Renal blood flow, glomerular filtration and plasma clearance

Normal functioning of the kidneys to subserve homeostasis and excretion of metabolic waste products depends on: an adequate blood supply; production of an ultrafiltrate of plasma; and the ability to modify the composition of the filtrate through reabsorption from and secretion into the tubule. About 25% of cardiac output (> 1 litre/min) is directed to the kidneys, but the distribution is not uniform. Almost 100% supplies the cortex, through the glomerular capillaries and peritubular capillaries that surround the proximal and distal convoluted tubules. Only about 10% enters the medulla, of which less than 3% reaches the inner medulla. The magnitude and selectivity of the filtration process is possible because of the arrangement of the capillaries within Bowman’s space, the structure of the capillary wall, and the visceral epithelial layer (podocytes) of Bowman’s capsule with which it is in intimate contact.