Heart's Work Output: Viscosity & Vein Length

[michaelwoodco]

I saw an example where the work of a heart was found to be 2 watts based on the height of a person and the liters of blood that it pumped throughout the day. Since we have closed circulation systems (unless we have a cut, annerism, etc. ) I thought height would not matter, and in order to achieve the best accuracy we would consider instead viscosity and vein length. Or perhaps take the pressure difference before & after the heart at the current flow rate and you could use that to calculate the resistance to flow. In theory with no viscosity wouldn't our blood just flow forever because although on one side of the loop the blood has to go up against gravity on the other side gravity is pulling it down?

 

[Curl]

Yeah I don't see why they would take into account height. Gravity doesn't affect the circulation work done since it is a conservative force.

 

[michaelwoodco]

How exactly would I calculate the energy needed for the heart to pump blood then, take into account viscosity and vein length/diameter? I'm guessing I could make a rough calculation by using average vein diameter?

 

[Curl]

Eh, its too difficult since each time the flow changes diameters there is a "minor loss" which is based on velocity squared, geometry, etc.
And wall friction factor isn't at all linear with diameter or velocity, so taking an average will give a poor result. Curvature in veins also contributes to "minor losses" which are impossible to determine.

Doing a "rough" calculation will probably end up being off by a factor of 100.

 

[PJC01]

Thank you for sharing your thoughts on the work output of the heart and the factors that may affect it. I would like to provide some insights on this topic.

Firstly, it is important to note that the work output of the heart is not solely determined by the height of a person and the liters of blood pumped throughout the day. Other factors such as heart rate, blood pressure, and overall health can also play a role.

Secondly, while height may not directly impact the work output of the heart, it can indirectly affect it through factors such as body mass and muscle mass, which can impact the overall workload of the heart.

In terms of viscosity and vein length, these factors can indeed play a role in the work output of the heart. Viscosity, or the thickness of the blood, can affect the resistance to flow and therefore impact the amount of work the heart needs to do to pump blood throughout the body. Additionally, vein length can also play a role in the overall workload of the heart, as longer veins may require more work to pump blood through.

In regards to your suggestion of using pressure difference and flow rate to calculate resistance to flow, this is a valid approach and is commonly used in cardiovascular research. However, it is important to note that this is just one aspect of the complex system of the heart and other factors such as the size and strength of the heart muscle also play a role in its work output.

Lastly, your theoretical question about the impact of viscosity on blood flow is an interesting one. While it is true that in a completely frictionless system, blood may flow indefinitely, in reality, there are other factors at play such as the pumping action of the heart and the presence of valves in the veins that prevent blood from flowing backwards.

In conclusion, the work output of the heart is a complex and multifactorial process that involves various physiological and physical factors. While considering viscosity and vein length can provide valuable insights, it is important to also consider other factors in order to fully understand the work output of the heart.