The human heart consists of 4 chambers - two upper atria and two lower ventricles separated by muscular walls. The left atrium opens to the left ventricle and right atrium to the right ventricle, and valves between the chambers ensure blood flows only in one direction from the atria to the ventricles. Arteries carry blood away from the heart while veins bring blood back to the heart, and valves in the arteries also prevent backflow into the heart.