Cardiac and renal physiology are intimately intertwined. Impaired cardiac function can lead to kidney injury and vice versa. Furthermore, many cardiovascular therapeutic interventions may cause renal injury. Impaired renal function is common among patients with heart disease and is one of the strongest predictors of worse outcome for these patients. Reports suggest that renal disease is underdiagnosed among patients with heart disease, which increases the risk of avoidable adverse outcomes.1,2,3,4
Renal Physiology and Important Concepts
The main functions of the kidneys are to maintain water balance and body fluid homeostasis and to eliminate metabolic waste products. This is achieved by filtration of plasma in the glomerulus, as well as by secretion and reabsorption of water and solutes along the renal tubules. An average human filters 150 to 200 L of plasma across the glomeruli and produces 0.5 to 2.5 L of urine per day. Urine output greater than 2.5 L/d is termed polyuria; urine output less than 400 mL/d is termed oliguria; and urine output less than 100 mL/d is termed anuria.
The concept of renal clearance is based on Fick’s principle, predicated on the law of conservation of mass. A substance that is neither synthesized nor metabolized within the kidney must enter and leave the kidney in equal amounts. A substance can enter the kidney only via the renal artery but can leave the kidney either via the renal vein or via the urine. Renal clearance refers to the excretory function of the kidney and therefore considers only the rate at which a substance is excreted into the urine, not the rate at which it is returned to the circulation through the venous system. In terms of mass balance, the excretion rate of substance y (Uy*V), where Uy is the concentration of y in the urine and V is urine volume/time, is proportional to the plasma concentration of y (Py). In a steady state, the rate at which y is moved from the plasma to the urine is the renal clearance (Cy):
Hence, renal clearance represents a volume of plasma from which the substance of interest has been removed and excreted into the urine per unit time. For substances that are eliminated from the body predominantly through the kidney, plasma concentrations in blood samples from peripheral venous blood will closely approximate arterial concentrations. Depending on the properties of the substance measured, renal clearance can be used to estimate glomerular filtration rate (GFR) and renal plasma flow (RPF). Substances such as inulin, which are only filtered by the glomerulus, are the gold standard for measurement of clearance, but because they need to be administered intravenously are not practical for routine clinical use. ...