Tubular Function

Published on 27/03/2015 by admin

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Chapter 522 Tubular Function

Rajasree Sreedharan, Ellis D. Avner

Water and electrolytes are freely filtered at the level of the glomerulus. Thus, the electrolyte content of ultrafiltrate at the beginning of the proximal tubule is similar to that of plasma. Carefully regulated processes of tubular reabsorption and/or tubular secretion determine final water content and electrolyte composition of urine. Bulk movement of solute tends to occur in the proximal portions of the nephron, and fine adjustments tend to occur distally (Chapter 52).

Sodium

Sodium is essential in maintaining extracellular fluid balance and, thus, volume status. The kidney is capable of effecting large changes in sodium excretion in a variety of normal and pathologic states.

There are 4 main sites of sodium transport. Approximately 60% of sodium is absorbed in the proximal tubule by coupled transport with glucose or amino acids, 25% in the ascending loop of Henle (mediated by NKCC2, the bumetanide-sensitive sodium-potassium 2 chloride transporter), and 15% in the distal tubule (mediated by NCCT, the thiazide-sensitive sodium chloride cotransporter) and collecting tubule (mediating by EnaC, the epithelial sodium channel).

The urinary excretion of sodium normally approximates the sodium intake of 2-6 mEq/kg/24 hr for a child consuming a typical American diet, minus 1-2 mEq/kg/24 hr required for normal metabolic processes. However, in states of volume depletion (dehydration, blood loss) or decreased effective circulating blood volume (septic shock, hypoalbuminemic states, heart failure), there may be a dramatic decrease in urinary sodium excretion to as low as 1 mEq/L. Changes in volume status are detected by baroreceptors in the atria, afferent arteriole, and the carotid sinus and by the macula densa, which detects changes in chloride delivery.

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