Hormonal control of the kidney

The kidneys' work is adjusted by hormones, in a feedback system involving the hypothalamus, the JGA, and to some extent the heart.

When the body loses too much water (say on a hot day): special sensors, the osmoreceptors, detect it, and the hypothalamus releases ADH (antidiuretic hormone, also called vasopressin) from the back part of the pituitary (neurohypophysis). ADH makes the later parts of the tubule reabsorb more water, so less urine is passed. This prevents excess urine loss (diuresis). When there is enough body fluid again, ADH release is switched off. ADH also narrows blood vessels, raising blood pressure, which raises GFR.

When blood flow or pressure in the glomerulus falls (or GFR drops): the JG cells release renin. Renin turns angiotensinogen in the blood into angiotensin I, and then into angiotensin II. Angiotensin II is a powerful vasoconstrictor (it narrows blood vessels), which raises the glomerular blood pressure and so the GFR.

Angiotensin II also tells the adrenal cortex to release aldosterone. Aldosterone makes the distal parts of the tubule reabsorb more Na⁺ and water, which raises blood pressure and GFR further. This whole chain is the renin-angiotensin mechanism.

There is a check on it. When more blood flows into the atria of the heart, they release Atrial Natriuretic Factor (ANF). ANF widens blood vessels (vasodilation) and lowers blood pressure, working against the renin-angiotensin mechanism.