Aldosterone is a hormone produced in the outer section (cortex) of the adrenal glands, which sit above the kidneys. It plays a central role in the regulation of blood pressure mainly by acting on organs such as the kidney and the colon to increase the amount of salt (sodium) reabsorbed into the bloodstream and to increase the amount of potassium excreted in the urine. Aldosterone also causes water to be reabsorbed along with sodium; this increases blood volume and therefore blood pressure. Aldosterone is part of a group of linked hormones, which form the renin–angiotensin–aldosterone system. Activation of this system occurs when there is decrease in blood flow to the kidneys following loss of blood volume or a drop in blood pressure (e.g. due to a haemorrhage). Renin is an enzyme that leads to a series of chemical reactions resulting in the production of angiotensin II, which in turn stimulate aldosterone release. Aldosterone causes an increase in salt and water reabsorption into the bloodstream from the kidney thereby increasing the blood volume, restoring salt levels and blood pressure.