The walls of the heart consist of layers of muscle, wound several times around the atria and the ventricles in a complicated arrangement. The layers of muscle around the ventricles are thicker than those around the atria. The heart muscle pumps blood by its cyclic contractions and relaxation. The contracted phase of the heart is called systole, and the relaxed phase is called diastole. Each contraction begins in the walls of the atria, and this squeezes the blood from the atria into the ventricles; a moment later, the walls of the ventricle contract. Squeezing the blood out of the heart into the ventricles; a moment later, the walls of the ventricle contract, squeezing the blood out of the heart into the arteries. Since the walls of both atria contract jointly, and the walls of both ventricles also contract jointly, the right and the left pumps in the heart always operate in unison. The layers of muscle around the left ventricle are much thicker than those around the right ventricle. T