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.