Understanding Heart Anatomy and Function

The heart, a vital organ responsible for pumping blood throughout the body, holds a special position in anatomical sciences. Unlike some organs that one can live without, the heart is indispensable for life. Expertise and compassion are essential when studying and understanding this complex organ. The heart's location is between the lungs in the middle of the chest, behind and slightly to the left of the breastbone (sternum). Health is more than doctor visits, and understanding your heart is a great way to promote your health.

The Heart's Structure: Chambers, Valves, and Great Vessels

The heart is a muscular organ that pumps blood around the body by circulating it through the circulatory/vascular system. It is found in the middle mediastinum, wrapped in a two-layered serous sac called the pericardium. The heart is shaped like a quadrangular pyramid, and orientated as if the pyramid has fallen onto one of its sides so that its base faces the posterior thoracic wall, and its apex is pointed toward the anterior thoracic wall. It weighs between 7 and 15 ounces (200 to 425 grams) and is a little larger than the size of your fist. By the end of a long life, a person’s heart may have beat (expanded and contracted) more than 3.5 billion times.

External Anatomy of the Heart

The heart has five surfaces: base (posterior), diaphragmatic (inferior), sternocostal (anterior), and left and right pulmonary surfaces. The left and right sides of the heart are separated by the interatrial and interventricular septa which are continuous with each other. Furthermore, the atria are separated from the ventricles by the atrioventricular septa. Blood flows from the atria into the ventricles through the atrioventricular orifices (right and left)-openings in the atrioventricular septa.

Chambers of the Heart

In a normal heart, there are four chambers:

Atria: The two upper chambers, the left and right atria, are the “collecting” chambers that receive blood returning to the heart. The right atrium receives oxygen-poor blood from the body via the superior and inferior vena cava. The left atrium receives oxygen-rich blood from the lungs via the pulmonary veins.
Ventricles: The two lower chambers, the left and right ventricles, are the “pumping” chambers. The ventricles are muscle chambers that pump blood out to the body or the lungs by way of the great arteries. The right ventricle pumps oxygen-poor blood to the lungs through the pulmonary artery, where it is oxygenated. The left ventricle, the largest and strongest chamber in your heart, pumps oxygen-rich blood out to the body through the aorta.

The atria and ventricles work together, contracting and relaxing to pump blood out of the heart. As blood leaves each chamber of the heart, it passes through a valve.

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Valves of the Heart

There are four valves inside the heart that separate the chambers. These valves are one-way “doors” in the heart. These valves open and shut between the atria and ventricles and between the ventricles and great arteries. The valves ensure that blood flows in only one direction through the heart. There are four heart valves within the heart:

Atrioventricular Valves: These valves are located between the atria and ventricles.
- The right atrioventricular valve, also known as the tricuspid valve, is between the right atrium and right ventricle.
- The left atrioventricular valve, also called the mitral valve, is between the left atrium and left ventricle. It is also called the mitral valve since it only has two cusps and resembles a miter in shape.
Semilunar Valves: These valves are located between the ventricles and the great arteries.
- The pulmonary semilunar valve is between the right ventricle and the opening of the pulmonary trunk.
- The aortic semilunar valve is between the left ventricle and the opening of the aorta.

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.

In clinical practice, the heart valves can be auscultated, usually by using a stethoscope.

Great Vessels of the Heart

The great vessels of the heart are the: aorta, pulmonary artery, pulmonary vein, and superior and inferior vena cava. They are called the great vessels because they are large in size; the diameter of the ascending aorta is 2.1 centimeters, which is like the size of an American nickel (five-cent coin), and they all carry blood to and from the heart. Major branches of the aorta include the brachiocephalic trunk, the left common carotid artery and the left subclavian artery.

Aorta: The largest artery in the body, carrying oxygen-rich blood from the left ventricle to the rest of the body.
Pulmonary Artery: Carries oxygen-poor blood from the right ventricle to the lungs.
Pulmonary Veins: Carry oxygen-rich blood from the lungs to the left atrium.
Superior and Inferior Vena Cava: Large veins that carry oxygen-poor blood from the body back to the right atrium. The superior vena cava brings blood from the head and arms to the heart, and the inferior vena cava brings blood from the abdomen and legs to the heart.

Blood Flow Through the Heart

The blood flow through the heart is quite logical. It happens with the heart cycle, which consists of the periodical contraction and relaxation of the atrial and ventricular myocardium (heart muscle tissue). Systole is the period of contraction of the ventricular walls, while the period of ventricular relaxation is known as diastole. Note that whenever the atria contract, the ventricles are relaxed and vice versa. The right atrium contracts pushing blood through the right atrioventricular valve into the right ventricle.

