When the fish breaths the ventricle contracts and blood is forced out to the gills which act as lungs.

That is what makes the ventricular systole occur after atrial systole, and lets all the blood leave the atria before ventricle contracts (meaning squeeze).

The atrium contracts and then the ventricle contracts, like on the right side.

When the left ventricle (LV) contracts, it forces blood through the valve to the aorta and then to the rest of the body.

During systole, the ventricle contracts and pumps a certain portion of that blood out to your body.

In mitral valve prolapse, the valve billows back toward the atrium each time the ventricle contracts.

It then closes to keep blood from leaking back into the left atrium or lungs when the ventricle contracts (squeezes) to push blood out to the body.

When the left ventricle contracts to force blood into the aorta, the aorta expands.

When the left ventricle contracts, it sends the blood to the rest of the body.

It is composed of two leaflets, one anterior and one posterior, that close when the left ventricle contracts.