The relaxation phase of the heart is called diastole, states Dr. Dawn Tamarkin of Springfield Technical Community College. Diastole describes when either the two upper chambers of the heart, the atria, or the two lower chambers, the ventricles, relax.
Atrial diastole occurs at the beginning of the contraction of the ventricles, while ventricular diastole happens when most of the ventricular blood has been pumped to the body, according to Dr. Tamarkin. Sometimes, the heart muscle does not fully relax for various reasons. The heart muscle of the ventricles must completely relax between beats so that these muscular chambers can fully open to receive blood, asserts Harvard Health Publications from Harvard Medical School. By not being able to fully fill up with blood, the ventricles cannot pump enough blood to nourish the rest of the body.