ABSTRACT
The primary function of the kidneys is the regulation of fluid and electrolyte composition of the body. The kidneys have a high blood flow, and from this a very large volume of ultrafiltrate of plasma is produced in the renal corpuscles. Markers used to determine the extracellular fluid must cross capillaries but not cell membranes; these include inulin, mannitol, radiosodium, radiochloride and thiosulphate. The interstitial volume cannot be measured directly and is calculated by subtraction of the plasma volume from the extracellular volume. During water deprivation, the urine volume can be reduced. A protein-free filtrate of plasma is formed at the beginning of the nephron by the renal corpuscle, and the fluid then passes along the lumen, through the proximal convoluted tubule, loop of Henle and distal convoluted tubule to the collecting ducts. The renal corpuscles, in the cortex of the kidney, comprise tufts of glomerular capillaries in distinct loops that invaginate Bowman’s capsules.
