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Introduction to the Heart

What do you think of when you see a heart: strong emotions of love and affection or the throbbing pulse in your neck?

When a biologist sees a heart, she thinks of the important biological functions that the heart performs. Although it is only a little bigger than a fist, the heart is capable of pumping enough blood to fill the gas tanks of 100 minivans each day. Each beat provides the organs within the body the oxygen and nutrients they need to function properly.

Biology of the Heart

heart in bodyThe heart is a very powerful muscle that pushes blood through the arteries and veins. The blood needs to reach each organ, including the heart, in order to feed every tissue with oxygen and nutrients!..

Each beat of your pulse is caused by the contraction of the heart muscles. Unlike the muscles in your arms, legs, and other parts of the body, the heart muscles contract involuntarily, meaning that there is no way to consciously control when the heart beats. The muscular power of the heart circulates blood through the body and to organs such as the brain, bones, and even the heart.

How does the heart beat?

This is the surface of the heart, it is made of long muscle cells, cadiomyocites, and it is covered in small blood vessel, capillaries, which feed the cells.

The heart beats due to the synchronized beating of millions of cells across the cardiac muscle.

The cardiac muscle is a special muscle, it is extra strong and extra irrigated with oxygen. Its cells, called cardiomyocites, like all muscle cells, are able to contract and extend themselves, and therefore the muscle they belong to contracts or expands. In the case of the cardiomyocites, when they work together, they make the heart beat!

But a lot of muscles that you are more familiar with, like the skeletal muscles in your arms and legs that allow you to move, are voluntary. That means that they respond directly to a conscious command from the brain, that arrives in the form of an electrical impulse delivered by a nerve. But, the average heart rate of a person of about 20 years old is between 100-170 beats a minute, can you imagine if you had to instruct your heart to beat every time?
Your heart muscle cells, on the other hand,can actually self-stimulate! Even alone, a heart cell would tell itself to beat (contract and relax) rhythmically. The heart muscle cells are connected to their neighbors by special joints (called gap junctions) through which an electrical signal can be passed very easily that allows them to synchronized their beating, so the first one to beat, will lets its neighbor know and so the signal spreads!

This a bit similar to what happens between the cells of the nervous system!

This signal travels across the heart, allowing larger sections of tissue to react in a coordinated way. When all sections of heart tissue do this with the correct timing, it makes the heart beat!

Heart Anatomy

Roll your mouse over the heart below to see the names of the structures!

There are four cavities, or open spaces, inside the heart that the blood flows in and out of. Two of these cavities are called atria. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of your chest.

The left side of the heart houses one atrium and one ventricle. The right side of the heart houses the others. A wall, called the septum, separates the right and left sides of the heart. Valves connect the various cavities to one another.

How does blood circulate through the heart?
The blood does a lot of traveling throughout the body. Each trip begins in the heart. Blood that carries oxygen is pumped towards the aorta, and from there it is distributed through the whole body, bringing oxygen and nutrients to the cells and picking up their waste, such as carbon dioxide.

Blood that has given up all its oxygen is called de-oxygenated blood , and must return to the heart. To enter the heart, deoxygenated blood flows into the right atrium and travels down to the right ventricle. From the right ventricle, blood leaves the heart and enters the lungs through the pulmonary arteries. Within the lungs, the blood is exposed to fresh oxygen that breathing brings in!

The blood releases its carbon dioxide and absorbs oxygen. Now the blood is oxygenated and returns to the heart through the pulmonary veins. In the heart, the blood flows from the left atrium down to the left ventricle. Finally, the blood is pushed out of the left ventricle, through the aorta, and out to the rest of the body. Because the left ventricle must supply enough force to push blood all the way down to the toes, it contracts more forcefully than the right ventricle and is a bigger, stronger muscle. When you feel your pulse in your neck, you are feeling the contraction of your left ventricle!


The heart is a muscle made up of millions of cardiomyocyte cells that cause the heart to beat. The beats carry oxygen and nutrients all over the body.

Heart Injury

The main cause of damage to the heart tissue is heart attacks. A heart attack is an interruption of heart beats caused by damage to heart tissue cells.

Clot in blood vessel
Inside a blood vessel, a bursting plaque is surrounded by cells attempting to heal it!

What causes a heart attack?
The main cause of heart attacks is a build up of plaque in the blood vessels that supply the heart with blood. This plaque is made up of protein, calcium and a fat called cholesterol. Poor diet is one of the main causes of plaque build-up, but a number of other things contribute to the problem. Smoking, obesity, and even some inherited traits can increase a person’s risk of having detrimental plaque build-up. Plaque build-up also occurs in other blood vessels and causes other problems. For instance, plaque build up in the blood vessels that lead to the brain could cause a stroke.

Plaque becomes especially dangerous when it builds up to the point that it bursts. The thin fibrous cap that covers the plaque ruptures, exposing the cholesterol and other contents to the blood that flows through the capillary or other blood vessel. Blood cells treat the burst plaque as if it were broken tissue and create a blood clot in an attempt to heal the plaque deposit. When the blood cells clot, they block the flow of the other cells within the vessel.

If the blood vessels that feed the heart with blood become blocked by a clot, the heart cells that they supported don’t receive oxygen and nutrients, so they die. Now, remember how all the sections of the heart muscle beat in a coordinated way? When one area of cells dies suddenly, it can affect the whole muscle and the heart can briefly stop.

