Total body water is divided into intracellular and extracellular compartments separated by semipermeable membranes. Osmolarity—determined primarily by sodium, potassium, and glucose—drives water movement. ADH (antidiuretic hormone) regulates water reabsorption in the collecting duct, with osmoreceptors in the hypothalamus sensing plasma osmolarity. Aldosterone regulates sodium reabsorption in the distal tubule, indirectly affecting water balance.
From your study of renal filtration, you know the kidney processes roughly 180 liters of filtered plasma daily, reabsorbing almost all of it in a carefully regulated sequence. But what signals drive that reabsorption? And how does the kidney "know" how concentrated to make urine? The answer lies in osmolarity control — the body's system for maintaining stable solute concentrations despite wildly variable fluid intake and losses.
Osmolarity (measured in mOsm/kg) describes the total concentration of solutes in a fluid. Normal plasma osmolarity is approximately 285–295 mOsm/kg, with sodium and its accompanying anions accounting for roughly 90% of the total. Because water moves across semipermeable membranes down osmotic gradients — as you studied in osmosis — the osmolarity difference between compartments directly determines water distribution. The intracellular fluid (ICF) holds about two-thirds of total body water; the extracellular fluid (ECF), including plasma and interstitial fluid, holds the remaining third. Sodium is the dominant ECF cation; potassium dominates the ICF. Disrupting sodium concentration disrupts the ECF-to-ICF osmotic balance throughout every cell in the body — which is why sodium disorders are among the most dangerous electrolyte disturbances.
Two hormone systems regulate this balance, each responding to a different sensor. ADH (antidiuretic hormone, also called vasopressin) is secreted by the posterior pituitary when hypothalamic osmoreceptors detect plasma osmolarity rising above ~290 mOsm/kg, or when baroreceptors signal low blood volume. ADH inserts aquaporin-2 water channels into the collecting duct epithelium, dramatically increasing its water permeability. Water then flows osmotically from the tubular lumen into the hyperosmotic medullary interstitium (the gradient the loop of Henle created), concentrating the urine. When you are well-hydrated and plasma osmolarity falls, ADH secretion drops, aquaporins are removed from the membrane, and dilute urine is excreted. Aldosterone, secreted by the adrenal cortex in response to the renin-angiotensin system (triggered by low renal perfusion), acts on the distal tubule and collecting duct to increase sodium reabsorption via Na⁺/K⁺-ATPase and epithelial sodium channels (ENaC). Because water follows sodium osmotically, aldosterone indirectly retains water and expands ECF volume.
The two systems solve different problems: ADH regulates osmolarity (solute concentration per liter), while aldosterone regulates volume (total sodium content and ECF expansion). They can work together or at cross-purposes. In dehydration, both are activated — ADH concentrates urine, aldosterone retains sodium and water together. In pure water overload, ADH is suppressed but aldosterone may remain active if volume is normal. Clinical conditions arise when these systems are dysregulated: SIADH (syndrome of inappropriate ADH) causes water retention and dilutional hyponatremia; diabetes insipidus (absent ADH or renal resistance to it) causes failure to concentrate urine and hypernatremia. Understanding which sensor is driving which hormone is the key to predicting the direction of any clinical fluid-electrolyte disorder.