Chapter 31. Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
A student uses a variety of experimental approaches in a research project designed to analyze blood flow rates in healthy subjects. In which of the following types of blood vessel will flow be the slowest?
The correct answer is C. The velocity of blood flow depends on the pressure in a particular segment, which is highest in the arteries and next highest in the arterioles (rules out options A and B). Pressure in the venules and veins is even lower than in the capillaries, and these “capacitance vessels” serve as a reservoir for the majority of blood in the systemic circulation. However, flow is also determined by the cross-sectional area of the vessel bed in question. Although each capillary is small, in aggregate their cross-sectional area exceeds that of all other vessel types listed, in some cases, by a considerable margin (rules out options D and E).
In the experiment described in Question 1, which of the following statements will be true about the measured velocity of blood flow?
A. It is higher in the capillaries than the arterioles.
B. It is higher in the veins than in the venules.
C. It is higher in the veins than the arteries.
D. It falls to zero in the descending aorta during diastole.
E. It is reduced in a constricted area of a blood vessel.
The correct answer is B. The rate of blood flow depends on the pressure in a given vessel and inversely upon the total cross-sectional area. The pressure in the veins and venules is relatively comparable, but the latter have a larger cross-sectional area. The capillaries have the largest cross-sectional area and pressure is also lower here than in the arterioles, and so flow will be slowest through the capillaries (rules out option A). The cross-sectional area of the veins and arteries is comparable, but the arteries reflect the pressure imparted by the heart (rules out option C). In the descending aorta, the elasticity of the walls provides recoil during diastole, meaning that flow continues despite the cessation of outflow from the heart (rules out option D). Finally, when a blood vessel is constricted, the reduction in cross-sectional area will speed blood flow for a given pressure (rules out option ...