- #1
FranzDiCoccio
- 348
- 42
Hi,
once again I'm probably asking a question that is more about human physiology than physics (I recently asked a question that had to do with hearing).
I found a (definitely too hasty) reference to a ballistocardiograph in a high school textbook.
So I got curious about the way this apparatus works. Roughly speaking, this is an instrument to visualise heartbeat exploiting the conservation of momentum. Here you can see one in action.
A person lays still on a bed that is free to roll back and forth (head-feet direction) with little or (ideally) no friction. The person's heart pumps blood and, as a result of the action-reaction principle, the bed sways back and forth (going in the opposite direction as the blood flows).
Also, there's an old post about this apparatus, but it does not go into the details of how it works, and it is not open for discussion any more.
I'm having some trouble understanding the oscillation, because I assume that blood moves around a circuit.
Of course the blood system is not a single loop... but whatever blood going towards the head from the heart, there should be an equal amount of blood entering the heart (and in general, going the other way).
Perhaps dissipation of kinetic energy in the blood flow has a role. Possibly also the fact that the blood goes out through a big conduit (aorta) which then splits in smaller and smaller channels...
I'm assuming that blood is pumped away from the heart in one direction. If I'm right, I'm not very sure as to what causes the bed to oscillate back.
Perhaps it is the "U" shape of the aorta.
I thought of this analogy: I'm on a long platform that can slide frictionlessly on the floor. At the back end of this platform there is a wall.
If I jump towards the back wall the whole platform goes forward. Then I hit the wall and bounce back. This causes the platform to invert the direction of its motion.
I represent the blood, and the wall represents the "U" bend in the aorta, which causes the blood to invert its direction of motion? Would this be a reasonable analogy for what is happening?
There is still something I am missing... Because I think that whatever amount of blood is pumped towards the bend in the aorta, this pushes an equal amount of blood that is beyond the bend in the opposite direction...
once again I'm probably asking a question that is more about human physiology than physics (I recently asked a question that had to do with hearing).
I found a (definitely too hasty) reference to a ballistocardiograph in a high school textbook.
So I got curious about the way this apparatus works. Roughly speaking, this is an instrument to visualise heartbeat exploiting the conservation of momentum. Here you can see one in action.
A person lays still on a bed that is free to roll back and forth (head-feet direction) with little or (ideally) no friction. The person's heart pumps blood and, as a result of the action-reaction principle, the bed sways back and forth (going in the opposite direction as the blood flows).
Also, there's an old post about this apparatus, but it does not go into the details of how it works, and it is not open for discussion any more.
I'm having some trouble understanding the oscillation, because I assume that blood moves around a circuit.
Of course the blood system is not a single loop... but whatever blood going towards the head from the heart, there should be an equal amount of blood entering the heart (and in general, going the other way).
Perhaps dissipation of kinetic energy in the blood flow has a role. Possibly also the fact that the blood goes out through a big conduit (aorta) which then splits in smaller and smaller channels...
I'm assuming that blood is pumped away from the heart in one direction. If I'm right, I'm not very sure as to what causes the bed to oscillate back.
Perhaps it is the "U" shape of the aorta.
I thought of this analogy: I'm on a long platform that can slide frictionlessly on the floor. At the back end of this platform there is a wall.
If I jump towards the back wall the whole platform goes forward. Then I hit the wall and bounce back. This causes the platform to invert the direction of its motion.
I represent the blood, and the wall represents the "U" bend in the aorta, which causes the blood to invert its direction of motion? Would this be a reasonable analogy for what is happening?
There is still something I am missing... Because I think that whatever amount of blood is pumped towards the bend in the aorta, this pushes an equal amount of blood that is beyond the bend in the opposite direction...