where is the highest viscosity ? aorta or vena cava
Which of the following vessels contains the highest viscosity of blood in the body?
A. Aorta
B. Vena Cava
C. Vasa Recta
D. Pulmonary Vein
E. Pulmonar Artery
Ans: Vena Cava
Explaination:
Diameter of Vena cava 12mm
Diameter of Aorta -10mm
Large vein - 6mm
Large artery - 3mm
Although the effect of viscosity on fluid flow is given by Poiseuille-Hagen formula, the formula is valid only for Newtonian fluids (eg. water) and is not precisely applicable to blood which is not a Newtonian fluid. The viscosity of blood decreases with an increase in shear rate (speed of motion of the fluid with respect to nearby fluid elements). This is especially marked at low shear rate. This anomalous viscosity( that is, with change in shearing force, the viscosity also changes) of blood is attributable to axial streaming of blood cells at high shear rates. Axial streaming means that the cells occupy the central axis of the tube through which blood is flowing. This leaves a 5micro meter wide cell free zone (plasma) immediately adjacent to the vessel wall. This cell free zone produces less friction with the vessel wall and therefore, the viscosity is lower.
Axial streaming also explains the Fahreus-lindqvist effect i.e., the reduction in blood viscosity with a decrease in tube diameter. Since the cell free zone has a constant width of about 5micrometer, expressed as a percent of the the tube diameter, it is much greater for smaller tubes and therefore results in a lower viscosity.
Another consequence of axial streaming is plasma skimming i.e., a vessel that branches off from the main blood vessel at a larger angle carries way more plasma than cells because the cell free zone lies at the periphery of the flowing blood. It explains why the hematocrit of capillary blood is about 25% lower than the whole body hematocrit. In larger vessels, a rise in hematocrit causes an appreciable rise in viscosity. However, in vessels smaller than 100micrometer in dia, like in arterioles, capillaries and venules, the viscosity changes is much less than it is in larger vessels. This is why hematocrit changes have relatively little effect on the peripheral resistance except when the changes are large.
