Renin-Angiotensin-Aldosterone System (RAAS)

The renin-angiotensin-aldosterone system (RAAS) is the kidney's main blood-pressure-and-sodium-conservation pathway, centred on the juxtaglomerular apparatus (JGA) - a specialised patch of tissue at the afferent arteriole made up of macula densa cells (which sense the flow of fluid reaching the distal tubule) and granular cells (which manufacture and release the enzyme renin). A drop in glomerular blood flow, blood pressure or filtration rate activates the granular cells to release renin, which converts the liver-made plasma protein angiotensinogen into angiotensin I; angiotensin-converting enzyme (ACE) then converts angiotensin I into angiotensin II, the system's main active hormone. Angiotensin II raises blood pressure directly by constricting blood vessels and by increasing sodium reabsorption in the proximal convoluted tubule, and it also stimulates the adrenal cortex to release aldosterone, which acts on the distal convoluted tubule and collecting duct to increase sodium reabsorption (with potassium excretion) and, indirectly, …