The human ciculatory system can be thought of as a closed system of interconnecting pipes through which fluid is continuously circulated by two pumps the two pumps the right and left verticles of the heart, work as simple two-stroke force pumps. The muscles of the heart regulate the force by contracting and relaxing. the contraction (systole) lasts about 0.2s and a complete systole/diastole (contraction/relaxation) cycle lasts about 0.8s. For flood pressures and speeds in the normal range. the volume flow rate of blood through a blood vessel is directly proportional tot he pressure difference over a length of the vessel and to the fourth power of the radius of the vessel. The total mechanical energy per unit volume of blood just as it leaves the heart is `E//V=rhogh+P+rhov^(2)`
Q. Why is diastolic blood pressure muchh lower than systolic pressure?

A. because the heart exerts more force on the blood during diastole
B. Because the heart exerts no force on the blood during diastole
C. Because the radii of the blood vessels increase during diastole while the force exerted by the heart on the blood remains the same.
D. Because the radii of the blood vessels increase during diastole while the force exerted by the heart on the blood remains the same.

Options

A. because the heart exerts more force on the blood during diastole
B. Because the heart exerts no force on the blood during diastole
C. Because the radii of the blood vessels increase during diastole while the force exerted by the heart on the blood remains the same.
D. Because the radii of the blood vessels increase during diastole while the force exerted by the heart on the blood remains the same.

Answer

To solve the question of why diastolic blood pressure is much lower than systolic pressure, we can follow these steps:

### Step 1: Understand the Definitions
- **Systole**: This is the phase of the heartbeat when the heart muscle contracts and pumps blood from the chambers into the arteries. During this phase, the pressure in the arteries is at its highest, known as systolic pressure.
- **Diastole**: This is the phase when the heart muscle relaxes after contraction, allowing the chambers to fill with blood. During this phase, the pressure in the arteries is at its lowest, known as diastolic pressure.

### Step 2: Analyze the Heart's Function
- During systole, the heart exerts a significant force on the blood, which results in a higher pressure in the arteries. The contraction of the heart pushes blood into the circulatory system, increasing the pressure.
- During diastole, the heart relaxes, and there is no force being exerted on the blood. The heart chambers fill with blood, but the pressure in the arteries decreases because the heart is not actively pumping blood.

### Step 3: Relate Pressure to Force
- Pressure (P) is defined as the force (F) applied per unit area (A): \( P = \frac{F}{A} \).
- When the heart contracts (systole), the force applied increases, leading to higher pressure.
- Conversely, during diastole, the force applied by the heart is negligible, resulting in lower pressure.

### Step 4: Conclusion
- Therefore, the reason diastolic blood pressure is much lower than systolic pressure is that the heart exerts no force on the blood during diastole. This leads us to conclude that the correct answer is **B**: Because the heart exerts no force on the blood during diastole.

### Summary of Steps:
1. Define systole and diastole.
2. Analyze the heart's function during these phases.
3. Relate pressure to force.
4. Conclude that diastolic pressure is lower due to the absence of force during diastole.