The Heart and How It Works
The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system. Each day the average heart "beats" (expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times.

The circulatory system is the network of elastic tubes that carries blood throughout the body. It includes the heart, lungs, arteries, arterioles (small arteries), and capillaries (very tiny blood vessels). These blood vessels carry oxygen- and nutrient-rich blood to all parts of the body. The circulatory system also includes venules (small veins) and veins. These are the blood vessels that carry oxygen- and nutrient-depleted blood back to the heart and lungs. If all these vessels were laid end-to-end, they'd extend about 60,000 miles. That's enough to encircle the earth more than twice.

The circulating blood brings oxygen and nutrients to all the body's organs and tissues, including the heart itself. It also picks up waste products from the body's cells. These waste products are removed as they're filtered through the kidneys, liver and lungs.








What is the heart's structure?

The heart has four chambers through which blood is pumped. The upper two are the right and left atria. The lower two are the right and left ventricles. Four valves open and close to let blood flow in only one direction when the heart beats:

1. The tricuspid valve is between the right atrium and right ventricle.
2. The pulmonary or pulmonic valve is between the right ventricle and the pulmonary artery.
3. The mitral valve is between the left atrium and left ventricle.
4. The aortic valve is 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 let blood flow in just one direction. Blood flow occurs only when there's a difference in pressure across the valves that causes them to open.








How does the heart pump blood?
The heart pumps blood by a highly organized sequence of contractions of its four chambers. The heart works as follows:

The right atrium receives blood from the veins. This blood carries little oxygen and lots of carbon dioxide. That’s because it’s returning from the body’s tissues, where much of the oxygen was removed and the carbon dioxide added. Venous blood is darker than arterial blood because of the difference in dissolved gases. While the heart is relaxed, venous blood flows through the open tricuspid valve to fill the right ventricle. An electrical signal starts the heartbeat by causing the atria to con-tract. This contraction "tops off" the filling of the ventricle. Shortly after the atrium contracts, the right ventricle contracts. As this occurs, the tricuspid valve closes and the partially deoxygenated blood is pumped through the pulmonary valve, into the pulmonary artery and on to the lungs. In the lungs the blood gives up its carbon dioxide and gets oxygen before returning to the left atrium. This newly oxygenated blood is bright red.

At the same time the right atrium contracts, the left atrium contracts, topping off the flow of oxygenated blood through the mitral valve and into the left ventricle. Then a split second later the left ventricle contracts, pumping the blood through the aortic valve, into the aorta and on to the body’s tissues.

A heart’s four chambers must beat in an organized way. This is governed by the electrical impulse. A chamber of the heart contracts when an electrical impulse moves across it. Such a signal starts in a small bundle of highly specialized cells in the right atrium - the sinoatrial node (SA node), also called the sinus node. A discharge from this natural "pacemaker" causes the heart to beat. This pacemaker generates electrical impulses at a given rate, but emotional reactions and hormonal factors can affect its rate of discharge. This lets the heart rate respond to varying demands.

The electrical impulses generated by the SA node move throughout the right and left atrium, causing the muscle cells to contract. Shortly after both atria have contracted, the electrical signal travels down specialized fibers throughout the ventricles. The signal’s path causes the ventricles to contract together in a wringing motion, squeezing blood from them. The route of this electrical impulse is specific and produces the coordinated, sequential contraction of the heart’s four chambers that’s necessary for the heart to work properly.