MCAT Biology : Blood Vessels and Vasculature

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Blood Vessels And Vasculature

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Untitled

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H+ + HbO2 ←→ H+Hb + O2

 

The hemoglobin gene can be the site of catastrophic genetic changes, one of which is the change seen in sickle cell anemia. In this disorder, hemoglobin mutations cause red blood cells to take on a sickled appearance. These cells are less able to flow freely in the blood through tight spaces. Which of the following vessels is most likely to be the site of accumulation of these misshapen cells?

Possible Answers:

Veins

Arteries

Arterioles

Venules

Capillaries

Correct answer:

Capillaries

Explanation:

With morphological changes, cells are most likely to be caught in regions with the smallest cross sectional area. Though capiallary beds have the highest TOTAL cross sectional area of any vessel bed in the body, individual capillaries are smaller than any other type of blood vessel. The result is that misshapen red blood cells, such as those in sickle cell anemia, can easily get stuck in capillaries.

Example Question #61 : Circulatory And Respiratory Systems

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Untitled

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H+ + HbO2 ←→ H+Hb + O2

During exercise the flow of blood is changed, and blood flow is preferentially directed toward working muscles. These muscles are then able to utilize the oxygen carried by hemoglobin. What vessels are most likely to be directly mediating changes in blood flow?

Possible Answers:

Arteries

Capillaries

Venules

Arterioles

Veins

Correct answer:

Arterioles

Explanation:

Arterioles serve an important sphincter function at the entry point to capillary beds. During exercise, capillary beds that are not in need of perfusion at a given moment may enlist the use of arterioles to constrict and divert blood where it is more needed.

Example Question #62 : Circulatory And Respiratory Systems

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Untitled

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H+ + HbO2 ←→ H+Hb + O2

Hemoglobin takes time to unload oxygen at tissues that need it, as well as time to pick up carbon dioxide from working cells. Blood flow must slow down during the most active periods when hemoglobin is absorbing and releasing atoms. Since cross-sectional area is inversely proportional to flow velocity, which of the following vessel beds has the greatest total cross sectional area?

Possible Answers:

Arteries

Veins

Venules

Arterioles

Capillaries

Correct answer:

Capillaries

Explanation:

Capillary beds have the greatest TOTAL cross-sectional area, but are composed of individual vessels that are smaller than any other type in the body. Because velocity is dependent on overall cross sectional area, capillary blood velocity is the lowest.

Example Question #63 : Circulatory And Respiratory Systems

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

Untitled

Capillaries, such as the one depicted in the passage, not only exchange immune cells, but also gas and nutrients with neighboring tissues. Which of the following is true of capillaries? 

I.  Hydrostatic pressure is greater at the arteriolar end.

II.  Osmotic pressure is greater at the arteriolar end.

III.  There is a net loss of fluid to the extravascular tissue from the arteriolar to the venule end of the capillary.

Possible Answers:

I and III

II and III

III, only

I, only

I and II

Correct answer:

I and III

Explanation:

There is greater hydrostatic pressure pushing fluid out of the capillary at the arteriolar end, and greater osmotic pressure pulling fluid back in at the venule end. Despite this balance, it is not exact, and there is a net loss of fluid to the extravascular compartment over the length of the capillary. This extra fluid is captured by lymphatics and recycled via lymph vessels.

Example Question #851 : Systems Biology And Tissue Types

All of the following are examples of human capillary classifications except __________.

Possible Answers:

hypodermic

continuous

sinusoidal

fenestrated

Correct answer:

hypodermic

Explanation:

Capillary beds can be broken down into three types: continuous, fenestrated, and sinusoidal.

Continuous capillaries have no pores, fenestrated have small pores, and sinusoidal have large, leaky pores. Hypodermic describes the region just beneath the skin; while this is the location of many capillaries, it is not a classification.

Example Question #852 : Systems Biology And Tissue Types

Fluid from the blood often leaks from between the cells lining capillary walls, entering into the interstitium. If this fluid is not returned to circulation it can result in edema, or swelling of the region due to the excess fluid. What happens to this excess fluid in a healthy individual to prevent edema?

Possible Answers:

It enters the venules

It enters the lymphatic vessels

It enters the veins

It is incorporated into nearby cells

It re-enters the capillaries

Correct answer:

It enters the lymphatic vessels

Explanation:

The lymphatic system functions to return extra fluid that has leaked from the capillaries into interstitial space. It takes this fluid and returns in back into the blood circulation through the right atrium. Fluids from the lymphatic vessels pass through lymph nodes to be screened for pathogens before collection in the right atrium. The lymph does not return to the circulatory system through any other circulatory vessels.

Example Question #11 : Blood Vessels And Vasculature

Which of the following vessel types will have the slowest velocity of blood flow?

Possible Answers:

Capillaries

Veins

Arteries

Venules

Correct answer:

Capillaries

Explanation:

There is an inverse relationship between the cross sectional area of a blood vessel, and the velocity that blood travels through it. The below equation shows the relationship between flow rate, area, and velocity.

Flow rate is constant throughout a system: .

For this equation, it is important to compare the combined cross-section area of the vessel type, rather than a single vessel. Capillaries are extremely small, but have a very large combined area compared to other vessels. In order to keep the flow constant, a large area correlates with a slow velocity. Intuitively, since nutrient exchange occurs in the capillaries, it makes sense the blood would flow most slowly through these regions.

Example Question #854 : Systems Biology And Tissue Types

What characteristics of arteries and veins allow the heart to pump blood strong enough to travel through the body against gravity without backing up?

Possible Answers:

Small diameters

One-way valves in arteries; compliance in thick-walled veins

One-way valves in veins; elasticity in thin-walled arteries

One-way valves in veins; compliance in thick-walled arteries

One-way valves in arteries; elasticity in thin-walled veins

Correct answer:

One-way valves in veins; compliance in thick-walled arteries

Explanation:

Areteries need to be thick and able to withstand strong, sudden increases in pressure because they recieve blood more directly from the heart. The aorta in particular is very thick and able to expand with a large increase in blood volume. "Compliance" is the word that decsribes a vessel's ability to do this. Veins are thin and collapsable, and contain one-way valves to prevent blood from flowing backwards as it moves against gravity towards the heart.

Example Question #11 : Blood Vessels And Vasculature

Which of the following incorrectly matches the type of blood with the vessel or structure carrying it?

Possible Answers:

Deoxygenated blood—right ventricle

Deoxygenated blood—inferior vena cava

Deoxygenated blood—veins

Oxygenated blood—left atrium

Oxygenated blood—pulmonary artery

Correct answer:

Oxygenated blood—pulmonary artery

Explanation:

Arteries always carry blood away from the heart. Most carry oxygenated blood, but the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs so that it can reoxygenated and sent back to the heart.

Example Question #855 : Systems Biology And Tissue Types

Pressure throughout the body is lowest in the __________.

Possible Answers:

aorta

veins

capillaries

arteries

Correct answer:

veins

Explanation:

The aorta is where the blood pressure is greatest, in order to pump blood to the entire body. Capillaries have relatively low blood pressure, but have greater pressure than veins due to their small diameter. Veins carry blood that has traveled a greater distance from the heart; the flow is slower and the pressure is lower. In order to bring the blood back to the heart, the veins must contain valves which prevent the back flow of blood. Arteries, on the other hand, do not need valves because of their great pressure that keeps continuous flow in one direction.

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