Peptide hormones _______________.

Correct Answer: D

Rationale: The correct answer is D because peptide hormones are hydrophilic molecules that cannot pass through the cell membrane, so they bind to receptors on the cell surface to initiate signaling pathways within the cell. Therefore, they are unable to enter the target cell like lipid-soluble hormones do. By binding to specific receptors on the cell surface, peptide hormones trigger a series of events that ultimately lead to a cellular response. Therefore, all of the given statements are true, making D the correct choice.

Correct Answer: D

Rationale: The correct answer is D because hormone secretion is regulated by various factors. The nervous system can directly stimulate or inhibit hormone release. Other hormones can act as signals to trigger or suppress the secretion of specific hormones. Changes in blood composition, such as glucose levels or electrolyte concentrations, can also influence hormone production and release. Therefore, all of the options play a role in regulating hormone secretion, making choice D the correct answer.

Correct Answer: D

Rationale: The correct answer is D: second messengers. Protein or amino acid-based hormones are unable to pass through the cell membrane due to their large size and hydrophilic nature. Therefore, they bind to specific receptors on the cell surface, triggering a series of events that lead to the production of second messengers inside the cell. These second messengers, such as cyclic AMP or calcium ions, then transmit the hormonal signal to the target cell's interior to initiate a physiological response. Choices A, B, and C are incorrect because protein or amino acid-based hormones do not directly exert their effects through ions, deactivators, or nucleotides intracellularly.

Correct Answer: D

Rationale: The correct answer is D because the neurohypophysis stores hormones produced by the hypothalamus for release into the bloodstream. The neurohypophysis does not produce its own hormones but acts as a storage and release site. A is incorrect as the neurohypophysis does play a role in hormonal release through the release of hormones produced by the hypothalamus. B is incorrect as the neurohypophysis remains functional in adults. C is incorrect as the neurohypophysis does function as an endocrine tissue by releasing hormones into the bloodstream.

Correct Answer: B

Rationale: The correct answer is B: aldosterone. Aldosterone is a hormone produced by the adrenal glands that plays a key role in regulating electrolyte concentrations in extracellular fluids. It specifically acts on the kidneys to increase the reabsorption of sodium and water while promoting the excretion of potassium. This helps to maintain the balance of electrolytes in the body. Insulin, glucagon, and cortisol do not directly regulate electrolyte concentrations in extracellular fluids. Insulin regulates blood sugar levels, glucagon regulates glucose release from the liver, and cortisol is involved in stress response and metabolism. Therefore, aldosterone is the most important regulator of electrolyte concentrations in extracellular fluids.