The ________ is not an endocrine gland, but it has a role in endocrine function.

Choice A rationale

Oxygen-poor blood, returning from the systemic circulation, enters the right atrium, passes through the right AV (tricuspid) valve into the right ventricle, and then is pumped through the pulmonary valve into the pulmonary artery, leading to the lungs for oxygenation. Both valves are involved in this pathway.

Choice B rationale

The right AV (tricuspid) valve allows blood flow from the right atrium to the right ventricle. However, oxygen-poor blood must also pass through the pulmonary valve to exit the right ventricle and reach the pulmonary circulation. Therefore, only stating the tricuspid valve is incomplete.

Choice C rationale

The left AV (mitral) valve and aortic valve are on the left side of the heart. The left side handles oxygenated blood returning from the lungs, pumping it into the systemic circulation. Therefore, oxygen-poor blood does not pass through these valves.

Choice D rationale

The left AV (mitral) valve regulates blood flow from the left atrium to the left ventricle. This valve is involved with oxygenated blood, not oxygen-poor blood. Thus, it is incorrect to include it in the pathway for deoxygenated blood.

Choice E rationale

The pulmonary valve allows oxygen-poor blood to exit the right ventricle, while the aortic valve allows oxygenated blood to exit the left ventricle. Therefore, stating both pulmonary and aortic valves for oxygen-poor blood is incorrect, as the aortic valve handles oxygenated blood.

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.