The Urinary and Reproductive Systems
The urinary and reproductive systems domain tests nephron function, fluid and acid-base balance, gamete production and reproductive cycles. Questions often ask you to trace filtration, reabsorption or secretion, or to compare male and female reproductive physiology.
Core rules and concepts
- The nephron begins with the glomerulus and Bowman’s capsule, then proximal tubule, loop of Henle, distal tubule and collecting duct. Filtration occurs at the glomerulus; reabsorption returns useful solutes and water; secretion adds wastes to tubular fluid.
- The proximal tubule reabsorbs most filtered sodium, water, glucose and amino acids. The loop of Henle creates a medullary osmotic gradient. ADH increases collecting-duct water permeability; aldosterone increases sodium reabsorption and potassium secretion.
- The kidneys regulate blood volume, electrolyte balance, acid-base balance and red blood cell production through erythropoietin. They activate vitamin D.
- Spermatogenesis occurs in seminiferous tubules, with testosterone from Leydig cells and support from Sertoli cells. Sperm mature in the epididymis.
- Oogenesis begins before birth; one primary oocyte completes meiosis I near ovulation. The corpus luteum secretes progesterone. Estrogen and progesterone regulate the endometrial cycle.
- Fertilization usually occurs in the fallopian tube, and implantation occurs in the endometrium.
Worked example
Inulin is freely filtered and neither reabsorbed nor secreted. If plasma inulin is 1 mg/mL, urine inulin is 100 mg/mL, and urine flow is 1 mL/min, inulin clearance is (100 × 1) ÷ 1 = 100 mL/min. Because inulin clearance equals glomerular filtration rate, this value estimates GFR. Glucose clearance would be zero at normal plasma levels because filtered glucose is reabsorbed.
Traps that cost marks
Do not confuse filtration with reabsorption. Do not say ADH increases sodium reabsorption; it increases water permeability. Do not say the loop of Henle directly produces concentrated urine; it establishes the gradient that the collecting duct uses. Remember that the corpus luteum secretes progesterone, and that sperm mature in the epididymis, not the testes.
Worked example
If ADH is low, collecting ducts are less permeable to water, so more dilute urine is produced.