Contraction

The right and left atria contract in unison and the right and left ventricles contract in unison.

The heart muscle (myocardium) is a thick muscular layer with contractile and electrically excitable properties.

The left ventricle is the largest and thickest chamber, because it has the most work to do in overcoming the resistance of virtually the entire vasculature to force blood throughout the body.

If the heart muscle loses the ability to contract efficiently and becomes unable to pump sufficient blood to the body, then the condition is described as heart failure. Agents that improve cardiac muscle contractility are called inotropic drugs.