Why does the cardiac ventricular pressure change during isovolumetric contraction and relaxation. The volume ( both valves are closed) is constant, Does Boyle's Law not apply? P1V1= P2V2 at constant temperature.
Boyle's law doesn't apply to all fluids: only gasses, not liquids. Liquids like blood are (mostly) incompressible, so their volume does not change substantially when you add pressure.
During isovolumetric contraction, the ventricular walls are pushing in on the blood contained in the ventricle, causing the increase in pressure, but there is not yet sufficient pressure to force the blood out the pulmonary/aortic valve.
Hillegass, E. (2016). Essentials of cardiopulmonary physical therapy. Elsevier Health Sciences.
The pressure increase in Ventricle is due to the depolarization of Ventricle myocardial cells I.e. the continued action potential through the ventricle noted as QRS wave in ECG. This makes the ventricles contract and hence the pressure increases without changing the volume of the blood as all the valves during isovolumetric contraction are closed.