How does viscous blood affect flow according to Poiseuille's law?

• A. It increases flow
• B. It decreases flow
• C. It has no effect on flow
• D. It can fluctuate flow rates

Answer: (B)

Viscous blood affects flow in accordance with Poiseuille's law, which states that the flow rate (Q) of a fluid through a pipe is proportional to the fourth power of the radius of the pipe and inversely proportional to the fluid's viscosity (η). The relationship can be expressed mathematically as:

[ Q = \frac{\pi r^4 (P_1 - P_2)}{8 \eta L} ]

where ( r ) is the radius of the vessel, ( P_1 ) and ( P_2 ) are the pressures at each end of the vessel, ( \eta ) is the viscosity, and ( L ) is the length of the vessel.

When blood viscosity increases, it raises the value of ( η ) in the equation. This increase in viscosity leads to a reduction in flow rate because flow rate is inversely proportional to viscosity. Under higher viscosity conditions, the blood encounters greater internal resistance as it moves through blood vessels. Consequently, for a given pressure difference, less blood will flow through the vessel, demonstrating that increased viscosity decreases flow.

This principle is crucial in understanding various physiological conditions, such as those seen in diseases where blood becomes more viscous,