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)