Sample Problems about Beating Hearts

Note that the kinetic portion of the work done by your heart is the total work minus the increase in potential energy of the blood due to the rise in pressure from diastolic to systolic:

W _K = W _total - U _{blood pressure}

= V (P _systolic - P _residual) - V (P _systolic - P _diastolic)

= V (P _diastolic - P _residual).

We do this here because in every problem, the systolic pressure drops out as above. The fact that the energy available to move blood and distend arteries does not depend on the systolic pressure tells you why your doctor is much more concerned about your diastolic pressure than your systolic.

Note that you should NOT use 70% of the volume in these problems, but rather the whole volume: these are strict conservation of energy problems. You DO need to consider the elastic potential energy stored in the aortal walls.

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