How the Heart Works

The heart is an amazing organ. It starts beating about 22 days after conception and continuously pumps oxygenated red blood cells and nutrient-rich blood and other compounds like platelets throughout your body to sustain the life of your organs. Its pumping power also pushes blood through organs like the lungs to remove waste products like CO2. This fist-sized powerhouse beats (expands and contracts) about 100,000 times per day, pumping five or six quarts of blood each minute, or about 2,000 gallons per day. In general, if the heart stops beating, in about 4-6 minutes of no blood flow, brain cells begin to die and after 10 minutes of no blood flow, the brain cells will cease to function and effectively be dead. There are few exceptions to the above.

The heart works by a regulated series of events that cause this muscular organ to contract (squeeze to push blood) and then relax (refill with blood). The normal heart has 4 chambers that undergo the squeeze and relax cycle at specific time intervals that are regulated by a normal sequence of electrical signals that arise from specialized tissue. In addition, the normal sequence of electrical signals can be sped up or slowed down depending on the needs of the individual. For example, the heart will automatically speed up electrical signals to respond to a person running and will automatically slow down when a person takes a nap. This article is designed to help individuals learn the heart anatomy and the heart circulatory system and provide some insight about heart health. It is not designed to present the many problems that can occur with the heart.

Illustrations of Blood Flow to the Heart

The heart is located under the rib cage -- 2/3 of it is to the left of your breastbone (sternum) -- and between your lungs and above the diaphragm. It is about the size of a closed fist, weighs about 10.5 ounces and is somewhat cone-shaped. It is covered by a sack termed the pericardium or pericardial sack. The normal heart anatomy consists of a four-chambered, hollow organ. It is divided into the left and right side by a muscular wall called the septum. The right and left sides of the heart are further divided into two top chambers called the atria (also termed the right and left atrium), which receive blood and then pump it into the two bottom chambers called ventricles, which pump blood to the lungs and to the body.

The coronary arteries are on the heart surface (left main, right coronary). The coronary arteries and veins comprise the heart’s own mini-circulatory system. Two major coronary arteries branch off from the aorta near the point where the aorta and the left ventricle meet:

• Right coronary artery supplies the right atrium and right ventricle with blood. It branches into the posterior descending artery, which supplies the bottom portion of the left ventricle and back of the septum with blood.
• Left main coronary artery branches into the circumflex artery and the left anterior descending artery. The circumflex artery supplies blood to the left atrium, side and back of the left ventricle, and the left anterior descending artery supplies the front and bottom of the left ventricle and the front of the septum with blood.
• These arteries and their branches supply all parts of the heart muscle with blood.

Normal heart anatomy and physiology need the atria and ventricles to work sequentially, contracting and relaxing to pump blood out of the heart and then to let the chambers refill. When blood leaves each chamber of the heart, it passes through a valve that is designed to prevent backflow of blood. There are four heart valves within the heart:

• Mitral valve between the left atrium and left ventricle
• Tricuspid valve between the right atrium and right ventricle
• Aortic valve between the left ventricle and aorta
• Pulmonic valve (also called pulmonary valve) between the right ventricle and pulmonary artery

The heart valves work the same way as one-way valves in the plumbing of your home. They prevent blood from flowing in the wrong direction. Each valve has a set of flaps, called leaflets or cusps. The mitral valve has two leaflets; the others have three. The leaflets are attached to and supported by a ring of tough, fibrous tissue called the annulus. The annulus helps to maintain the proper shape of the valve. The leaflets of the mitral and tricuspid valves are also supported by tough, fibrous strings called chordae tendineae. These are similar to the strings supporting a parachute. They extend from the valve leaflets to small muscles, called papillary muscles, which are part of the inside walls of the ventricles.

The endocardium is the membrane composed of epithelial cells that line the heart chambers and valves. It provides a slick surface so that red blood cells, platelets and other substances in blood will not stick to the heart’s inner surface. It also contains Purkinje fibers (specialized muscle cells that can transmit electrical impulses that can cause heart muscle contraction) and collagen fibers to make the endocardium elastic.

In addition, a cluster of cells that are located in the upper right atrium is termed the SA (sinoatrial node or pacemaker), which generates electrical impulses. These impulses move down cells toward the AV node (atrioventricular node), another cluster of cells located near the center of the heart between the bottom of the right atria and the top of the ventricles. The AV node pauses the electrical impulse long enough to have the atria fully contract (squeeze blood out into the ventricles); then it allows the impulse to go into cells termed the bundle of His to the ventricles that split into the right and left bundle branches in the ventricles. The electrical impulse finally reaches Perkinje fibers and then cause the ventricles to contract to push blood into the lungs and aorta. The heart rate (pulse) and blood pressure are generated by ventricular contractions; the SA node impulse rate is influenced by the body’s autonomic nervous system. At rest, a normal heart beats around 50 to 99 times a minute. Exercise, emotions, fever, and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute.

The right and left sides of the heart work together. The pattern described below is repeated over and over (heart rhythm), causing blood to flow continuously to the heart, lungs, and body to supply oxygen and nutrients to the body cells and to deliver waste products to organs that remove them from your body. In general, veins return blood carrying CO2 while arteries usually contain O2 enriched red blood cells. However, the blood flow through the heart is a little different. For example:

Right side of the heart

• Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart.
• As the atrium contracts, blood flows from your right atrium into your right ventricle through the open tricuspid valve.
• When the ventricle is full, the tricuspid valve shuts. This prevents blood from flowing backward into the atria while the ventricle contracts.
• • As the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated. Note that oxygen-poor or CO2 containing blood goes through the pulmonary artery to the lungs where CO2 is exchanged for O2.

Left side of the heart (operating at the same time as the right side of the heart)

• The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium of the heart.
• As the atrium contracts, blood flows from your left atrium into your left ventricle through the open mitral valve.
• When the ventricle is full, the mitral valve shuts. This prevents blood from flowing backward into the atrium while the ventricle contracts.
• As the ventricle contracts, oxygen-enriched blood leaves the heart through the aortic valve, into the aorta and to the arteries and eventually into veins to complete the blood circulation in your body.

• Once blood travels through the pulmonic valve, it enters your lungs. This is called the pulmonary circulation.
• From your pulmonic valve, blood travels to the pulmonary artery to tiny capillary vessels in the lungs.
• Here, oxygen travels from the tiny air sacs in the lungs, through the walls of the capillaries, into the blood.
• At the same time, carbon dioxide, a waste product of metabolism, passes from the blood into the air sacs.
• Carbon dioxide leaves the body when you exhale.
• Once the blood is purified and oxygenated, it travels back to the left atrium through the pulmonary veins.

• According to the American Heart Association, no matter what age you are, your heart can benefit from a healthy diet and adequate physical activity.
• In the reference below, there are numerous specific suggestions about how you can decrease your risk for heart disease.