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The average 70-kg person has 42 L of water divided into the intracellular space (ICF) of 28 L and the extracellular space (ECF) of 14 L. The ICF is subdivided into the red blood cell (RBC) mass of 2 L and the visceral mass of 26 L; the ECF is subdivided into a plasma volume (PV) of 3 L and an interstitial fluid space (IFS) volume of 11 L. The cardiac output (CO) in this 70-kg person is 5 L/min with 20% of this flow going to kidneys; thus the kidneys, with a combined weight of about 600 g, have a renal blood flow (RBF) of 1250 mL/min or more than 2 mL/min/g of renal parenchymal. This unusually large RBF reflects the vital renal role in regulating the ICF and ECF, controlling fluid and electrolyte balance, modulating acid-base balance, and excreting undesirable catabolyes.1,2 Protection of renal function is essential for recovery after a shock or septic insult. This chapter reviews normal renal physiology, the renal response to shock and sepsis, guidelines for prevention and treatment of acute renal failure (ARF).
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NORMAL RENAL FUNCTION
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The 1250 mL/min of RBF passes through the renal artery into the interlobar, the arcuate, and, finally, the intralobar arteries; 85% of the RBF perfuses the outer cortical glomeruli; the remaining 15% of RBF perfuses the juxtamedullary glomeruli (Fig. 59-1). The glomeruli (Bowman’s capsules) are like capillaries except that proteins, normally, are not filtered.2,3 While passing through the glomeruli, 20% of the plasma is filtered as a cell-free, protein-free filtrate. The effective renal plasma flow (ERPF) through these tubular vessels is determined by the clearance of para-aminohippurate (CPAH) that is filtered and secreted but not reabsorbed by the renal tubules. Ninety-one percent of PAH is cleared in one passage; 9% remains bound to the plasma protein. Renal oxygen consumption parallels ERPF which averages 650 mL/min. True renal plasma flow (TRPF) is calculated by dividing ERPF by 0.91 and averages 710 mL/min. The extraction ratio of PAH (EPAH), however, may vary with injury and sepsis; true EPAH requires renal vein sampling to accurately measure true renal plasma flow (TRPF) (TRPF = ERPF/EPAH).2,3,4,5 TRBP may be calculated by correcting the TRPF for hematocrit: TRBF = TRPF/(1-Hct).
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The distribution of RBF can be measured by isotopic disappearance of radioactive xenon-133 (133Xe) or krypton-85 that can be graphically portrayed as a cumulative slope composed of four separate subslopes reflecting ...