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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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.