Urea Plays a Special Role in the Concentrating Mechanism

It has long been known that animals or humans on low-protein diets have an impaired ability to maximally concentrate the urine. A low-protein diet is associated with a decreased [urea] in the kidney medulla.

Figure 23.26 shows how urea is handled along the nephron. The proximal convoluted tubule is fairly permeable to urea and reabsorbs about 50% of the filtered urea. Fluid collected from the distal convoluted tubule, however, has as much urea as the amount filtered. Therefore, urea is secreted in the loop of Henle.

The thick ascending limb, distal convoluted tubule, connecting tubule, cortical collecting duct, and outer medullary collecting duct are relatively urea-impermeable. As water is reabsorbed along cortical and outer medullary collecting ducts, the [urea] rises. The result is the delivery to the inner medulla of a concentrated urea solution. A concentrated solution has chemical potential energy and can do work.

The inner medullary collecting duct has a facilitated urea transporter, which is activated by AVP and favors urea diffusion into the interstitial fluid of the inner medulla. Urea may reenter the loop of Henle and be recycled (see Fig. 23.26), building up its concentration in the inner medulla. Urea is also added to the inner medulla by diffusion from the urine surrounding the papillae (calyceal urine). Urea accounts for about half of the osmolality in the inner medulla. The urea in the interstitial fluid of the inner medulla counterbalances urea in the collecting duct urine, allowing the other solutes (e.g., NaCl) in the interstitial fluid to counterbalance osmotically the other solutes (e.g., creatinine, various salts) that need to be concentrated in the urine.

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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