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Renin-angiotensin and aldosterone system also known as RAAS is one of the major regulatory systems of the human body. It plays a major role in maintaining blood pressure and volume and regulating blood sodium levels. Mechanism of secretion of renin: Renin is an enzyme/hormone that is secreted by extra glomerular mesangial cells/ Polkessien cells or juxtaglomerular granular cells. These cells are present in afferent arteriole of each nephron of kidneys and some in efferent arteriole too. They are the specialized smooth muscle which secrete renin whenever blood flow is less. When there is normal blood flow, there is normal filtration on the glomerulus, about 60% of sodium is reabsorbed and 25% is reabsorbed in the ascending loop of Henle. The remaining 10 % remains in the distal convoluted tubule ( DCT). In the DCT there are specialized cuboidal cells called macula densa cells which detect the concentration of sodium ions in the DCT. Juxtaglomerular apparatus: Nephron