The Heart: Anatomy, Function, and Conditions

The heart—the primary organ of the cardiovascular system—is a muscle that contracts regularly, via a natural pacemaker that produces electrical impulses. The heartbeat drives the transport of blood throughout the body, which provides oxygen and nutrients to all the body’s cells, tissues, and organs. Although the heart is a complicated organ, in essence, it is a pump that continuously works to circulate blood throughout the body. 

Anatomy

The heart is an organ that weighs approximately 350 grams (less than one pound). It’s nearly the size of an adult’s clenched fist. 

It's located in the thorax (chest)—between the lungs—and extends downward between the second and fifth intercostal (between the ribs). It is fairly protected from many types of trauma because it is surrounded by the rib cage.

Together, the heart and the circulatory system comprise the cardiovascular system. The blood vessels carry the blood through a network of arteries, arterioles, and capillaries. Once the oxygen is absorbed by the tissues, blood is then returned to the heart via the veins.

Interestingly, if all of the blood vessels in the cardiovascular system were laid in a straight line—end-to-end—the total distance they would cover would be over 60,000 miles.

Blood Flow Through the Heart

The human heart is primarily comprised of four chambers. The two upper chambers are called the atria, the remaining two lower chambers are the ventricles.

The right and left sides of the heart are separated by a muscle called the “septum.” Both sides work together to efficiently circulate the blood. 

Each chamber has its unique job in blood circulation:

• The right atrium: Receives oxygen-poor blood from the body, then pumps it to the right ventricle.
• Right ventricle: Pumps the deoxygenated blood to the lungs to receive oxygen.
• The left atrium: Receives the oxygenated blood from the lungs (via the pulmonary vein) and pumps it to the left ventricle.
• Left ventricle: The largest and most powerful of all the heart’s chambers pumps the oxygen-rich blood through the aorta (the largest artery in the body) to be circulated through the body’s arteries, arterioles, and capillaries to deliver oxygen to all the cells, tissues and organs throughout the body. Even though the walls of the left ventricle are only around a half-inch in thickness, the left ventricle has enough muscle power to push the blood out of the heart, through the aortic valve, and onto the rest of your body.

The Heart Valves

Heart valves help control blood flow and direction.

Picture blood flowing from the atria to the ventricles. Once the ventricles are full, the mitral and tricuspid valves must close before the powerful contraction pumps out the blood. If the valves stay open, blood will be forced back up into the atria, instead of being sent out of the heart to circulate throughout the body.

Four heart valves function to regulate blood flow through the heart, these include:

• The tricuspid valve: Regulates the flow of blood between the right atrium and the right ventricle.
• The pulmonary valve: Controls the flow of blood from the right ventricle into the pulmonary arteries.
• The mitral valve: Regulates the flow of oxygenated blood from the left atrium into the left ventricle.
• The aortic valve: Allows oxygenated blood to flow from the left ventricle into the aorta to be circulated out of the body to all the tissues. 

Structure

The heart is comprised of layers, including:

• The myocardium: The layer made up of cardiac muscle cell.
• The endocardium: The inner lining.

The heart is encased in a membrane called the pericardium. The pericardium’s job is to anchor the heart and keep it from overexpanding.

Location

The heart’s location is in the middle of the thorax (chest), slightly to the left, and behind the sternum (breastbone). The space that houses the heart is called the mediastinum. Inside of the mediastinum, the heart is separated from other structures by the pericardium. 

The backside of the heart is positioned near the vertebrae, the front side sits behind the sternum (breastbone). Attached to the superior (upper) portion of the heart are the great veins (the superior vena cava and the inferior vena cava) and the great arteries (the aorta and pulmonary trunk).

Anatomical Variations

There are many genetic and congenital defects that can affect the heart. 

