Sodium - I.DavidWeiner,M.D 3746102...

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Figure 1. Drawing of renal glomerulus showing afferent arteriole (AA), efferent arteriole (EA), glomerulur tufts, Bowman’s space (BS) and the initial proximal tubule (PT). Sodium transport in the kidney I. David Weiner, M.D. 374 6102 [email protected] A major function of the kidney is regulating extracellular fluid composition . Normal variation in the intake of substances such as water, salt (NaCl), potassium and glucose, and in the generation of metabolic waste products, such as urea, requires the ability to regulate the excretion of solutes. Sweat and stool content of these solutes are low and relatively constant. Therefore other mechanisms are used to regulate the body's content of these. In general, the primary mechanism through which the body regulates its content of water soluble compounds is through variations in urinary excretion. The kidney, for most solutes, uses a two step mechanism to regulate body fluid composition. The first stage filters blood, separating components that should never be excreted into the urine from those that need to be excreted under either some or all conditions. Examples of blood components that should never be in the urine are cellular components (RBC, WBC and platelets), albuminandotherproteins,andlipids. Examplesofcomponents routinely present in the urine include water (H 2 O), sodium (Na + ), potassium (K + ), glucose and urea. Glomerular filtration (Figure 1) separates these components (see other lectures). Fluid that enters Bowman’s space and then into the lumen of the proximal tubule consists of water (H 2 O), and the majority of water soluble minerals, electrolytesandothersolutespresentinblood. The second step is to reabsorb the water and electrolytes from this glomerular filtrate in the amounts needed to maintain health. Table 1 shows quantitative considerations of renal filtration and reabsorption of water and other filtered minerals and molecules. In general, the kidneys adjust reabsorption so that excretion balances the difference between intake, typically oral intake, and extra renal excretion. For example, if someone is dehydrated, GFR Substance Units Filtered Excrete Reabsorbed Amount % Water L/day 180 1 179 99.4% Na + mEq/d 25,200 150 25,050 99.4% K + mEq/d 720 100 620 86.1% Ca +2 mEq/d 540 10 530 98.1% HCO 3 mEq/d 4,320 4 4,316 99.9% Cl mEq/d 18,000 150 17,850 99.2% Glucose mmol/d 800 0 800 100.0% Table 1. Filtration and reabsorption of ions and other molecules by the kidney.
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Sodium transport in the kidney Page 2 of 5 ~ 3 Na + 2 K + [Na + ] ~10-20 mEq/L -70 mV Figure 3. Effect of basolateral Na + K + ATPase on intracellular sodium concentration and membrane potential. Figure 4. Sodium reabsorption along the nephron. PT Proximal tubule; TAL thick ascending limb of the loop of Henle; DT distal connecting tubule; CCD cortical collecting duct; IMCD inner medullary collecting duct.
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Sodium - I.DavidWeiner,M.D 3746102...

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