Kidney: Pharmacology of Contrast Media

Abstract

Water accounts for 60% of the body weight of normal man, varying inversely (10%) with the presence or absence of fat [14]. Body water is distributed between the intracellular (40%) and the extracellular (20%) spaces. The transcellular space well be ignored in this review. Water moves between these spaces to balance osmotic and hydrostatic pressures. The movement of solutes is restricted by cell membranes and is mediated by active transport processes. The extracellular space may be further subdivided into a plasma (or vascular) component and an interstitial fluid component. Water and solutes distribute themselves freely between these compartments by diffusion, without the constraint of crossing cell membranes. The diffusion of solutes is influenced, to varying degrees, by factors such as molecular weight and protein binding. One quarter of the cardiac output perfuses the kidneys, providing a renal blood flow of 1250 ml/min or, expressed as renal plasma flow, 600 ml/min. Blood passing through the glomeruli produces a filtrate equivalent to 125 ml/min of plasma of 116 ml/min of water. Solutes are removed from and added to the filtrate by active tubular transport processes. The movement of solutes is accompanied by the passive movement of water in response to osmotic effects. The proximal tubule is the principal site of water reabsorption. In health, the final concentration of the filtrate is adjusted during its passage through the distal segments of the nephron, by the action of antidiuretic hormone, in response to the overall hydration status. The range of water reabsorption varies from 110 to 115 ml/min, leaving a residual urinary output of 6.0–0.5 ml/min.