Biomechanics 3
Hemorheology - rheology characteristics of the blood. Dependence of blood viscosity on the hematocrit, deformability and aggregation of the red blood cells.
Although plasma is about 90% water, blood is a non-Newtonian liquid. This is because blood viscosity is not independent of flow at all flow velocities.
In some instances blood exhibits Newtonian characteristics. Eg when flowing through very large vessels, much larger than the size of the components of blood, blood acts as a homogenous fluid and the blood viscosity exhibits Newtonian characteristics.
Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow.
The most important factor in vascular resistance is friction between blood and the vessel walls. The amount of friction depends on the length and diameter of the vessel.
Vessel length: Increasing the length of a blood vessel increases friction, the longer the vessel the larger is the surface area in contact with blood.
Vessel diameter: The effects of friction on blood act primarily in a narrow zone closest to the vessel wall. In a small-diameter vessel, nearly all the blood is slowed down by friction with the walls. Resistance is therefore relatively high. Blood near the center of a large-diameter vessel does not encounter resistance from friction with the walls, so the resistance is relatively low.
Differences in diameter have much more significant effects on resistance than differences in length do. With two vessels of equal length, one twice the diameter of the other, the narrower one will offer 16 times as much resistance to blood flow.
More significantly, there is no way to control vessel length, but vessel diameter can change quickly through;
- 1. Vasoconstriction; increased contraction of circular smooth muscle in the arteriolar wall, which leads to increased resistance and decreased flow through the vessel.
- 2. Vasodilation; decreased contraction of circular smooth muscle in the arteriolar wall, which leads to decreased resistance and increased flow through the vessel.
Relative viscosity is the ratio of the viscosity of solution to the viscosity of water at some temperature. Blood plasma relative viscosity is 1.8. Hematocrit on blood viscosity: Increased hematocrit value = increase in relative viscosity of blood. The relationship is shown in this Einstein equation:
The increase is non-linear, so if you double hematocrit value you more than double the relative viscosity. Normal HCT value for men is 45% and for women 40%.