11.12: The Juxtaglomerular Apparatus
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The juxtaglomerular apparatus (JGA) controls the rate of the glomerular blood flow depending on various factors such as hydration, blood volume and ion concentration. The JGA includes the macula densa and juxtaglomerular cells. The macula densa is a short segment of the distal convoluted tubule that is located directly adjacent to the afferent and efferent arterioles at the vascular pole of the glomerulus. The juxtaglomerular cells are within the interstitium between the macula densa and the vascular pole. The macula densa cells sense sodium chloride concentration in the lumen and stimulates the juxtaglomerular cells to synthesize and release renin into the bloodstream. This happens in states of hypovolemia or low sodium chloride concentration in the blood (and in the urinary filtrate). This activates the renin-angiotensin-aldosterone system (RAAS) and triggers several key important steps to help improve blood pressure. The release of renin in the bloodstream causes hydrolysis of angiontensinogen to form angiotensin I. Angiotensin I is converted by angiotensin-converting enzyme (ACE) to angiotensin II. Angiotensin II triggers the adrenal glands to produce and release aldosterone, whichacts on tubules to upregulate sodium and water reabsorption back into the blood.