The relationship between renin and aldosterone occurs within a physiological pathway that regulates blood pressure. Released by the kidneys, the enzyme renin activates production of angiotensin, a protein messenger. This chemical signals the adrenal secretion of aldosterone, a hormone that decreases the amount of sodium and water excreted by the kidneys, increasing blood volume and pressure. This pathway is an important factor in conditions like high blood pressure and is monitored in cases of cardiovascular disease.
Renin and aldosterone are components of a physiological feedback mechanism that regulates blood volume and cardiac output by control of vascular resistance, especially arterial blood pressure. They are components of the renin-angiotensin-aldosterone system (RAAS). Renin is an enzyme circulated by the juxtaglomerular apparatus of the kidneys, released in response to signals from the sympathetic nervous system, or when blood volume is sufficiently low. Increased amounts of renin in the bloodstream trigger production of angiotensin, a chemical messenger that increases blood pressure through constricting arteries and veins.
Once activated, angiotensin induces the adrenal glands to secrete the aldosterone, a steroid hormone. When this messenger circulates in the bloodstream, it causes sodium and water to be reabsorbed in the kidneys rather than excreted. Potassium is released as well, increasing the overall blood volume. The primary effect of renin and aldosterone working together — increased blood pressure — is enhanced by this mechanism. Sometimes the sympathetic nervous system will also raise the heart rate by releasing epinephrine simultaneously, which reinforces the actions of the RAA system.
Regulation of renal mineral exchange by substances like renin and aldosterone is an especially important step in control of blood pressure. Both the sympathetic nervous system and cells in the kidneys themselves influence the pathway. Neurological feedback slows down renal excretion of sodium and water, while local sensors in the juxtaglomerular apparatus respond by releasing more renin. The retention of sodium in the body does not just occur in the kidneys. Aldosterone prevents the loss of sodium in sweat as well by inducing an exchange with potassium ions.
Renin and aldosterone laboratory tests are used to determine if sufficient or excess levels of hormone are being produced, and help indicate the causes of malfunctioning blood pressure regulation. Increased dietary sodium, obesity, and many other factors can make the RAA system work too much, as can disorders that cause the adrenal glands to secrete too much aldosterone. This results in high blood pressure, also known as hypertension. Some medicines used to treat this condition block aldosterone receptors, decreasing arterial pressure and reducing the effects of the feedback pathway.