   MCAT Review Problem #9

Use the following to answer questions 1 -- 6:

Human blood varies substantially from person to person, with state of health, etc. Typical values of the density of blood plasma and blood cells are 1025 and 1125 , respectively. The viscosity of blood at C is about 2.7 centipoise, much greater than the viscosity of water at the same temperature (about 0.7 centipoise). The flow of blood in the human circulatory system, which consists of nearly circular cross section blood vessels, can be described quite well using Poiseuille's equation: V/t is the volume of blood flow per unit time, is the pressure difference between the ends of a tube of radius R and length L. is the fluid's coefficient of viscosity, or more simply the viscosity.

1. What is the mass of 500 ml of blood plasma which has a density of 1025 ?
1. 205 g
2. 488 g
3. 512 g
4. 762 g
2. What is the apparent weight (i.e., net downward force) on a blood cell of volume 150 and density 1125 suspended in plasma of density 1025 ? ( )
1. 0.15 pN
2. 0.30 pN
3. 0.45 pN
4. 0.60 pN
3. When blood moves from a blood vessel with diameter 5.0 mm to a blood vessel of diameter 3.0 mm, the speed of flow increases by a factor of
1. 1.3
2. 1.7
3. 2.0
4. 2.8
4. According to Bernouilli's law the concentration of blood cells is greater near the center than near the wall of a blood vessel because
1. both the fluid speed and pressure are lower near the center.
2. the speed is greater but the pressure is lower near the center.
3. the speed is smaller but the pressure is greater near the center.
4. both the fluid speed and pressure are greater near the center.
5. The viscosity of a fluid can be determined by measuring the time taken for a given volume of the fluid to flow through a fixed length of capillary tube of fixed radius. The pressure difference is the fluid static pressure difference over the height of the tube. According to Poiseuille's equation, the relative flow time for a liquid of density d in this type of viscometer is directly proportional to

1. the viscosity.
2. the viscosity divided by the density.
3. the viscosity times the density.
4. the density divided by the viscosity.
6. According to Poiseuille's equation, the volume of blood flow per unit time is reduced to what percent of the initial value when the blood that was flowing in a 5.0 mm diameter blood vessel enters a 3.0 mm diameter blood vessel, the pressure difference per unit length remaining constant?

1. 13 %
2. 22 %
3. 36%
4. 50 %   