The heart has four valves, used to prevent backflow of blood during contraction of each chamber. The two atrioventricular valves separate the atria from the ventricles, and the two semilunar valves separate the ventricles from the exiting arteries. The right atrium and ventricle are separated by the tricuspid valve, while the left atrium and ventricle are separated by the bicuspid, or mitral, valve. The aortic semilunar valve separates the aorta from the left ventricle and the pulmonary semilunar valve separates the pulmonary arteries from the right ventricle.

Veins are responsible for returning blood to the heart, while arteries take blood away from the heart. The two veins listed are the pulmonary vein and the inferior vena cava. The pulmonary vein returns oxygenated blood to the heart and dumps it into the left atrium. The inferior vena cava returns deoxygenated blood to the heart and dumps it into the right atrium.

The heart has two ventricles which pump blood out of the heart and to either the lungs or the rest of the body. The right ventricle pumps blood into the pulmonary arteries which go to the lungs, and the left ventricle pumps blood into the aorta, which sends blood to the tissues of the body.

Many people have the misconception that arteries only carry oxygenated blood that has been pumped out fo the heart. The truth is that arteries are responsible for carrying all blood away from the heart, whether it be oxygenated or deoxygenated. For example, the aorta is an artery that carries oxygenated blood from the heart to the tissues, however, the pulmonary arteries carry deoxygenated blood from the heart to the lungs. Any vessel that travels away from the heart is classified as either an artery or an arteriole, regardless of the blood it contains.

Action potentials are spontaneously conducted so that the heart can pump automatically, without necessary stimulation from the central nervous system. These spontaneous action potentials are created by a group of cardiac cells called the sinoatrial node. Because it determines the heart rate, the sinoatrial node is considered the pacemaker of the heart.

After generation by the sinoatrial node, action potentials will cause the atria to contract and travel to the atrioventricular node. The atrioventricular node introduces a delay, which prevents the ventricles from contracting during atrial systole, which could push blood backward from the ventricle to the atrium. The atria relax and the signal is passed from the atrioventricular node to the bundle of His in the atrioventricular septum before spreading to the ventricles and causing ventricular systole.

Blood vessels can be constricted or dilated in order to adjust blood pressure and reroute blood to areas in need of nutrients and oxygen. This constriction is done by smooth muscle, which is primarily found wrapped around arterioles.

Arteries also have a thick lining of smooth muscle, but are generally too large in diameter to be useful in directing blood flow. Capillaries have no smooth muscle and cannot be used to direct blood. Venules may have small layers of smooth muscle, but are not nearly as effective as arterioles.