The juxtaglomerular apparatus is the region in each nephron where the afferent arteriole comes into contact with the last portion of the thick ascending limb of the loop (fig. 17.25). Under the microscope, the afferent arteriole and tubule in this small region have a different appearance than in other regions. Granular cells within the afferent arteriole secrete the enzyme renin into the blood; this enzyme catalyzes the conversion of angiotensino-gen (a protein) into angiotensin I (a ten-amino-acid polypeptide).
Secretion of renin into the blood thus results in the formation of angiotensin I, which is then converted to angiotensin II (an eight-amino-acid polypeptide) by angiotensin-converting enzyme (ACE). This conversion occurs primarily as blood passes through the capillaries of the lungs, where most of the converting enzyme is present. Angiotensin II, in addition to its other effects (described in chapter 14), stimulates the adrenal cortex to secrete aldosterone. Thus, secretion of renin from the granular cells of the juxtaglomerular apparatus initiates the renin-angiotensin-aldosterone system. Conditions that result in increased renin secretion cause increased aldosterone secretion and, by this means, promote the reabsorption of Na+ from cortical collecting duct into the blood.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.