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- Blood flow is the flow of blood in the cardiovascular system.
- The discovery that blood flows is attributed to William Harvey.
- The flow in healthy vessels is generally laminar, however in diseased (e.g. atherosclerotic) arteries the flow may be transitional or turbulent.
Factors Affecting the Blood Flow
- Blood flows from one site to another proportionally to the difference of pressures between these sites and inversely proportionally to the resistance of conduits (which are the vessels) in which blood is circulating.
- This is the same concept of Ohm's law and it can be illustrated in the following formula:
- Flow = Difference in Pressure/Resistance
- Difference in pressure= Pressure at the first site - Pressure at the second site
- Resistance= 8 x viscosity of blood x length of vessels/ Pi x radius of vessels^4
- Circulation is influenced by the resistance of the vascular bed against which the heart is pumping.
The radius of the blood vessels:
- The vessels actively change diameter under the influence of physiology or therapy:
- Vasoconstrictors decrease vessel radius and increase resistance and hence decrease the blood flow.
- Vasodilators increase vessel radius and decrease resistance and hence increase the blood flow.
- It is important to note that resistance to flow changes dramatically with respect to the radius of the tube. This is important in angioplasty, as it enables the increase of blood flow with balloon catheter to the deprived organ significantly with only a small increase in radius of a vessel.
- Blood viscosity affects blood resistance and hence alters blood flow:
- Blood is an inhomogeneous medium consisting mainly of plasma and a suspension of red blood cells.
- Red cells tend to coagulate when the flow shear rates are low, while increasing shear rates break these formations apart, thus reducing blood viscosity.
- This results in two non-Newtonian blood properties, shear thinning and yield stress.
- In healthy large arteries blood can be successfully approximated as a homogeneous, Newtonian fluid since the vessel size is much greater than the size of particles and shear rates are sufficiently high that particle interactions may have a negligible effect on the flow.
- In smaller vessels, however, non-Newtonian blood behavior should be taken into account.