Normally, the heart stretches as it fills with blood (during diastole), then contracts to pump out the blood (during systole). The main pumping chambers in the heart are the ventricles. Heart failure due to systolic dysfunction usually develops because the heart cannot contract normally. It may fill with blood, but the heart cannot pump out as much of the blood it contains because the muscle is weaker or because a heart valve malfunctions. As a result, the amount of blood pumped out to the body and to the lungs is reduced, and the ventricles usually enlarge. Heart failure due to diastolic dysfunction develops because the heart muscle stiffens (particularly the left ventricle) and may thicken so that the heart cannot fill normally with blood. Consequently, blood backs up in the left atrium and lung (pulmonary) blood vessels and causes congestion. Nonetheless, the heart may be able to pump out a normal percentage of the blood it receives (but the total amount pumped out may be less). The heart chambers always contain some blood, but different amounts of blood may enter or leave the chambers with each heartbeat as indicated by the thickness of the arrows.