The primary function of the kidneys is to maintain body fluid and ionic homeostasis by tightly regulating excretion and reabsorption of these substances ( Fig 1 ).
Although comprising only 0.5 per cent of total body weight, the kidneys receive between 20 and 25 per cent of the cardiac output. This constitutes a greater blood flow per unit weight of tissue than other vital organs and ensures efficient filtration of the blood. The volume of daily urinary excretion is the residuum of an ultrafiltration of roughly 180 liters of plasma per day and the reabsorption of more than 99 per cent of this ultrafiltrate. Despite wide fluctuations in renal arterial perfusion pressure (80-180 mmHg), renal plasma flow is strictly preserved, governed by autoregulation of the renal vascular resistance (afferent and efferent arterioles).
Renin-angiotensin, prostaglandins, thromboxanes, and the sympathetic nervous system are the putative principal mediators of arteriole tone ( Neild 19.90).
Approximately 20 per cent of the daily renal plasma flow (600 ml/min) is filtered via the glomeruli, resulting in a glomerular filtration rate of 120 ml/min. Glomerular filtration rate is contingent on the rate of plasma flow, the balance of hydrostatic and oncotic pressures traversing the capillary wall, and the permeability and total surface area of the filtering capillaries. Creatinine clearance and plasma creatinine are rough approximations of the glomerular filtration rate. The kidney also serves as a selective mechanical and electrostatic barrier to plasma protein excretion, effectively restricting the passage of molecules larger than inulin (5200 Da) and negatively charged proteins (albumin).
Fig. 1 Anatomical and physiological features of the kidney comparing cortex and medulla.
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