Final answer:
The difference in stroke volume (SV) when the body is upright or lying down is due to the effect of gravity on the circulation of blood.
Step-by-step explanation:
When the body is upright, the stroke volume (SV), which is the amount of blood pumped out of the heart with each contraction, can double. However, when lying down, the SV only increases by 20%. This difference is due to the effect of gravity on the circulation of blood.
When standing upright, the heart has to work against gravity to pump blood upwards to the brain. This requires a greater force, which leads to a larger SV. On the other hand, when lying down, gravity helps the blood flow to the brain, reducing the workload of the heart and resulting in a smaller increase in SV.
Therefore, the change in body position affects the SV differently because of the effect of gravity on blood circulation.