Discuss the role of the kidneys in the physiological regulation of blood volume and blood pressure.

When blood volume is low renin, excreted by the kidneys, stimulates production of angiotensin I, which is converted into angiotensin II, which has many effects, including increase in blood pressure due to its vasoconstrictive properties.

The cells that excrete renin are called juxtaglomerular cells. When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I.

Aldosterone secretion from the adrenal cortex is induced by angiotensin II and causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood, thereby increasing blood volume and blood pressure.