Schema for the kidney blood volume pressure feedback mechanism adapted from the work of Guyton and colleagues . Positive relations are indicated by a plus sign; inverse relations are indicated by a minus sign. The block diagram shows that increases in extracellular fluid (ECF) volume result from increases in sodium chloride (NaCl) and fluid intake or decreases in kidney volume output. An increase in ECF volume increases the blood volume, thereby increasing the venous return to the heart and cardiac output. Increases in cardiac output increase arterial pressure both directly and by increasing peripheral vascular resistance (autoregulation). Increased arterial pressure is sensed by the kidney, leading to increased kidney volume output (pressure diuresis and pressure natriuresis), and thus returning the ECF volume to normal. The inset shows this relation between mean arterial pressure (MAP), renal volume, and sodium excretion . The effects of acute increases in arterial pressure on urinary excretion are shown by the solid curve. The chronic effects are shown by the dotted curve; note that the dotted line is identical to the curve in Figure 2-3. Thus, when the MAP increases, urinary output increases, leading to decreased ECF volume and return to the original pressure set point. UNaV—urinary sodium excretion volume.
Sodium (Na) reabsorption along the mammalian nephron. About 25 moles of Na in 180 L of fluid daily is delivered into the glomerular filtrate of a normal person. About 60% of this load is reabsorbed along the proximal tubule (PROX), indicated in dark blue; about 25% along the loop of Henle (LOH), including the thick ascending limb indicated in light blue; about 5% to 7% along the distal convoluted tubule (DCT), indicated in dark gray; and 3% to 5% along the collecting duct (CD) system, indicated in light gray. All Na transporting cells along the nephron express the ouabain-inhibitable sodium-potassium adenosine triphosphatase (Na-K ATPase) pump at their basolateral (blood) cell surface. (The pump is not shown here for clarity.) Unique pathways are expressed at the luminal membrane that permit Na to enter cells. The most quantitatively important of these luminal Na entry pathways are shown here. These pathways are discussed in more detail in Figures 2-15 to 2-19. CA—carbonic anhydrase; Cl—chloride; CO2—carbon dioxide; H—hydrogen; H2CO3—carbonic acid; HCO3—bicarbonate; K—potassium; OH—hydroxyl ion.
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