| The glomerular filtration rate or GFR, is the rate (usually per day) at which plasma is filtered through the fenestrated portions of the glomerular capillaries into Bowman's Space. This ultrafiltrate eventually passes through the renal tubules of the nephrons in the kidneys.
It is often desirable in clinical diagnosis of kidney problems to know this rate. If it is too low, there may be a problem of hypotension, or a blockage of the renal arteries. There may also be problems with the kidneys, especially in the filtration barrier.
GFR is most often calculated using one of three chemicals. These are:
See the nodes of these items for detailed information, but briefly, the first and third are not natuarlly found in the body, and must be infused, usually with an IV drip. The second is produced as a waste product of the muscles naturally. All of these are freely filtered from the body, so a knowledge of the amount in the urine, the urine flow rate, and the plasma concentration of any of these will give a good estimate of glomerular filtration rate. The equation for this is as follows:
GFR = ( U * V ) / P
Where U is the urine concentration of the solute, V is the urine flow rate, and P is the plasma concentration of the solute. Note that inulin and PAH are more accurate than creatinine, which generally yields only an estimate due to how the kidney handles it.
This rate is entirely dependent on the pressure difference between the glomerular capillary and Bowman's Capsule, and therefore the blood pressure entering that capillary. This pressure is regulated by the afferent arteriole, and the need to maintain filtration may lead to hypertension in pathological situations.
These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano) | 
