When the left ventricle contracts, the ________ valve is pushed open and the ________ valve closes.

Choice C rationale

Negative feedback inhibition is the most common mechanism by which target organs regulate pituitary hormone secretion. When the concentration of a hormone secreted by a target gland reaches a sufficient level, it inhibits the release of stimulating hormones from the pituitary and/or hypothalamus, thereby maintaining hormonal homeostasis and preventing overproduction.

Choice A rationale

Positive feedback is less common in endocrine regulation and typically leads to an amplification of the initial stimulus, rather than a return to homeostasis. An example is the surge of luteinizing hormone during ovulation, where estrogen stimulates more LH release.

Choice B rationale

Direct nervous stimulation occurs in some cases, such as the adrenal medulla's release of catecholamines in response to sympathetic preganglionic neurons. However, for target organ regulation of the pituitary, feedback loops involving hormones are the predominant mechanism, not direct nervous stimulation.

Choice D rationale

Reverberation refers to a type of neural circuit where neurons re-excite themselves or upstream neurons, leading to a sustained output. This concept is primarily relevant to neural networks and not a direct mechanism for endocrine feedback from target organs to the pituitary gland.

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.