The damage caused by a heart attack doesn't affect only the heart. If the heart stops even for a brief period of time, then during that period of time blood isn’t being pumped tp other tissues that need oxygen and nutrients. Without a constant supply of blood, oxygen, and nutrients, the cells within other tissues can also begin to die.

After a heart attack, the body cannot replace the dead heart tissue with new heart tissue because it only makes a very small number of new heart cells very slowly. Instead, it forms scar tissue on the heart. Scar tissue is a weaker and more disorganized version of the original tissue. Although the heart can return to its normal function, it is now weaker because there is less tissue and may beat irregularly.

Frighteningly, heart attacks are the leading cause of death around the world. In order to fight this statistic, doctors and organizations are showing people that taking simple preventative measures could reduce their risk of heart attack. Measures include exercising regularly, eating healthily, and cutting down on bad habits such as smoking and drinking excessively. Doctors are targeting adults in their forties and fifties because older people are at a greater risk of having a heart attack. 


A build up of plaque in the blood vessels that supply the heart with blood can cause a heart attack.

Modern Treatments for the Heart

How do doctors help someone who just had a heart attack?

Those who do suffer a heart attack receive immediate care to get blood flowing back into the heart. A patient must have blood clots removed so that blood flow to the heart resumes. For a few hours after a heart attack, the blood clot can be removed with the help of an injection that dissolves clots. If it has been more than a few hours since the heart attack, the clots need to be removed in surgery.
There are two types of treatment a person needs after a heart attack, doctors can try to prevent another attack and can try to replace the weakened tissue. Medication-based treatments can help reduce clots and prevent further plaque build up. In cases of severe injury, doctors can also try to compensate for the damage done to the heart with more invasive methods.

Replacing injured tissue
Someone with a seriously damaged heart may need to have it replaced. This is called a heart transplant, and this technique was developed by Dr. Christiaan Barnard, a South African Surgeon. There are about 2,000 of these transplants each year in the United States, but that is not nearly enough. Thousands more wait on long lists for donor hearts. Sadly, some of these people die.

Ventricular Assist Device

There are other methods to help heal diseased hearts. If the heart is damaged enough that it cannot beat correctly, a heart surgeon may install a ventricular assist device (VAD). The most common type of device is a left ventricular assist device. It is used most often because the left ventricle is responsible for pumping blood to the entire body and needs a lot of healthy muscle tissue to do so. The right ventricle only pumps blood to the lungs and back, which is comparatively a much shorter distance. A left ventricular assist device (LVAD) is a battery-powered pump that is attached to the left ventricle of the heart. In order to help a weakened left ventricle pump blood to the body, the LVAD takes blood from the left ventricle and pumps it to the aorta. By doing this, the LVAD provides the force to pump blood to the body and makes up for the weakened heart muscle. A patient could also have a right ventricular assist device or both. All VADs give the heart extra time to heal its damaged tissue and gives a patient more time to wait for an available transplant.

How can a doctor help prevent a future heart attack?
By reducing plaque and blood clots from the blood vessels with special medication and by suggesting lifestyle changes.

Milk and broccoli
A healthy diet and lifestyle is the number one way of preventing heart attacks!

For everybody, heart attack survivors and healthy adults alike, one of the best ways to prevent heart attacks is to lead a healthy lifestyle by exercising regularly, eating healthy foods, quitting smoking, and limiting alcoholic drinks.


A doctor can aid a heart weakened by a heart attack with a Ventricular Assist Device or replace it altogether with a heart transplant. Anyone can prevent a heart attack with proper diet, exercise, and other healthy habits!

Regenerative Medicine in the Heart

regenerative medicine

What is regenerative medicine?
Regenerative medicine can help people heal faster by enhancing the body's own natural healing process. Most treatments that are currently under investigation use stem cells that can be found in the center of bone ends, in a structure called red bone marrow. These stem cells have the potential to become new blood, brain, bone, or heart cells. To find out more about the bone marrow, visit the Biology of the Bone page.

How can regenerative medicine help patients recover from a heart attack?

Stem cells are taken from the bone marrow of the patient and exposed to growth factors in a lab that cause the stem cells to multiply and turn into heart cells. These cells are then placed on a scaffold, a mesh patch. This is a flexible patch that is placed on the heart, with a surgical procedure. Over time the cells on the patch grow into heart muscle cells and replace the damaged heart cells, allowing the heart to continue its normal rhythms. Eventually, the mesh patch is absorbed into the body.

Another treatment involves a simple injection. This injection stimulates the body to produce millions of stem cells in the bone marrow. These stem cells then circulate throughout the body in blood vessels and replace damaged heart muscle. The stem cells can find the damaged muscle and heal it on their own, by picking up signals from the environment (called the extracellular matrix – see Bone Biology page) that tell them to become heart cells.

See a movie of new heart tissue!

Watch this movie of a patch of heart muscle that was created in the lab and is able to beat on its own! Please note, this movie might disturb some viewers.


Regenerative medicine allows your body to re-grow damaged heart tissue where the body would normally produce scar tissue that weakens the heart.

Additional Resources

Agent Guy Simplant: The of the Ailing Heart
An online, interactive game that teaches students about the functions of the heart and how factors such as diet, exercise, and food choice affect a person’s chance for developing heart disease.

National Institute of Health Office of Science Education

Science Education Partnership Award

American Heart Association - Cholesterol
Revised website on cholesterol

Creative ideas by the NFL and American Heart Association to get kids active

The Go Healthy Challenge (American Heart Association)

American Heart Association's How to Help Your Kids

NICHD Kids Page

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