When a blood pressure cuff is used to force a hydraulic resistance onto an artery, the pressure drop that causes sounds (until the pressure before the narrowing has equilibrated with that after, the "diastolic pressure") is analogous to what produces heat in resistors in electronic circuits, or, in hydraulic circuits other than blood system such as "hydraulic heaters" (that usually employ very viscous fluid such as oil. ) The pressure drop across the blood pressure cuff releases acoustic energy, or in a resistor thermal energy - heat - energy is conserved, the law of conservation of energy.
