Which of the following provides lubrication of the vagina during sexual excitement?

A. Myelin is a fatty substance that wraps around axons in many neurons. It acts as an insulative covering that speeds up the transmission of nerve impulses by enabling the signal to jump between gaps in the myelin (called nodes of Ranvier). This is the correct answer.

B. The synapse is the junction between two neurons where signals are transmitted. It is not involved in the insulative covering of a neuron.

C. The axon is the long, slender part of a neuron that transmits electrical signals, but it requires myelin to efficiently conduct those signals.

D. Dendrites are branched extensions of a neuron that receive signals from other neurons. They do not have an insulative function.

A. Excessive aldosterone causes increased reabsorption of sodium and water, and the excretion of potassium. Therefore, excessive aldosterone would typically decrease plasma potassium levels, not increase them.

B. Aldosterone stimulates the kidneys to reabsorb sodium and excrete potassium, leading to increased plasma sodium levels. This is the correct effect of excessive aldosterone secretion.

C. Phosphorus levels are not directly affected by aldosterone; its primary role is in regulating sodium and potassium balance.

D. Aldosterone does not directly regulate calcium levels in the blood, so plasma calcium levels are not significantly affected by aldosterone secretion.