Examples of anomalies in adults include:

• Familial [hereditary] dilated cardiomyopathy: An inherited condition that causes the heart muscle to weaken and become thin. Familial dilated cardiomyopathy can lead to heart failure (HF), which is a condition in which the heart cannot effectively pump blood.
• Familial Hypercholesterolemia: An inherited disease that causes hypercholesterolemia (high blood cholesterol). This inherited condition can lead to coronary heart disease. 
• Familial hypertrophic cardiomyopathy: A hereditary heart disorder involving thickening of part or all of the heart muscle, which can raise the pressure inside the heart’s chamber and interfere with normal blood flow from the heart. This makes the heart work harder to circulate the blood. In severe instances, hypertrophic cardiomyopathy can cause death.

Function

The average heart rate is around 75 beats (contractions) per minute. The heart beats nearly:

• 108,000 times each day
• 37 million times per year
• 3 billion times in an average lifetime

Associated Conditions

There are many medical conditions that affect the heart.

Atherosclerosis is perhaps the most common condition that impacts the heart. Coronary artery disease (CAD) is atherosclerosis of the arteries that supply oxygen to the heart muscle. It is the leading cause of death and illnesses across the globe.

Cardiac arrhythmias are conditions in which the normal rate or rhythm of the heartbeat is disrupted.

Heart Failure (HF)

Heart failure (HF) is a commonly misunderstood term. Unlike the name suggests, the heart does not completely stop beating when a person has heart failure. Rather, it is not able to efficiently pump the blood to supply adequate oxygen and nutrients to the body’s cells, tissues, and organs.

Heart failure is not a single disorder—it's a continuum of signs and symptoms that can develop quickly, or can be chronic. 

Endocarditis

Endocarditis is an infection or inflammation of the inner surface of the heart; this type of infection involves the heart valves. The infections can be hard to clear and continually seed the bacteria in the blood, leading to a serious, uncontrolled systemic infection.

The infections can permanently damage valves and can lead to heart failure.

Pericarditis

Pericarditis is inflammation of the pericardium (the membranous sac that encases the heart). Pericarditis can be caused by an infection, but not all pericarditis is infection-related. It can cause excess fluid accumulation, called pericardial effusion. 

Pericarditis can impact a person at any age, but it is more common in men aged 16 to 65.

Tests

Tests commonly ordered to diagnose cardiac and vascular disease include:

Physical Examination

Before your physical examination, your healthcare provider will take a thorough history to assess symptoms of heart disease. 

The healthcare provider may ask about symptoms such as:

• Trouble breathing
• Wheezing
• Exercise intolerance
• Any type of chest pain, such as angina
• Pain, numbness, or weakness in the arms or other extremities
• Fluttering in the chest
• Fainting or dizziness
• Severe fatigue
• Weight loss

A physical examination can detect signs and symptoms of cardiovascular disease.

Physical examination abnormalities that can indicate heart disease include:

• Poor circulation in the extremities
• An irregular heartbeat
• Changes in heart or lung sounds
• Swelling
• High or low blood pressure

Blood Tests

Blood tests can help diagnose some of the problems that can lead to heart disease.

Examples of blood tests you may have during an evaluation for heart disease include:

• A lipid profile
• Blood glucose levels
• A complete blood count
• A BUN test

Non-Invasive Imaging Tests

Non-invasive imaging tests can include:

• Echocardiogram: Sometimes referred to as an “echo,” is an ultrasound of the heart. 
• Doppler ultrasound: A specific type of ultrasound that uses a special device to look at the arteries or the veins and the velocity of blood flowing through the imaged vessel.
• Electrocardiogram (EKG or ECG): A common test performed to evaluate the rhythm, rate, and activity of the heart.
• Stress EKG (exercise or treadmill test): A test utilized to evaluate the blood supply in the coronary arteries when the heart is working (due to physical activity).

Invasive Tests

An invasive test is one that is usually performed during a surgical procedure. This includes a cardiac catheterization, which is when a thin, hollow tube—called a catheter—is inserted into a large blood vessel that leads to the heart. A contrast dye is injected through the catheter, which serves to illuminate the area so that X-rays can be taken. Taking images of the blood vessels during cardiac catheterization is referred to as an angiography (picture of a blood vessel).