Antidiuretic hormone (ADH) increases water reabsorption by the kidneys, reducing urine volume and helping maintain blood volume and osmotic balance. During exercise, its release helps conserve water lost through sweating.
The Mechanism
ADH is a peptide hormone synthesized in the hypothalamus and stored and released by the posterior pituitary gland. Its secretion is controlled mainly by osmoreceptors in the hypothalamus, which detect changes in the solute concentration of the blood.
When sweating causes water loss, blood solute concentration rises and blood volume may fall. Osmoreceptors stimulate greater ADH release into the bloodstream. ADH then binds to receptors in the kidney's collecting ducts, causing more aquaporin water channels to be inserted into their cell membranes.
The collecting ducts consequently become more permeable to water. More water moves from the kidney filtrate back into the blood by osmosis, producing a smaller volume of more concentrated urine. As blood water content returns toward its normal level, ADH secretion decreases through negative feedback.
| Body condition | ADH response and result |
|---|---|
| Dehydration, exercise or increased blood solute concentration | More ADH is released; water reabsorption increases and urine becomes concentrated. |
| High water intake or decreased blood solute concentration | Less ADH is released; water reabsorption decreases and urine becomes dilute. |
| Reduced blood volume or blood pressure | ADH release increases to help conserve water and support blood volume. |
A common misconception is that ADH adds water to the body. It does not create water; it reduces water loss by increasing renal water reabsorption. Although ADH can cause vasoconstriction at high concentrations, its key SEHS role is water conservation and homeostasis.
Exam Technique
For an IB SEHS question on inter-system communication, link the endocrine and urinary systems in sequence: stimulus, receptor, hormone release, target organ, response and negative feedback. State that ADH decreases urine volume but increases urine concentration; do not say that it decreases both.
