Fluidbalance

Fluid balance is the dynamic equilibrium between total body water intake and loss, maintaining stable hydration and osmolar status necessary for cellular function and hemodynamic stability.

Body water is distributed across compartments: intracellular fluid makes up about two-thirds of total body water, while extracellular fluid accounts for about one-third (plasma around 5% and interstitial fluid about 25%). Water movement is governed by osmotic gradients and hydrostatic pressures. Regulation is mediated by thirst and antidiuretic hormone (vasopressin), which promotes water reabsorption in the kidneys; the renin-angiotensin-aldosterone system promotes sodium and water retention; atrial natriuretic peptide promotes excretion. Insensible losses from skin and lungs contribute to daily fluid needs.

Fluid inputs come from beverages and food; outputs include urine, feces, sweat, and insensible losses through respiration. Maintenance fluid requirements in adults are typically about 25–30 mL/kg/day, varying with age, climate, activity, and illness. In clinical care, precise intake/output monitoring helps assess fluid status and guide management.

Disturbances of fluid balance lead to dehydration or overhydration. Hypernatremia or hyponatremia reflect shifts in osmolality; edema can result from excess total body water or altered capillary forces. Common contributors include vomiting, diarrhea, kidney disease, heart failure, liver disease, and certain medications.

Management aims to restore balance and treat underlying causes. Mild dehydration may be addressed with oral rehydration; hypovolemia is typically treated with intravenous isotonic fluids, with careful electrolyte correction. Hypervolemia may require diuretics and fluid/sodium restriction. Ongoing assessment relies on daily weights, accurate intake/output recording, and monitoring of electrolytes and clinical signs.