When the dartos muscle contracts, it causes which of the following?

A. decreases surface area of the scrotum, increases its thickness, and prevents heat loss: The dartos is a layer of smooth muscle within the superficial fascia of the scrotum. When it contracts in response to cold temperatures, it causes the scrotal skin to wrinkle and tighten. This reduction in surface area and the resulting thickening of the wall minimizes radiant heat loss to the environment.

B. allows scrotum and testes to hang lower from the body: This occurs when the dartos and cremaster muscles relax, typically in response to warm temperatures. Moving the testes away from the pelvic cavity promotes cooling by increasing the distance from core body heat. Relaxation is a heat-dissipating mechanism rather than a result of muscle contraction.

C. elevates the scrotum: While the dartos contributes to the tightening of the sac, the primary muscle responsible for the vertical elevation of the testes toward the inguinal canal is the skeletal cremaster muscle. The dartos primarily affects the texture and surface area of the skin itself. Elevation is a coordinated effort but is specifically the cremasteric reflex.

D. increases surface area of the scrotum, decreases its thickness, and allows heat loss: This description characterizes the physiological state of the scrotum during heat exposure. When the dartos muscle is flaccid, the skin becomes thin and smooth, maximizing the area available for cooling. This is the opposite effect of the contraction described in the question stem.

A. antidiuretic hormone: This hormone acts primarily on the principal cells of the collecting ducts to increase water permeability via aquaporin insertion. It does not directly stimulate the macula densa cells. The macula densa is part of the feedback loop that regulates the filtration rate.

B. changes in pressure in the tubule: Pressure changes are primarily sensed by the granular cells of the afferent arteriole, which act as baroreceptors. The macula densa cells are specialized chemoreceptors rather than mechanoreceptors. They monitor the chemical composition of the fluid rather than its physical pressure.

C. aldosterone: This steroid hormone acts on the distal tubules and collecting ducts to increase sodium reabsorption and potassium secretion. It is a downstream product of the renin-angiotensin-aldosterone system. It does not serve as a primary stimulus for the macula densa chemoreceptors.

D. changes in Na+ content of the filtrate: Macula densa cells are located in the distal ascending limb and monitor the concentration of sodium chloride in the filtrate. High salt levels indicate a high filtration rate, triggering tubuloglomerular feedback. This causes afferent arteriole vasoconstriction to reduce the glomerular filtration rate.

E. renin concentration: Renin is an enzyme secreted by the granular cells in response to signals from the macula densa. Therefore, renin is an output of the juxtaglomerular apparatus rather than the stimulus for the macula densa. These cells initiate the cascade that leads to renin release.