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juxtaglomerular apparatus (JGA) A region of tissue found in each nephron in the kidney that is important is regulating blood pressure and body fluid and electrolytes. It is located where the distal convoluted tubule passes close to the afferent arteriole supplying the Bowman's capsule, near to the glomerulus, and contains two types of specialized cells. Large smooth muscle cells in the wall of the afferent arteriole form the juxtaglomerular cells, or granular cells; these contain granules of the proteolytic enzyme renin, which are released when the juxtaglomerular cells detect decreased blood pressure in the arteriole. The JGA also includes chemoreceptor cells of the adjacent region of the distal tubule, which form a tightly packed array called the macula densa. This detects low concentrations of sodium ions in the filtrate inside the kidney tubule (indicative of reduced plasma sodium levels) and triggers release of renin from the juxtaglomerular cells. Release of renin into the bloodstream leads to increased levels of angiotensins, which raise blood pressure and also stimulate the secretion of aldosterone from the adrenal cortex and antidiuretic hormone from the posterior pituitary. Aldosterone promotes reabsorption of sodium ions from the distal tubule, and antidiuretic hormone promotes water reabsorption.
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