Which of the following statements is true? a. The longer the vessel, the lower the resistance and the greater the flow. b. As blood volume decreases, blood pressure and blood flow also decrease. c. Increased viscosity increases blood flow. d. All of the above are true.
The correct answer and explanation is:
The correct answer is:
b. As blood volume decreases, blood pressure and blood flow also decrease.
Explanation:
To understand why option b is true and the others false, let’s analyze each statement in detail.
a. The longer the vessel, the lower the resistance and the greater the flow.
This statement is false.
- According to Poiseuille’s law, resistance to blood flow in a vessel is directly proportional to the vessel’s length.
- The longer the vessel, the greater the resistance.
- Increased resistance reduces blood flow if other factors remain constant.
Therefore, a longer vessel actually increases resistance and tends to reduce flow, not increase it.
b. As blood volume decreases, blood pressure and blood flow also decrease.
This statement is true.
- Blood volume is a major determinant of blood pressure.
- A decrease in blood volume (e.g., due to bleeding, dehydration) lowers the amount of fluid circulating in vessels.
- This reduction leads to a decrease in blood pressure because less volume presses against vessel walls.
- Lower blood pressure generally results in reduced blood flow to tissues.
Hence, decreasing blood volume causes a drop in both blood pressure and blood flow.
c. Increased viscosity increases blood flow.
This statement is false.
- Blood viscosity refers to the thickness or stickiness of blood.
- Higher viscosity means the blood is thicker, which increases resistance to flow.
- According to fluid dynamics principles, increased viscosity decreases flow if pressure and vessel diameter remain the same.
Thus, increased viscosity reduces blood flow.
d. All of the above are true.
This is incorrect because statements a and c are false.
Summary:
- Vessel length is directly proportional to resistance, so longer vessels increase resistance.
- Blood volume directly affects blood pressure and flow, so decreasing blood volume reduces both.
- Increased blood viscosity raises resistance, decreasing flow.
Therefore, only statement b is true.