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- TL;DR
- A seemingly simple question, but not quite. The classic answer, that in a horizontal position the heart does less work against gravity, meaning the liquid column is lower, doesn't seem plausible.
The system is closed, and as much blood rises to a certain height, an equal amount descends from the same height, so the two liquid columns create equal but opposite pressures, and their effect on the heart is 'neutralized.' Therefore, if we ignore friction in the blood vessels, it shouldn’t matter at what height the heart pumps. It is known in engineering that it does not matter to what height a pump raises the liquid, as long as the same liquid is returned to the pump from the same height, i.e. circulates in a circle and both pipes (rising and descending) are always filled with liquid. A longer path only increases friction in the system and increases the load on the pump. But now we are not talking about friction, but about gravity and the pressure caused by a column of liquid. It should be the same with the heart.
I believe the answer is more likely related to the elasticity of the arteries. Let’s not forget that blood vessels (arteries) are not rigid tubes; they are elastic and expand under pressure. Since the overall resistance in the body is very high, at the moment of a heartbeat, the arteries expand, and this expansion accommodates the larger volume of pumped blood. After a second, the elasticity of the arteries moves (pumps) the blood into the smaller vessels, and the arteries contract back. Now, let’s imagine that we are standing, and the arteries in our legs are already stretched by the weight of the blood, due to gravity. In this case, it will be more difficult for the heart to pump blood into these already stretched arteries, as they cannot stretch indefinitely.
What do you think?
I believe the answer is more likely related to the elasticity of the arteries. Let’s not forget that blood vessels (arteries) are not rigid tubes; they are elastic and expand under pressure. Since the overall resistance in the body is very high, at the moment of a heartbeat, the arteries expand, and this expansion accommodates the larger volume of pumped blood. After a second, the elasticity of the arteries moves (pumps) the blood into the smaller vessels, and the arteries contract back. Now, let’s imagine that we are standing, and the arteries in our legs are already stretched by the weight of the blood, due to gravity. In this case, it will be more difficult for the heart to pump blood into these already stretched arteries, as they cannot stretch indefinitely.
What do you think?