Glomerular filtration is a rather nonselective process, since both useful and waste substances are filtered. By contrast, tubular transport is selective; different substances are transported by different mechanisms. Some substances are reabsorbed, others are secreted, and still others are both reabsorbed and secreted. For most, the amount excreted in the urine depends in large measure on the magnitude of tubular transport. Transport of various solutes and water differs in the various nephron segments. Here we describe transport along the nephron and collecting duct system, starting with the proximal convoluted tubule.
The proximal convoluted tubule comprises the first 60% of the length of the proximal tubule. Because the proximal straight tubule is inaccessible to study in vivo, most quantitative information about function in the living animal is confined to the convoluted portion. Studies on isolated tubules in vitro indicate that both segments of the proximal tubule are functionally similar. The proximal tubule is responsible for reabsorbing all of the filtered glucose and amino acids; reabsorbing the largest fraction of the filtered Na+, K+, Ca2+, Cl—, HCO3—, and water and secreting various organic anions and organic cations.
Tubular fluid (or urine) [inulin]/plasma [inulin] ratio as a function of collection site (data from micropuncture experiments in rats). The increase in this ratio depends on the extent of tubular water reabsorption. The distal tubule is defined in these studies as beginning at the macula densa and ending at the junction of the tubule and a collecting duct and it includes distal convoluted tubule, connecting tubule, and initial part of the collecting duct. (Modified from Giebisch G, Windhager E. Renal tubular transfer of sodium, chloride, and potassium. Am J Med 1964;36:643-669.)
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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.