Question

what has the greatest impact on the resistance to blood flow?

a.) viscosity

b.) diameter of blood vessel

c.) length of blood vessel

d.) hemoglobin concentration of the blood

Answer 1

The Hagen - Poiseuille Equation

Answer: Diameter of the blood vessel

In the Hagen-Poiseuille equation, where; r = radius of the blood vessel

h = Blood viscosity, l = length of blood vessel, pD = pressure gradient

Out of these,h (blood viscosity), which is inversely proportional to the rate of blood flow. As the viscosity decreases (like anemia) the blood flow increases. But viscosity will be generally constant over short period of time.Hemoglobin concentration in the blood will affect the viscosity of the blood proportionally. But, it is almost constant for a normal human body unless any blood disorder is there.

l (length of the blood vessel) is generally constatnt over a long period of time. It is inversely proportional to the rate of blood flow. E.g. In case of a fat body, there will be long vessels to accomodate to the proportional body area. So there will be more resistance and blood flow will be less.

pD (Pressure gradient) will influence the rate of flow proportionally.

however, the most varying factor among all these is r (Radius of the blood vessel). VESSEL RADIUS IS QUICKLY REGULATED. It is major contributor for the change in the resistance.Since its power is 4 (r4) it has an enormous potential to change the resistance of the blood vessel, thereby rate of blood flow too. It is primarily regulated in arterioles via sympathetic nervous system.

E.g. if sympathetic nervous sysytem is stimulated over the arterioles, there will be vasoconstriction, by which the diameter of the blood vessel and thereby the radius of the blood vessel decreases.Further the flow of blood will be drastically reduced due to increase in the vessel resistance.