Test: Circulatory System - 1 - MCAT MCQ

Blood from the vena cava enters the right atrium. Blood from the atria flow into their respective ventricles.
Blood in the left leg would first enter the venous system to drain back towards the heart, first encountering the vena cava.
Blood flow from the vena cava to the right atrium, to the right ventricle, to the lungs, left atrium, left ventricle and finally the aorta.

• The papillary muscles contract during systole to prevent blood from flowing backwards within the heart.
• Blood flows from high pressure to low pressure. The papillary muscles are only necessary in areas where this fact may propagate flow in a backwards direction.
• The pressure generated by ventricular systole is substantial enough that it overcomes the pressure of the blood vessels they supply. This force opens the pulmonary and aortic valves, which then shut passively when the pressure of the ventricles is equal to or less than the pressure upstream.
• The mitral and tricuspid valves are both atrioventricular valves.
• Semilunar valves are not associated with the papillary muscles, and are not actively closed.

The first heart sound, also known as S1, is produced by the closure of the mitral and tricuspid valves. These valves close at the beginning of ventricular systole, which is the contraction phase of the ventricles.

During ventricular systole, the pressure in the ventricles increases as they contract, causing the blood to be forcefully ejected into the pulmonary artery and aorta. In order to prevent backflow of blood into the atria during this contraction, the mitral and tricuspid valves close tightly.

As the ventricles contract, the pressure within the ventricles rises above the pressure in the atria, causing the mitral and tricuspid valves to close simultaneously, generating the first heart sound.

Therefore, at the end of the first heart sound, both the mitral and tricuspid valves are closed (option D). This marks the beginning of ventricular systole and the ejection of blood into the circulation.

The second heart sound (S2) occurs when the semilunar valves (pulmonary valve and aortic valve) close at the end of ventricular systole. After the closure of the semilunar valves, all four heart valves (both atrioventricular valves and both semilunar valves) are closed. This marks the beginning of ventricular diastole, during which all valves remain closed momentarily before the next cardiac cycle begins.

During ventricular diastole, the atrioventricular valves (mitral valve and tricuspid valve) are closed to prevent the backflow of blood from the ventricles to the atria. Simultaneously, the semilunar valves are closed to prevent the backflow of blood from the aorta and pulmonary artery back into the ventricles.

Therefore, at the instant following the second heart sound, all heart valves are closed (option D). This brief period of closure allows the ventricles to relax and refill with blood before the next contraction.

Resistance in a tube is equal to ,  where L is the length of the tube, n is the viscosity, and r is the radius.
Therefore, the resistance is proportional to 1/r⁴

= The new resistance would be 1/8

The tunica media is the middle layer of the arterial wall and is composed primarily of smooth muscle cells. The smooth muscle cells in the tunica media are responsible for regulating the diameter (vasoconstriction and vasodilation) of the arteries.

In large systemic arteries, such as the aorta and the major arterial branches, the tunica media is relatively thicker compared to smaller arteries and arterioles. This increased thickness provides a greater amount of smooth muscle tissue.

Smooth muscle contraction is regulated by intracellular calcium concentrations. When intracellular calcium levels increase, it triggers the contraction of smooth muscle cells, leading to vasoconstriction and narrowing of the arterial diameter. On the other hand, when intracellular calcium levels decrease, smooth muscle relaxation occurs, resulting in vasodilation and widening of the arterial diameter.

Due to the greater amount of smooth muscle tissue in the thick tunica media of large systemic arteries, these arteries are more responsive to changes in intracellular calcium concentrations. Even small changes in calcium levels can have a significant effect on smooth muscle contraction and arterial diameter.

Therefore, the relatively thick tunica media in large systemic arteries makes them more responsive to changes in intracellular calcium concentrations (option B).

Blood pressure refers to the force exerted by the blood against the walls of blood vessels. The pressure in the circulatory system varies throughout the different types of blood vessels.

The aorta is the largest artery in the body and receives blood directly from the left ventricle. It experiences the highest pressure generated by the heart during systole. Therefore, the aorta has the highest average pressure.

