Fluid Compartments & Homeostasis

• Distribution: Water is 50%–60% of adult weight (varies with fat). ICF holds 2/3 of water; ECF holds 1/3 (interstitial fluid/plasma). Weight change is the best fluid shift indicator (1 kg = 1 L).
• Capillary Exchange: Governed by hydrostatic (pushes out) and plasma oncotic (albumin pulls in) pressures. Edema occurs if hydrostatic pressure rises, oncotic falls, or lymphatics are blocked. Third-spacing traps nonfunctional fluid.

Regulatory Mechanisms

• Hormones: Dehydration triggers thirst and pituitary ADH, raising renal water reabsorption. Aldosterone promotes sodium retention and potassium excretion. Atrial stretch releases natriuretic peptides (ANP/BNP) to excrete sodium/water.