Descending and Ascending Limbs Differ in Water Permeability

Tubular fluid entering the loop of Henle is isosmotic to plasma, but fluid leaving the loop is distinctly hypoos-motic. Fluid collected from the earliest part of the distal convoluted tubule has an osmolality of about 100 mOsm/kg H2O, compared with 285 mOsm/kg H2O in plasma because more solute than water is reabsorbed by the loop of Henle. The loop of Henle reabsorbs about 20% of filtered Na+, 25% of filtered K+, 30% of filtered Ca2+, 65% of filtered Mg2+, and 10% of filtered water. The descending limb of the loop of Henle (except for its terminal portion) is highly water-permeable. The ascending limb is water-impermeable. Because solutes are reabsorbed along the ascending limb and water cannot follow, fluid along the ascending limb becomes more and more dilute. Deposition of these solutes (mainly Na+ salts) in the interstitial space of the kidney medulla is critical in the operation of the urinary concentrating mechanism.

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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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