Glomerular Filtration Rate
Introduction
The glomerular filtration rate or GFR is the amount of fluid filtered from the capillaries into the Bowmans capsule per unit time. The GFR can be expressed as the following formula:
Kf = the filtration coefficent
Kf can furthermore be expressed by the following formula
The GFR is practically proportional to metabolic body mass. Therefore the bigger the animal the greater the GFR.
Regulation of the GFR
The following formula helps us to understand GFR and how various factors affect it. Whilst reading this article you may find it useful to refer back to it:
Q = Flow, PA = Pressure in afferent arteriole, PE = Pressure in efferent arteriole, R = Resistance
There are two major forces opposing GFR. These are the hydrostatic pressure in the Bowmans space and the plasma protein osmotic pressure. These are not under physiological control. The filtration coefficient is also beyond the realms of physiological control. On the other hand the hydrostatic pressure in the capillaries and the renal blood flow are under physiological regulation and adjust filtration according to the bodies needs.
Regulation of Renal Blood Flow and Capillary Hydrostatic Pressure
These two factors are determined by the arterial blood pressure coupled with the contraction of both the afferent and efferent arterioles. The total resistance of the afferent and efferent arterioles, which is determined by the contraction of them, determines the renal blood flow and any particular arterial pressure. Therefore it is important that they change with arterial pressure in order to maintain a steady renal blood flow.
Constriction of the Afferent and Efferent Arterioles
Normally the afferent arteriole is of larger diameter than the efferent. This means there is high resistance as the blood is forced from a wider vessel to a narrower one and this promotes filtration. If the arterial blood pressure remains constant then contracting either vessel reduces blood flow as it increases resistance. However contracting either has opposite effects on the filtration pressure. If you contract the afferent arteriole there will be less of a pressure difference between the afferent and efferent arteriole so there will be reduced filtration pressure. However if you constrict the efferent arteriole you are increasing the pressure difference between the two and filtration pressure increase.
Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure. (Both of these statements are assuming a constant blood pressure). The fact that both can be altered allows independent regulation of both GFR and blood flow.
Physiological Regulators of GFR
The main systems which regulate renal blood flow and GFR are: