Poiseuille's Law
Rate of Flow = (P1
- P2)(3.14R4)
8Ln
From the equation above, the rate of flow is clearly shown to be dependant upon the dimensions of the tube, viscosity of the fluid and the pressure difference.
The variables in the equation
are as follows:
R = the radius of the tube (radius
of capillary)
L = length of the capillary
n = coefficient of viscosity
P1 - P2
= difference in pressure of different ends of capillary
For instance, if the pressure difference across the tube increases or the tube radius increases, the rate of flow increases. In addition, if the viscosity of the fluid increases or the length of the tube increases the rate of flow of the fluid decreases.
This equation applies to the rate of flow of your blood in the capillaries in your body. Blood with a high concentration of red blood cells has a high viscosity and therefore, greater pressure is required to pump the blood from the heart in order to keep this blood in circulation. Similarly if something causes constriction of the capillary, thus making the radius of the capillary smaller, the heart is required to produce a greater pressure in order to maintain the rate of normal blood flow.