fluid balance

Meaning: Fluid balance means water intake (drinking, food) is matched by water loss (urine, sweat, breath, feces) while plasma osmolarity and blood volume are held near set points. The kidney does most of the adjusting by changing how much water and Na⁺ it reabsorbs.

Two main loops operate. Osmolarity loop: hypothalamic osmoreceptors detect high plasma osmolarity, which triggers thirst and ADH release from the posterior pituitary. ADH inserts aquaporins in the collecting duct, so more water is reabsorbed and urine is small and concentrated. Volume loop: low blood pressure or volume makes the juxtaglomerular cells release renin, leading to angiotensin II and aldosterone from the adrenal cortex. Aldosterone increases Na⁺ reabsorption in the distal tubule and collecting duct, and water follows by osmosis. Atrial natriuretic peptide, released when the atria are stretched, opposes this by promoting Na⁺ and water excretion.

On the MCAT: Passages describe dehydration, a salty meal, alcohol intake or a hormone defect, then ask you to predict ADH level, urine volume or urine concentration. Dehydration gives high ADH and low-volume, concentrated urine. Alcohol suppresses ADH, giving large, dilute urine.

Common confusion: Students swap ADH and aldosterone. ADH changes water permeability and mainly corrects osmolarity. Aldosterone moves Na⁺ and mainly corrects volume and pressure; because water follows Na⁺, it changes osmolarity little.