Explain the Glomerular Filtration Rate (GFR)

The kidneys receive a large amount of blood flow. The kidney blood flow is approximately 1200 ml per minute. One-tenth of this is filtered as the blood flows through the glomeruli i.e., about 120 ml. Filtration takes place through the semi-permeable walls of the glomerular capillaries, which are almost inlpermeable to proteins and large molecules. The filtrate is thus virtually free of proteins and has no cellular elements. The glomerular filtrate is formed by squeezing the fluid through the glomerular capillary bed.

The driving hydrostatic pressure (the pressure exerted by a liquid as a result of its potential energy) is controlled by the afferent and efferent arterioles (the muscular walled vessels leading to and from each glomerulus, and is provided by the arterial pressure, that is, the pressure of the circulating blood on the arteries. The glomerular filtration rate decreases with age and disease. In order to keep the renal blood flow and GFR relatively constant, the hydrostatic pressure in the glomerulus has to be kept fairly constant. When there is a change in the arterial blood pressure, there is a constriction or dilatation of the afferent and efferent arterioles. This process is called as autoregulation and can be more precisely defined as the tendency of the blood flow to organ to remain constant in spite of the pressure changes in the artery that delivers blood to that organ.