The renin angiotensin aldosterone system (RAAS) is a hormone system that regulates blood pressure and fluid balance. The juxtaglomerular cells are a group of baroreceptors located in the afferent arterioles of the kidney, which can sense if the blood volume is low, these cells will then secrete renin directly into circulation. Plasma renin then converts angiotensinogen released by the liver to angiotensin I. Angiotensin I is then converted to angiotensin II in the lungs by an enzyme called, angiotensin converting enzyme or ACE. Angiotensin II is a strong vasoactive peptide that causes blood vessels to constrict leading to increased blood pressure, and it mainly works on the efferent arterioles of the kidney. Angiotensin II also stimulates the secretion of the hormone aldosterone from the zona glomerulosa in the adrenal cortex. Aldosterone then acts on the distal convoluted tubules and the cortical collecting ducts of the kidneys, increasing the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure.
See more at: http://www.medical-institution.com/renin-angiotensin-aldosterone-system/