From the aorta, blood flows into smaller arteries, which maintain relatively high pressure. Arteries carry blood away from the heart and distribute it to various parts of the body. Arteries have strong, elastic walls that help maintain blood pressure and facilitate the smooth flow of blood.

Arterioles are smaller branches of arteries that regulate blood flow into capillaries. While they have smaller diameters than arteries, arterioles still maintain relatively high pressure compared to other blood vessels.

Capillaries are the smallest and thinnest blood vessels where gas exchange and nutrient exchange occur between blood and surrounding tissues. Capillaries have low average pressure due to their small size and high total cross-sectional area, allowing for efficient exchange.

After capillaries, blood flows into venules, which are small veins that collect blood from capillaries. Venules have lower pressure compared to arterioles but higher pressure compared to veins.

Veins are blood vessels that carry blood back to the heart. They have relatively low average pressure due to their larger diameter, thin walls, and the presence of valves that prevent backflow and assist in venous return.

Therefore, the correct order of average pressure from highest to lowest is: Aorta > artery > arteriole > capillary > venule > vein (option B).

The endocardium is the innermost layer of the heart wall, lining the chambers and valves of the heart. It consists of a thin layer of endothelial cells supported by connective tissue.

Infections caused by bacteria circulating in the blood, known as bacterial endocarditis, primarily affect the endocardium. This condition occurs when bacteria enter the bloodstream and adhere to damaged or abnormal heart valves or other areas of the endocardium. The bacteria can form colonies (vegetations) on these surfaces, leading to inflammation and potential damage to the endocardium.

The endocardium is susceptible to infections because it comes into direct contact with circulating blood. It is also more prone to damage and exposure to bacteria due to the turbulent flow of blood through the heart, especially around the valves.

On the other hand, the epicardium (outermost layer), myocardium (middle muscular layer), and pericardium (fibrous sac surrounding the heart) are not in direct contact with the circulating blood. While infections can occasionally affect these layers, they are less immediately susceptible to bacterial infections compared to the endocardium.

The pulmonary artery is the blood vessel that carries deoxygenated blood away from the heart. It is the only artery in the body that carries deoxygenated blood. The pulmonary artery originates from the right ventricle of the heart and divides into two branches, which lead to the left and right lungs.

The deoxygenated blood is pumped from the right ventricle into the pulmonary artery and is then transported to the lungs. In the lungs, the blood undergoes oxygenation through the process of gas exchange, where carbon dioxide is released, and oxygen is taken up by the red blood cells. After oxygenation, the blood returns to the heart through the pulmonary veins as oxygenated blood.

Coronary arteries, on the other hand, are responsible for supplying oxygenated blood to the heart muscle itself. They originate from the aorta, the main artery leaving the heart, and supply the heart with oxygen and nutrients.

The circulatory system can promote heat retention/conservation on a cold day through vasoconstriction. Vasoconstriction refers to the narrowing of blood vessels, specifically the arterioles, in response to signals from the nervous system or local factors.

During vasoconstriction, the smooth muscles in the walls of blood vessels contract, reducing the diameter of the blood vessels. This constriction reduces blood flow to the skin and peripheral tissues, diverting more blood towards the body's core and vital organs. By reducing blood flow to the skin, less heat is lost through radiation, conduction, and convection from the body surface to the environment.

When the external temperature is cold, vasoconstriction helps minimize heat loss from the body. By conserving heat within the core of the body, the circulatory system helps maintain body temperature and prevents hypothermia.

On the other hand, options A, B, and C do not promote heat retention/conservation on a cold day:

• Decreasing tunica media contraction (option A) would actually result in vasodilation, which increases blood flow and heat loss to the skin.
• Increasing capillary surface area (option B) would also increase blood flow to the skin, promoting heat loss rather than heat conservation.
• Vasodilation (option C) refers to the widening of blood vessels, which enhances blood flow and promotes heat loss from the body.

Therefore, the correct answer is D. Vasoconstriction, which helps promote heat retention/conservation in the circulatory system on a cold day.