Sexual reproduction entails the union of two gonads to form a zygote.

 

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.

A. the prostate: The prostate gland contributes approximately 25 to 30 percent of the total seminal volume. Its secretion is a milky, slightly acidic fluid containing citrate, enzymes, and prostate-specific antigen. While vital for sperm activation and liquefaction, it does not provide the majority of the fluid volume.

B. the pituitary: The pituitary gland is an endocrine organ located in the sella turcica of the sphenoid bone. It secretes gonadotropins like follicle-stimulating hormone and luteinizing hormone to regulate the reproductive system. It does not produce any components of the liquid semen ejaculated from the male reproductive tract.

C. the epididymis: The epididymis serves as the primary site for sperm maturation and storage where they acquire motility. While it adds a very small amount of fluid to support the sperm, its contribution to the final volume of ejaculate is negligible. It is a ductal structure rather than a major secretory gland.

D. the seminal vesicles: These paired glands produce about 70 percent of the total semen volume. The secretion is a viscous, alkaline fluid rich in fructose, prostaglandins, and clotting proteins. This fluid provides the primary energy source for sperm and helps neutralize the acidic environment of the female reproductive tract.

E. the bulbourethral glands: Also known as Cowper's glands, these small structures secrete a clear, alkaline mucus during sexual arousal. This pre-ejaculate lubricates the glans penis and neutralizes residual acidic urine in the urethra. Their contribution to the final volume of semen is usually less than 5 percent.