Oxygen-poor blood passes through the:

Choice D rationale

Venous pooling, particularly in the lower extremities, commonly occurs when standing still for prolonged periods due to the effects of gravity and reduced skeletal muscle pump activity. This pooling decreases venous return to the heart, leading to reduced cardiac output and consequently a drop in cerebral blood flow, which can result in syncope (fainting).

Choice A rationale

Increased respiratory rate (hyperventilation) typically leads to a decrease in carbon dioxide levels, causing respiratory alkalosis. While severe hyperventilation can lead to cerebral vasoconstriction and lightheadedness, it is not the primary physiological mechanism for syncope after prolonged standing.

Choice B rationale

Vasoconstriction, particularly systemic arteriolar vasoconstriction, would generally help maintain blood pressure by increasing total peripheral resistance, counteracting a drop in blood pressure. Therefore, it would typically prevent rather than cause syncope in this scenario, unless it was a compensatory, insufficient response.

Choice C rationale

An increased heart rate is a compensatory mechanism to maintain cardiac output when venous return or stroke volume is reduced, such as during orthostatic stress. While the heart rate does increase, it is often insufficient to prevent the fall in blood pressure that leads to syncope if venous pooling is severe.

Choice A rationale

Oxygen-poor blood from the systemic circulation enters the right atrium, then passes through the right AV (tricuspid) valve into the right ventricle. From the right ventricle, it is pumped through the pulmonary valve into the pulmonary artery, which carries it to the lungs for oxygenation. This pathway ensures deoxygenated blood reaches the pulmonary circuit.

Choice B rationale

This choice is incomplete because while oxygen-poor blood does pass through the right AV (tricuspid) valve, it must also pass through the pulmonary valve to exit the right ventricle and reach the lungs for oxygenation. Omitting the pulmonary valve provides an incomplete description of the path.

Choice C rationale

The left AV (mitral) valve and aortic valve are components of the left side of the heart, which handles oxygenated blood. Oxygenated blood from the lungs enters the left atrium, passes through the mitral valve into the left ventricle, and then through the aortic valve into the aorta for systemic distribution.

Choice D rationale

This choice describes only the entry of oxygenated blood into the left ventricle from the left atrium via the left AV (mitral) valve. It does not account for the oxygen-poor blood pathway, which involves the right side of the heart.

Choice E rationale

The pulmonary valve allows oxygen-poor blood to leave the right ventricle. However, the aortic valve allows oxygenated blood to leave the left ventricle. Therefore, the combination of pulmonary and aortic valves does not exclusively describe the path of oxygen-poor blood.