When blood enters the heart from systemic circulation, it is first collected in the right atrium. It then passes through the tricuspid valve into the right ventricle. To understand where the blood travels next, we must remember that this blood is deoxygenated after its passage through the body; it must pass to the lungs for oxygenation. It does so by entering the pulmonary arteries, which carry the blood away from the heart to the lungs. The pulmonary veins later return the oxygenated blood to the left side of the heart.

The major function of heart valves between the chambers of the heart is to restrict blood flow to one direction. This unidirectional flow prevents backflow and mixing of blood between chambers. This allows blood to deliver nutrients and oxygen to the peripheral tissues, return carbon dioxide to the lungs, become reoxygenated in the lungs, and maintain the circulatory system cycle without traveling backward at any point in the process. The patterns of valve opening and closing ensure that the contraction of a chamber will only expel blood in one direction, rather than allowing it to exit from both opening in the chamber.

The amount of pumped blood, also known as cardiac output, is controlled by the strength and rate of heart contractions. Since the heart valves are not constructed from muscle, they are unable to contract and propel blood. The valves are passive structures composed of connective tissue. 

Gap junctions allow the same action potential to be experienced by multiple neighboring cardiac muscle cells via electrical synapses. This simultaneous electrical stimulus allows for a more unified and powerful contraction by the heart.

Intercalated discs, a unique structure to cardiac muscle, also play a key role in synchronizing contraction.

The atria and ventricles of the heart are separated by two valves, one on each side of the heart. The left atrium and left ventricle is separated by the mitral valve, also known as the bicuspid valve. The right atrium and right ventricle is separated by the tricuspid valve, named for its three flaps that work together to form the valve.

The semilunar valves separate the aorta from the left ventricle and the pulmonary artery from the right ventricle. They are commonly called the "aortic valve" and "pulmonary valve."

The right atrium receives blood that is returning to the heart from the body. The vena cavae are responsible for collecting the blood from the rest of the body and depositing it in this heart chamber. The superior vena cava collects blood from the head and upper extremities, while the inferior vena cava collects blood from the lower trunk and lower extremities.

The aorta is the artery that exits the left ventricle to deliver blood back to the body's tissues. The carotid artery carries blood to the head; the left branch is derived from the aorta, while the right branch is derived from the brachiocephalic artery.

The left ventricle is the chamber responsible for pumping oxygen-rich blood throughout the entire system, as opposed to the right ventricle, which only pumps oxygen-poor blood to the lungs. This requires the cardiac muscle that makes up the walls of the left ventricle to be much thicker, and thus stronger, than that of the rest of the heart chambers. 

The coronary arteries supply blood to the heart. These vessels wrap around the heart muscle. Heart attacks often occur when these blood vessels become clogged, thus inhibiting blood flow to the heart, resulting in necrosis of cardiac cells. Note that the blood in the chambers of the heart is not involved in any nutrient exchange within the heart, rather, it must be pumped through capillaries where blood can supply nutrients to cells.

Oxygen-poor blood leaves the heart towards the lungs via the pulmonary arteries and returns oxygen-rich blood from the lungs to the heart via the pulmonary veins. Any blood vessel that transports blood away from the heart is an artery. Any blood vessel that transports blood towards the heart is a vein. 

Blood flows from the vena cavae into the right atrium, then into the right ventricle through the tricuspid valve. From here, blood is pumped into the pulmonary artery through the pulmonary semilunar valve. Oxygen-rich blood returns to the left atrium via the pulmonary veins. Blood flows from the right atrium into the right ventricle through the bicuspid valve. From the left ventricle, blood is pumped into the aorta, through the aortic semilunar valve.

The heart is composed of cardiac muscle. This is the only location in the entire body that contains this type of muscle. Skeletal muscles are those attached to bones, and are under voluntary control. While contraction and relaxation of smooth muscle is involuntary, it is not found in the heart. Cardiac muscle is unique in that the cells are connected via gap junctions, which allows rapid electrical conduction between them, producing wavelike contractions of the heart, essential to its function.