffering blood pH and acts to transport substances in the blood.
Name three substances or mechanisms that help regulate plasma volume.
- osmotic pressure
- antidiuretic hormone (ADH)
- aldosterone
High plasma osmolarity draws water into the blood from the tissues; low plasma osmolarity causes water to flow out of the blood. Both ADH and aldosterone act on the kidneys, ADH by directly increasing water reabsorption and aldosterone by increasing the reabsorption of salt.
Define:
erythrocyte
Also known as red blood cells, these are the most common cells found in the blood. Their main responsibility is to carry oxygen to body tissues.
Red blood cells transport oxygen using hemoglobin. For the MCAT, note that these cells do not contain nuclei (or any other membrane-bound organelles), and thus cannot undergo mitosis.
In what part of the body are erythrocytes produced?
In the bone marrow from stem cells.
This process is referred to as erythropoiesis and is stimulated by decreased oxygen in the blood, which causes the kidneys to release erythropoietin (EPO).
What organs are responsible for the destruction of old and damaged erythrocytes?
The spleen and liver are the principal organs that destroy erythrocytes and process their parts.
When heme is broken down, iron is first released and recycled. The remainder of the molecule is converted into bilirubin, some of which becomes a component of bile and the rest of which is immediately excreted.
How do the kidneys respond to low oxygen levels in the blood?
They produce erythropoietin (EPO), which stimulates bone marrow to produce more red blood cells. This increases the oxygen carrying capacity of the blood.
Note that EPO is a peptide hormone.
Anemia is marked by a low value of what blood-related measurement?
Hematocrit
Hematocrit describes the percentage of a person’s total blood volume that is composed of red blood cells. A low hematocrit signifies that fewer RBCs are being produced, generally resulting in a decreased ability to transport oxygen.
What features characterize the structure of hemoglobin?
Hemoglobin has four protein subunits, each composed of a heme group and a globin protein. Together, these monomers form a hemoglobin tetramer.
To which part of blood does oxygen bind in order to be transported to tissue?
It binds to the iron in the heme group of hemoglobin, which is located in red blood cells.
Oxygen can travel in and out of a red blood cell via diffusion.
What conditions can alter the oxygen affinity of hemoglobin?
High temperature, low pH, and high carbon dioxide concentration lower the oxygen affinity of hemoglobin. This decrease allows oxygen to be more easily released to body tissue.
These conditions can be remembered as effects that occur during exercise, when the body is starved for oxygen. During such situations, hemoglobin must be able to readily drop off oxygen in the muscle and other tissue.
What term describes the basic shape of the oxygen-hemoglobin dissociation curve?
sigmoidal
or S-shaped
This shape stems from the ability of hemoglobin to utilize cooperative binding. Once a single oxygen molecule is bound, it becomes much easier for hemoglobin to bind to additional molecules, creating a steep slope.
What is the Bohr effect?
It states that the oxygen binding affinity of hemoglobin decreases as carbon dioxide concentration increases. In other words, O2 binding and plasma [CO2] are inversely related.
Since high CO2 concentration results in a lowered pH, acidic blood also correlates with lower binding affinity.
What happens to the oxygen binding curve of hemoglobin when a person exercises?
The curve shifts to the right.
A rightward shift represents a decrease in oxygen affinity. Exercise causes an increase in carbon dioxide, a drop in pH, and an increase in temperature, resulting in a lowered affinity for oxygen. This makes it easier to release the oxygen in muscle tissue.
Define:
myoglobin
It is the iron-containing protein that binds oxygen in skeletal muscle cells. It is closely related to hemoglobin, but contains only one monomer instead of four.
Because it picks up the oxygen that hemoglobin releases in the tissues, myoglobin has a higher O2 affinity than hemoglobin.
What is the basic shape of the oxygen binding curve of myoglobin?
hyperbolic
For the MCAT, it is most important to know that unlike hemoglobin, it is not sigmoidal.
The myoglobin curve lacks a sigmoidal shape because, as a protein composed of a single subunit, it cannot undergo cooperative binding.
How do adult and fetal hemoglobin differ in their oxygen affinity?
Fetal hemoglobin has a greater affinity for oxygen than adult hemoglobin.
Specifically, fetal hemoglobin must be more prone to binding oxygen than maternal hemoglobin. This phenomenon allows a developing fetus to accept oxygen from its mother’s partially deoxygenated blood.
Define:
leukocyte
Also known as the white blood cell, it is an immune cell that combats infection in the body.
Unlike red blood cells, leukocytes do contain nuclei, and are also larger and less numerous.
Which main component of blood is required for proper clotting?
platelets
In addition to physically blocking lacerations, they activate proteins in the blood called clotting factors.
Platelets are not full cells, but tiny cell fragments derived from larger megakaryocytes. As such, they do not contain nuclei.
Name the primary protein responsible for blood clotting.
fibrin
Fibrin is derived from the soluble precursor fibrinogen.
Another protein, thrombin, cleaves fibrin into its active form. Thrombin also forms from a precursor, called prothrombin.
Which organ is responsible for producing clotting factors?
liver
These factors, including fibrinogen and prothrombin, circulate in the bloodstream and are activated by platelets after tissue damage.
Define:
coagulation
It is the process by which a liquid becomes a gel. When referring to blood, this process is also called clotting.
Coagulation functions to prevent immediate blood loss from a damaged vessel. It also facilitates eventual repair of the tissue.
How does the blood respond when the endothelium of a vessel is damaged?
First, platelets plug the area of the wound and activate a coagulation cascade. This pathway ends in the activation of fibrin from fibrinogen. Finally, fibrin forms an insoluble crosslinked mesh that seals the laceration.
