MCAT Biology : Pulmonary and Systemic Circuits

Study concepts, example questions & explanations for MCAT Biology

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Example Question #1 : Pulmonary And Systemic Circuits

What is the proper path of a drop of blood through the vascular system, starting in the right atrium?

Possible Answers:

Right atrium, right ventricle, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae, left atrium, left ventricle, pulmonary arteries, lungs, pulmonary veins

Right atrium, left atrium, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae, right ventricle, left ventricle, pulmonary arteries, lungs, pulmonary veins

Right atrium, right ventricle, pulmonary arteries, lungs, pulmonary veins, left atrium, left ventricle, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae

Right atrium, left atrium, pulmonary arteries, lungs, pulmonary veins, right ventricle, left ventricle, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae

Correct answer:

Right atrium, right ventricle, pulmonary arteries, lungs, pulmonary veins, left atrium, left ventricle, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae

Explanation:

The correct path of a drop of blood through the vascular system is right atrium, right ventricle, pulmonary arteries, lungs, pulmonary veins, left atrium, left ventricle, aorta, arteries, arteriorles, capillaries, venules, veins, vena cavae.

The right atrium and ventricle transfer deoxygenated blood to the lungs via the pulmonary arteries. Blood is oxygenated and returned to the left artium via the pulmonary veins. The left ventricle then pumps the oxygenated blood to the body, exiting the heart through the aorta. Systemic circulation flows through arteries, then arterioles, then capillaries where gas exchange occurs to tissues. Blood is then returned to the heart through venules and veins, which merge into the superior and inferior vena cavae and empty into the right atrium to complete the circuit.

Example Question #2 : Pulmonary And Systemic Circuits

Which heart chamber is more muscular and why?

Possible Answers:

The left ventricle requires more muscle because it must pump more blood than the right ventricle

The left ventricle requires more muscle because it must keep blood in the aorta at a high pressure

The right ventricle requires more muscle because it must keep blood entering the lungs at higher pressure than blood entering the aorta

The right ventricle requires more muscle because it must pump more blood than the left ventricle

Correct answer:

The left ventricle requires more muscle because it must keep blood in the aorta at a high pressure

Explanation:

The left ventricle is more muscular than the right ventricle because it must keep the blood in the aorta at high pressure. The high blood pressure in the aorta helps to continue pushing the rest of the blood in the general circulation through the body and back to the heart. The blood in the pulmonary artery is actually at lower pressure than blood in the aorta, since pulmonary capillaries would easily rupture otherwise.

Note that both ventricles pump the same volume of blood, as any blood passing through one ventricle will ultimately return to the other. Blood can be thought of as the current flow in a series circuit.

Example Question #3 : Pulmonary And Systemic Circuits

Which structures contain deoxygenated blood?

Possible Answers:

Pulmonary veins and right ventricle

Right and left atria

Superior vena cava and left ventricle

Right ventricle and pulmonary arteries

Correct answer:

Right ventricle and pulmonary arteries

Explanation:

When blood returns to the heart via the superior and inferior vena cavae, it is deoxygenated. It remains this way as it passes through the right atrium, the right ventricle, and the pulmonary arteries, through which it travels to the lungs to conduct gas exchange with the alveoli. Both the right ventricle and the pulmonary artery contain deoxygenated blood.

All of the other answer choices contain at least one component that carries oxygenated blood.

Example Question #4 : Pulmonary And Systemic Circuits

Which of the following paths correctly orders blood flow through the systemic circuit of the circulatory system? 

Possible Answers:

Left ventricle, aorta, pulmonary veins, right atrium

Left ventricle, aorta, vena cavae, right atrium

Right ventricle, aorta, vena cava, left atrium

Right ventricle, pulmonary arteries, pulmonary veins, left atrium

Correct answer:

Left ventricle, aorta, vena cavae, right atrium

Explanation:

The heart is composed of two circuits: the pulmonary circulation on the right side of the heart, and the systemic circulation on the left side of the heart. Keep in mind that these simplified pathways are ignoring the arterioles, capillaries, and venules that are present in each circulation.

Pulmonary circulation is ordered from the right ventricle to the pulmonary arteries, through the lungs, to the pulmonary veins, and reenters the heart in the left atrium.

Systemic circulation is ordered from the left ventricle to the aorta, through the structures of the body, to the superior or inferior vena cava, and reenters the heart in the right atrium.

