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Why is it easier on the heart in a horizontal position?
Thread starterEratosten
Start date: Today, 12:59 AM
TL;DR Summary A seemingly simple question, but not quite.
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.'
Reference: https://www.physicsforums.com/threads/why-is-it-easier-on-the-heart-in-a-horizontal-position.1066940/
When you stand up, blood flows to the lower extremities. This is partly due to the fact that blood flows to any muscle being worked and partly due to the effects of gravity.
In the supine position, the weight of the heart and abdominal organs compresses the lung, increasing the risk for hypoxemia and ventilator-induced injury.
Impaired Oxygen and Carbon Dioxide Exchange
The greatest distribution of blood flow is in the dependent (back) area of the lung.
TL;DR Summary 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.
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.
Why Does Your Heart Rate Increase When You Are Vertical Vs. Horizontal?
Body PositionWhen you stand up, blood flows to the lower extremities. This is partly due to the fact that blood flows to any muscle being worked and partly due to the effects of gravity.
- Lying down eliminates both these factors.
When you are standing up, the heart has to work harder, against gravity, to get the blood pumped through the body and back to the heart. It beats faster to overcome the effects of gravity.
- Lying down neutralizes gravity, so your heart doesn't have to work as hard anymore. Your heart rate slows down.
Why is the supine position harmful for ventilated patients?
In the supine position, the weight of the heart and abdominal organs compresses the lung, increasing the risk for hypoxemia and ventilator-induced injury.
Impaired Oxygen and Carbon Dioxide Exchange
The greatest distribution of blood flow is in the dependent (back) area of the lung.
- Unfortunately for patients with respiratory failure in the supine position, this area is not only compressed but can be filled with fluid.
- The lungs are then unable to exchange oxygen and carbon dioxide, which contributes to worsening hypoxemia and pulmonary shunt.
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