Nightingale Anatomy Practice Exam

Urine volume is shaped by hormones that control water and salt reabsorption in the kidneys. Antidiuretic hormone (ADH) directly reduces urine output by making the collecting ducts more permeable to water, so more water is reabsorbed back into the body and the urine becomes more concentrated and smaller in volume. Aldosterone also decreases urine output, but indirectly: it increases sodium reabsorption in the distal tubule and collecting ducts, and water follows by osmosis, which expands blood volume and reduces the amount of urine produced. The combination of these two hormones is what most strongly decreases urine volume. By contrast, atrial natriuretic peptide promotes diuresis (increases urine), and renin is an enzyme that drives RAAS rather than a direct hormone that lowers urine output. Therefore, the test associated with decreasing urine production involves both aldosterone and ADH.