isovolumetric contraction
Isovolumetric contraction is the brief moment when the ventricle squeezes but nothing comes out yet — like tightening your grip on a sealed bottle, building pressure before the cap is off. The volume stays the same while the pressure shoots up.
It begins the instant the inlet valves (mitral and tricuspid) close, marking the start of systole. The ventricle is now a sealed chamber: both inlet and outlet valves are shut. The muscle contracts, but because no blood can leave or enter, the volume is fixed and pressure climbs rapidly.
The phase ends the moment ventricular pressure rises above the pressure in the aorta or pulmonary artery, which forces the outlet valves open and lets ejection begin. Its job is purely to generate the pressure needed to overcome afterload.
Isovolumetric contraction is energetically costly: the heart is doing work and consuming oxygen without yet moving blood. Conditions that raise afterload lengthen and intensify this phase, increasing the heart's oxygen demand.
On a pressure–volume loop, isovolumetric contraction appears as a straight vertical line on the right side: pressure rises sharply while volume holds constant.
The vertical right limb of the PV loop is isovolumetric contraction.
Isovolumetric means equal-volume: the chamber's blood volume does not change because all four valves around the ventricle are momentarily closed. A mirror-image phase, isovolumetric relaxation, occurs at the start of diastole.