Example Question #5 : Pulmonary And Systemic Circuits

Blood velocity is slowest through which of the following vessels?

Possible Answers:

Aorta

Capillaries

Femoral artery

Vena cava

Venules

Correct answer:

Capillaries

Explanation:

The velocity of blood is inversely proportional to the size of the vessel. Although capillaries are the smallest individually, they have the largest combined cross-sectional area. Blood is slowest therefore slowest in the capillaries, which is necessary for gas and nutrient exchange to occur. Blood is fastest in the aorta, and as the vessels branch and have a larger cross sectional area the velocity deceases.

Example Question #6 : Pulmonary And Systemic Circuits

Which vessel carries blood away from the right ventricle of the heart?

Possible Answers:

Superior vena cava

Pulmonary veins

Pulmonary arteries

Aorta

Correct answer:

Pulmonary arteries

Explanation:

The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs for oxygenation. Arteries always carry blood away from the heart, while veins always carry blood toward the heart. The pulmonary arteries are the only arteries to carry deoxygenated blood. After traveling to the lungs, blood is returned to the left atrium via the pulmonary veins, the only veins to carry oxygenated blood.

The aorta carries blood from the left ventricle to the body for systemic circulation. The vena cavae return the blood from systemic circulation to the right atrium. The superior vena cava returns blood from the head upper extremities, while the inferior vena cava returns blood from the abdomen and lower extremities.

Example Question #7 : Pulmonary And Systemic Circuits

Which of the following vessels is not involved in the systemic circulation?

Possible Answers:

Pulmonary veins

Hepatic portal capillaries

The vasa recta

Femoral artery

Correct answer:

Pulmonary veins

Explanation:

The systemic circulation refers to the path that carries blood from the left ventricle, through the body, back to the right atrium. In contrast, the pulmonary circuit refers to the path from the right ventricle, through the lungs, and back to the left atrium.

The femoral artery is a major systemic artery found in the leg and thigh. The hepatic portal system delivers oxygenated blood to the liver. The vasa recta refers to the capillaries surrounding the nephrons in the kidney, which help to regulate the ion gradient responsible for concentrating urine.

The pulmonary veins carry oxygenated blood from the lungs to the left atrium, and are part of the pulmonary circuit.

Example Question #7 : Pulmonary And Systemic Circuits

An obstruction in the pulmonary artery would cause an immediate increase in blood pressure which region?

Possible Answers:

Right atrium

Pulmonary veins

Left ventricle

Right ventricle

Correct answer:

Right ventricle

Explanation:

When an obstruction causes a restriction of flow, increased pressure will occur upstream of the blockage. In the cardiopulmonary system blood flows from the right atrium to the right ventricle, then through the pulmonary artery, lungs, and pulmonary vein, before re-entering the heart at the left atrium.

Should a blockage occur in the pulmonary artery, blood will pool behind the blockage (upstream) in the right ventricle, increasing the pressure in this chamber.

Example Question #8 : Pulmonary And Systemic Circuits

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 optimized 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 binding to oxygen is dependent on oxygen partial pressure, as depicted in the above graph. Where is oxygen partial pressure likely to be the highest?

Possible Answers:

Venules

Tissue arterioles

Capillaries

Lung capillaries

Aorta

Correct answer:

Lung capillaries

Explanation:

Oxygen partial pressure is likely to be highest in the lung capillaries, as this is where oxygen will be "loaded" on to hemoglobin molecules for transportation to the tissues. Since binding affinity increases with oxygen partial pressure, one would also expect red blood cells in lung capillaries to bind the strongest to oxygen, which allows hemoglobin saturation in the lungs.

Example Question #9 : Pulmonary And Systemic Circuits

A man is diagnosed with increased pulmonary capillary resistance. As a result, which part of the heart would be expected to increase in muscle mass?

Possible Answers:

Right ventricle

Right atrium

Left atrium

Right ventricle and left atrium

Left ventricle

Correct answer:

Right ventricle

Explanation:

Increased pulmonary resistance means that it will be more difficult to pump blood into the lungs. The right ventricle, which performs this function, will compensate by increasing in muscle mass. The left atrium will not increase in muscle mass because it receives blood from the lungs and pumps blood into the left ventricle; its muscle mass will likely be unaffected.

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