The human heart is a remarkable organ. Slightly larger than a human fist the heart must continuously beat over person's lifetime. Beating 80,000 to 100,000 times and pumping approximately 2,000 gallons a day the heart will have beat 2-3 billion times and pumped 50-65 million gallons of blood over a 70-90 year lifespan. The human heart is made of specialized muscle capable of sustaining continuous beating. This muscle is different than skeletal muscle that powers the arms and legs.

The human heart is made up of four chambers, valves, and joining blood vessels:

• Chambers: right atrium (RA), right ventricle (RV), left atrium (LA), left Ventricle (LV)
• Vessels: inferior (IVC) & superior vena cava (SVC), aorta, pulmonary artery (PA), pulmonary veins (PV)
• Valves: tricuspid (Tv), aortic (Ao), mitral (MV), pulmonic (Po)

For the heart to function properly, the four chambers must beat in an organized manner. The four heart chambers fill with and pump blood. The right and left atria pump blood to the right and left ventricles respectively.

The four heart valves are: 1. the tricuspid valve, located between the right atrium and right ventricle; 2. the pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery; 3. the mitral valve, between the left atrium and left ventricle; and 4. the aortic valve, between the left ventricle and the aorta. Each valve has a set of flaps (also called leaflets or cusps). The mitral valve has two flaps; the others have three. Under normal conditions, the valves permit blood to flow in only one direction. Blood flow occurs only when there's a difference in pressure across the valves that causes them to open.

Blood returning to the heart from the body (venous blood that has already had oxygen taken from it) enters the right atrium. Blood flows and is pumped from the right atrium across the open tricuspid valve into the right ventricle.

As the right ventricle starts to contract the tricuspid valve closes (blood can only be pumped forward) the pulmonic valve opens and blood pumped into the pulmonary arteries. These arteries carry blood to the lungs to be oxygenated.

Oxygenated blood is returned to the heart by pulmonary veins. This oxygenated blood enters the left atrium. Blood from the left atrium flows across an open mitral valve to enter the left ventricle. As the left ventricle starts to contract the mitral valve closes and the aortic valve opens as blood is pumped across it into the aorta.The aorta and arteries that branch from it carry blood to the entire body. The left ventricle is the largest and most forcefully contracting chamber of the heart. It must pump oxygen rich blood to the whole body.

The heartbeat cycle consists of two components: diastole and systole

• Diastole occurs when the heart is relaxed and not contracting. During diastole blood fills each of the atria and begins filling the ventricles.
• Systole occurs when electrical impulse traveling down specialized conducting fibers trigger the heart to contract. The left and right atria contract at nearly the same time pumping remaining blood into the left and right ventricle. Systole continues as the right and left ventricle contract, pumping blood to the lungs and body, several tenths of a second after the right and left atria have contracted.

Systole and diastole continuously alternate as long as the heart continues to beat.