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The right and left sides of the heart work together. The pattern described below is repeated over and over, causing blood to flow continuously to the heart, lungs, and body.

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.
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.

Left Side of the Heart

The now-oxygen-rich blood travels through your pulmonary vein and into your left atrium.
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, blood leaves the heart through the aortic valve, into the aorta, and to the body.

This cycle is repeated with every heartbeat. The sound of your heartbeat is the sound of different heart valves closing.

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.

The Heart's Electrical System

The atria and ventricles work together, alternately contracting and relaxing to make the heart beat and pump blood. The electrical system of your heart is the power source that makes this possible. Your heartbeat is triggered by electrical impulses that travel down a special pathway through your heart.

The impulse starts in a small bundle of specialized cells called the SA node (sinoatrial node), in the right atrium. This node is known as the heart's natural pacemaker. The electrical activity spreads through the walls of the atria and causes them to contract.
A cluster of cells in the center of the heart between the atria and ventricles, the AV node (atrioventricular node) is like a gate that slows the electrical signal before it enters the ventricles. This delay gives the atria time to contract before the ventricles do.
The His-Purkinje network is a pathway of fibers that sends the impulse to the muscular walls of the ventricles, causing them to contract.

At rest, a normal heart beats around 50 to 99 times a minute in an adult. Exercise, emotions, fever, and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute.

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Coronary Arteries: Supplying the Heart Muscle

The heart must also be supplied with oxygenated blood. Heart muscles work constantly (thank goodness!), so the heart has a very high nutrient need. Like all organs, your heart is made of tissue that requires a supply of oxygen and nutrients. Although its chambers are full of blood, the heart receives no nourishment from this blood. The heart receives its own supply of blood from a network of arteries called the coronary arteries. The coronary arteries arise from the aortic sinuses at the beginning of the ascending aorta, and then circle the heart-giving off several branches. In this way, oxygenated blood reaches every part of the heart. Venous blood from the heart is collected into the cardiac veins: middle, posterior, and small.

Two major coronary arteries branch off from the aorta near the point where the aorta and the left ventricle meet:

Right Coronary Artery (RCA): Supplies the right atrium and right ventricle with blood. It usually 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.
- Circumflex Artery: Supplies blood to the left atrium, side, and back of the left ventricle.
- Left Anterior Descending Artery (LAD): Supplies blood to 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.

When the coronary arteries narrow to the point that blood flow to the heart muscle is limited (coronary artery disease), a network of tiny blood vessels in the heart that aren't usually open called collateral vessels may enlarge and become active. This allows blood to flow around the blocked artery to the heart muscle, protecting the heart tissue from injury.

Fetal Circulation

In a developing fetus, several special "fetal" connections exist. For some babies with heart conditions, these fetal connections become important even after baby is born. The patent ductus arteriosus (PDA) is a special fetal blood vessel that sends blood from the mother’s placenta directly to the baby’s body, past the fetal lungs. In babies with a normal heart, the PDA begins to close within a few days after birth because it is no longer needed. The patent foramen ovale (PFO) is a normal fetal opening between the two atrial chambers. Typically, the PFO closes shortly after birth because it is no longer needed.

Common Heart Disorders

There are many disorders that can affect the heart and its adjacent structures.

Angina Pectoris: A pain in the chest that comes and goes and is due to the lack of oxygenation of the myocardium. Stable angina is the most common form and occurs because of the severe narrowing of the coronary arteries.
Infective Endocarditis: A bacterial or fungal infection of the heart and can include but is not limited to the cardiac valves.
Congenital Heart Disease: The variations of congenital heart disease usually have an unknown etiology. Certain syndromes that occur due to chromosomal abnormalities usually have secondary heart defects which can include atrial septal defects, ventricular septal defects, a patent ductus arteriosus and even in some cases transposition of the great vessels.
Cardiomyopathy: is when the heart muscle becomes enlarged, thick, or rigid. in one or more of the layers of tissue in the heart, including the pericardium, myocardium, or endocardium. This can lead to serious complications, including heart failure, cardiogenic shock, or irregular heart rhythm.
Heart Failure: Usually the left side of the heart fails. When the left ventricle of the heart can no longer pump enough blood around the body, this leads to buildup in the blood vessels that take blood away from the lungs. This is usually caused by heart disease, heart attack, or long-term high blood pressure. Symptoms include shortness of breath or trouble breathing. The right side can also fail (it can't pump enough blood to the lungs), causing buildup of blood in the veins. Often, failure of the right ventricle is caused by failure of the left ventricle first.
Heart Defect: heart defect can happen at any point during development of an unborn baby, or embryo, inside the pregnant mother.

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