Why is it easier on the heart in a horizontal position?

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The discussion centers on the relationship between heart function, blood pressure, and the effects of gravity and friction within the circulatory system. It argues that in a closed system, the heart's pumping action is not significantly affected by height, as equal pressures are created by liquid columns. The conversation highlights the importance of arterial elasticity, noting that arteries expand to accommodate blood volume during a heartbeat and contract afterward. This elasticity is crucial, especially when considering the effects of gravity on blood flow in standing positions, where already stretched arteries make pumping more challenging. Additionally, there is contention regarding the role of friction, with some participants emphasizing its significance in both horizontal and vertical body positions. The discussion concludes with a reference to hydrostatic pressure and a note that previous contributions were deemed partially correct or speculative, leading to the thread's closure.
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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?
 
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As a physicist, I think it's wrong to ignore friction, because its significant.
 
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The friction is the same in the horizontal and vertical position of the body, because nothing about the "architecture" of the system has been changed.
 
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