Sperm make up just 10% of semen volume.
 

A. 4; 1: During spermatogenesis, a single primary spermatocyte undergoes two meiotic divisions to produce four functional, haploid spermatozoa. In contrast, oogenesis involves asymmetrical cytokinesis, where one primary oocyte yields only one functional secondary oocyte and three non-functional polar bodies. This allows the single ovum to retain the cytoplasm necessary for early embryonic life.

B. 4; 2: Oogenesis does not result in two functional ova under normal physiological conditions. While two polar bodies may be visible at different stages, only one cell matures into the gamete capable of being fertilized. Producing two functional eggs would be the result of a double ovulation event rather than the standard meiotic process.

C. 2:4: This ratio is the reverse of biological reality. Males produce significantly more gametes per meiotic event than females to ensure a higher probability of fertilization. Females prioritize the quality and nutrient content of a single cell over the quantity of gametes produced. This ensures the zygote has an adequate supply of organelles.

D. 4; 4: If oogenesis produced four functional ova, human multiple births would be the standard rather than the exception. The energy cost of producing four nutrient-rich eggs is too high for the female reproductive strategy. Asymmetrical division is a specific evolutionary adaptation to ensure the survival of a single fertilized zygote.

E. 1; 4: This choice suggests males produce fewer gametes than females per meiotic cycle, which is incorrect. A single spermatogonium eventually leads to the production of four spermatids through the process of meiosis. Oogenesis is the pathway that results in a single functional gamete due to the formation of polar bodies.

A. sodium and hydrogen: Sodium is primarily reabsorbed by active transport mechanisms throughout the nephron to maintain blood volume and osmotic pressure. While hydrogen is actively secreted to regulate acid-base balance, sodium is almost exclusively recovered from the filtrate. This choice incorrectly pairs a primarily reabsorbed cation with a secreted one.

B. potassium and hydrogen: These two cations are the primary substances moved via tubular secretion into the filtrate. Potassium secretion occurs in the distal convoluted tubule and collecting duct under the influence of aldosterone to prevent hyperkalemia. Hydrogen ion secretion is essential for maintaining systemic pH by eliminating metabolic acids through the renal system.

C. sodium and potassium: Sodium is the main extracellular cation and is strictly conserved by the kidneys through extensive reabsorption in the proximal tubule and loop of Henle. While potassium is secreted, sodium is not typically a secreted electrolyte. Their renal handling is diametrically opposed to ensure proper electrolyte balance and resting membrane potentials.

D. calcium and potassium: Calcium is predominantly reabsorbed in the proximal tubule and distal convoluted tubule, a process often regulated by parathyroid hormone. It is not a major secreted electrolyte in the renal tubules. Although potassium is secreted, calcium is treated as a valuable nutrient that the body aims to retain from the filtrate.