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).
NORMAL RENAL FUNCTION
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).
The kidney is divided into three components. The outer cortex (CI) contains 85% of the glomeruli. The inner cortex/outer medulla (CII) contains the remaining juxtamedullary glomeruli whose peritubular vessels extend to the vasa recta in the inner medulla (CIII) that establishes the hyperosmolality within the loops of Henle.
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 ...