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Some background to set up the problem. Blood flows into the left atrium via the mitral valve and leaves through another valve, the aortic valve. There is a moment in the cardiac cycle where these two valves are closed and the muscle that forms the left ventricle squeezes the blood inside. It’s called isvolumetric contraction because the same volume of blood is trapped by these two closed valves. Pressure in the ventricle increases.

My question is how? How does pressure is a liquid increase when it is squeezed in a confined space? My specific confusion is the assumption that liquids are incompressible so I do not see how squeezing it could change the number of collisions against the walls and therefore increase pressure. Another thought of mine would be that the elastic walls surrounding the liquid are doing work on the contained volume which is increasing the energy of the molecules and causing more collisions but wouldn’t that essentially be saying the liquids raises its temperature? I don’t think either of these things are right.

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  • $\begingroup$ Liquid "incompressibility" is a simplification. They have low compressibility, not zero. $\endgroup$ – BowlOfRed Jan 17 at 1:27
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I believe the better model to understand the effect of pressure in liquids is the potential curve between atoms/molecules.

When studying the interaction between 2 Hydrogen atoms by variational principle in QM, the bonding energy tends to zero at infinity, reaches a minimum (greater negative value) at the equilibrium position, and increases sharply for smaller distances.

$F = -dE/dx$ => strong separation force for smaller distances.

The interaction of molecules in a liquid follows a similar pattern, and any mechanical attempt to reduce molecular distance results in a great increase of separating force, what is measured macroscopically as pressure.

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