Water Balance: ADH, Aldosterone and the Kidney

Osmoregulation is the control of water and dissolved substances. In this lesson, watch whether the question is asking about water directly or sodium first.

The USARIEM result above happened inside the kidney. Each kidney contains about one million nephrons, the functional filtration units. Blood is filtered at the start of each nephron, and the filtrate then flows through a series of tubule segments that reclaim water and solutes back into the blood, a process called reabsorption.

For water balance, two segments matter. The distal tubule (DCT) is where aldosterone acts to increase Na⁺ reabsorption, with water following by osmosis. The collecting duct is the final segment, where ADH acts by inserting aquaporin water channels into the membrane. The more aquaporins present, the more water returns to the blood.

At baseline, with no ADH present, the collecting duct is relatively impermeable to water. Most of the filtrate passes through and is excreted as dilute urine. When ADH arrives, aquaporins flood into the collecting duct membrane, water reabsorption rises sharply, and urine becomes concentrated. This permeability switch is what sets urine concentration.

Nephron structure showing filtration, reabsorption and secretion

Water balance regulation showing ADH, aldosterone and kidney function

When blood becomes too concentrated, ADH increases water reabsorption. This is negative feedback: urine volume falls and the blood is diluted back toward normal.

The loop above is worth slowing down, because each step earns a mark. The stimulus is blood osmolarity rising above about 295 mOsm/kg, from sweating, a salty meal, or not drinking. Osmoreceptors in the hypothalamus detect the rise: they shrink slightly as water leaves them by osmosis, and that shrinkage is the detection event.

The hypothalamus then signals the posterior pituitary by nerve impulses, and the pituitary releases ADH into the bloodstream. ADH is synthesised in the hypothalamus but stored and released from the posterior pituitary, and exams reward students who separate the synthesis site from the release site.

ADH travels in the blood to the kidney and binds to collecting duct cells, triggering aquaporin channels to move into the tubule membrane. Water now leaves the filtrate by osmosis and re-enters the blood. Urine volume falls and concentration rises, and as osmolarity drops back toward 285 to 295 mOsm/kg, ADH secretion falls. The loop switches itself off: negative feedback.

The reverse pathway, overhydration

When blood osmolarity falls below about 285 mOsm/kg after drinking a large volume of water, the same loop runs backwards. Osmoreceptors detect the dilution, ADH secretion falls, aquaporins are withdrawn from the collecting duct membrane, and water permeability drops. Less water is reabsorbed, so the kidneys excrete large volumes of pale, dilute urine until osmolarity returns to range.

ADH corrects concentration; aldosterone corrects volume and pressure. When blood pressure falls, through dehydration, blood loss or low salt intake, juxtaglomerular cells in the kidney wall detect the pressure drop in the afferent arteriole and release the enzyme renin. This starts the renin-angiotensin-aldosterone system, usually shortened to RAAS.

Renin converts angiotensinogen, a liver protein circulating in the blood, into angiotensin I, which is then converted to angiotensin II, the active form, in the lungs. Angiotensin II stimulates the adrenal cortex, the gland sitting on top of each kidney, to release aldosterone into the bloodstream.

Aldosterone acts on the distal tubule and increases Na⁺ reabsorption from the filtrate back into the blood. It does not move water directly. As sodium leaves the filtrate, the blood becomes slightly more concentrated than the filtrate, and water follows down that osmotic gradient by osmosis. Blood volume rises, and blood pressure with it, without a large change in osmolarity.

Rising pressure is then detected and renin release is suppressed, so aldosterone falls and Na⁺ reabsorption returns to baseline. This pathway is also the target of two major blood pressure drug classes: ACE inhibitors such as ramipril block angiotensin II formation, and aldosterone antagonists such as spironolactone block the hormone's receptors. Both reduce Na⁺ retention, blood volume and pressure.

Homeostasis is coordinated by two systems that divide the labour by what each does best. Neural coordination uses electrical impulses along nerve fibres: millisecond-fast, precisely targeted, but brief, ending as soon as the impulse stops. Hormonal coordination uses chemical signals in the bloodstream: slower, in seconds to minutes, but broader and sustained while the hormone circulates.

Water balance uses both in sequence. The neural component is the hypothalamus sending nerve impulses to the posterior pituitary, a fast and specific trigger. The hormonal component is ADH then travelling in the blood to every collecting duct in both kidneys, a slower signal that persists for as long as blood osmolarity stays high. The neural trigger starts the response; the hormone carries out the sustained correction.

The same pattern runs through this module. In temperature regulation, hypothalamic nerve signals activate sweat glands within seconds, while thyroid hormone adjustment to a cold climate takes days. Speed is a trade-off, not a hierarchy: neither system is better, and most homeostatic responses pair a fast neural trigger with a sustained hormonal follow-through.