The juxtaglomerular complex is a set of specialized cells at beginning of the distal convoluted tubule, near the glomerular arterioles. When glomerular arteriole blood pressure is low (as with low blood volume), DCT fluid flow is reduced. Macula densa cells in the juxtaglomerular complex activate renin release when tubular flow decreases. These cells also mediate autoregulation, releasing a vasopressin that locally vasoconstricts the afferent arteriole when tubular flow is increased due to increased blood pressure.
Renin secretion leads to production of aldosterone in a cascade of reactions called the renin-angiotensin-aldosterone system. Renin converts inactive hepatic angiotensinogen to angiotensin I. ACE (angiotensin-converting enzyme) in the endothelium (of which the lungs have a great abundance) activate angtiotensin I to angiotensin II. Angiotensin II causes vasoconstriction and secretion of aldosterone from the adrenal cortex. Vasoconstriction of efferent arterioles maintains high glomerular pressure with low blood flow. Angiotensin II also stimulates brain thirst centers (volumetric thirst) driving the seeking of water.
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