How many oxygen molecules can be carried by one molecule of hemoglobin?
Hemoglobin has an iron-containing heme group for each of its four protein subunits, allowing it to carry four oxygen molecules total.
Each red blood cell, then, contains around 250 million molecules of hemoglobin, resulting in a carrying capacity of around one billion O2 molecules per erythrocyte.
In which form is most carbon dioxide carried in the blood?
Most carbon dioxide (70-80%) is carried in the form of bicarbonate ions, HCO3-.
These ions comprise an integral part of the blood buffer system.
The remaining CO2 is either bound to hemoglobin or dissolved in its original form in the plasma.
Define:
antigen
It is a substance that causes an immune response. Specifically, antigens are markers that can be recognized by antibodies.
The surface proteins that determine blood type (A and B) are examples of antigens.
Which characteristic of an individual’s blood cells is used to identify blood type?
Blood type is classified based on the surface antigens present on a person’s erythrocytes.
Human blood types include A, B, AB, and O. While A and B denote possible antigens, an O blood type implies that neither is present.
With regard to blood type, what is the Rhesus factor?
Also known as the Rh factor, this refers to a surface antigen on human blood cells that was first discovered in Rhesus monkeys.
Rh positive (Rh+) blood displays the Rh antigen, while Rh negative (Rh-) blood lacks it. Rh- blood can be given to individuals of either blood type, while Rh+ blood is only given to other Rh+ people.
Define:
antibody
An immune protein that recognizes a specific, potentially harmful antigen or class of antigens. Antibodies are produced by B lymphocytes.
For the MCAT, be sure to avoid confusion between antibodies and antigens. Antigens are attacked, while antibodies do the attacking.
What occurs when a person is given a blood transfusion that contains foreign antigens?
In a process called hemagglutination, the recipient’s antibodies cause the transfused blood to bind and clump together.
How does the circulatory system regulate body temperature?
Blood vessels can undergo vasoconstriction and vasodilation.
In cold temperatures, arterioles near the skin become narrow, or constrict, to limit heat loss. In hot temperatures, arterioles widen, or dilate, to allow heat to dissipate from the body’s surface.
Describe the general structure of the human heart.
It has four chambers, with the left and the right atrium positioned above the left and the right ventricle. The atria and ventricles are separated by valves.
The right side of the heart contains deoxygenated blood, while blood on the left side is oxygenated.
In order, list the structures that blood will contact between the vena cava and the aorta. Do not include valves.
After returning to the heart via the vena cava, blood will contact the:
1. Right atrium
2. Right ventricle
3. Pulmonary artery
4. Lung
5. Pulmonary vein
6. Left atrium
7. Left ventricle
From the left ventricle, it exits the heart through the aorta.
Name the two valves in the heart that separate an atrium from a ventricle.
- The tricuspid valve
- The bicuspid or mitral valve
The tricuspid valve is situated between the right atrium and the right ventricle.
The bicuspid or mitral valve separates the left atrium from the left ventricle.
Both of these structures are classified as atrioventricular valves.
After blood is oxygenated in the lungs, which vessel carries it back to the heart?
pulmonary vein
Unlike the vast majority of venous circulation, the pulmonary vein carries oxygenated blood. Remember that veins are defined as vessels that transport blood toward the heart, not as vessels that carry oxygen-depleted blood.
A certain heart malformation prevents the mitral valve from closing properly. How would this condition affect blood flow?
Some of the blood leaving the left atrium would leak back in instead of fully progressing to the left ventricle.
Such defects can cause the heart to work harder to counteract the defective valve, causing stress and long-term impairments in blood flow.
Name the main components present in arterial walls.
Arteries have thick walls that contain:
- endothelium
- elastic connective tissue
- smooth muscle
The elastic fibers allow arteries to stretch in diameter, while smooth muscle functions in vasoconstriction and vasodilation.
Describe the similarities between arterial and venous walls.
Like arteries, veins contain an inner lining of endothelium, as well as smooth muscle and connective tissue.
Which type of vessel connects arterioles to venules?
Capillaries
Capillaries, the smallest and most numerous blood vessels, arise from arterioles and converge into venules. They form the site of gas and nutrient exchange with the tissues.
What function do capillary beds serve?
They provide a site where oxygen, carbon dioxide, nutrients, and wastes are exchanged between blood and interstitial fluid.
Which two opposing pressures control the movement of fluid between capillaries and tissues?
- Hydrostatic pressure
- Osmotic pressure
Hydrostatic pressure is synonymous with blood pressure and promotes the movement of fluid out of the capillaries. Osmotic pressure is controlled by solute concentration and promotes the return of fluid from the tissues.
How do capillary walls differ from the walls of other types of blood vessels?
Capillary walls consist only of a single layer of endothelium. This thin-walled structure facilitates the diffusion of gases and solutes.
Since capillaries do not contain smooth muscle, they cannot vasoconstrict.
Which structures are connected by the ductus arteriosus?
the pulmonary artery to the aorta
Much of fetal blood does not even reach the pulmonary artery, but this adaptation ensures that any blood that does reach this vessel will still bypass the lungs.
Which structures are connected by the ductus venosus?
the umbilical vein to the inferior vena cava
This adaptation allows oxygenated blood from the placenta to bypass the liver.
What is the function of the foramen ovale?
This is a hole between the fetal atria that allows oxygenated blood to travel directly from the right atrium to the left atrium. (Normally, in adults, the blood in the right atrium is deoxygenated. In a fetus, however, this blood was already oxygenated in the placenta.)
Along with the ductus arteriosus, the foramen ovale causes fetal blood to bypass the lungs.
