WHAT DOES THE HEART DO?
The heart is a muscle about the size of a closed fist. The heart usually beats about 60 - 80 times per minute. With each beat, your heart pushes blood throughout your body. Blood doesn't flow on its own; it needs the heart to beat (contract) and push the blood through your blood vessels.

Each time your heart beats, your heart muscle is contracting. You might not think of your heart as a muscle that gets a big workout. But it does. Think about what the heart does every day in a healthy adult.

• It pumps about 1,900 gallons (7,200 liters) of blood
• It beats 100,000 times
• And it gets only a fraction of a second to rest between each beat!

The term cardiac refers to the heart. Your heart's walls are made mostly of strong muscle called the myocardium. The myocardium is the strongest, hardest-working muscle in your body. It has to be - your heart pumps blood from your head to your toes. None of your tissues or organis could survive without the oxygen and nutrients carried by your blood.

WHERE IS THE HEART LOCATED?
Your heart is located slightly to the left of the center of your chest. Your breastbone protects your heart. For further protection, your heart is positioned inside your ribcage, and betweeen your lungs. Although not really shaped like a valentine heart, your heart is slightly pointed at the lower end. The lower end is called the apex.

WHAT DO HEART VALVES DO?
Your heart has four valves that act like doors. Each valve has two jobs. They open to allow blood to flow in or out. They close to prevent blood from flowing where it shouldn't flow. The heart valves keep blood flowing in one direction thorugh your heart.

ATRIOVENTRICULAR VALVES
Atrioventricular valves are located at the base of the atria. They control blood flow between your heart's upper and lower chambers. The valve between the righ atrium and the right ventricle is the tricuspid valve. The valve between the left atrium and the left ventricle is the mitral valve.

SEMILUNAR VALVES
These valves are at the top of the ventricles. Semilunar valves control blood flow out of your heart. Blood flows out of the right ventricle to the lungs through the pulmonary valve. Blood flows out of the left ventricle to your body through the aortic valve.

When you listen to your heartbeat through a stethoscope you hear "lubb-dubb, lubb-dubb." That sound is your heart valves closing. Although your heart has four valves, the valves open and close two at a time. In other words, the atrioventricular valves open and close at the same time. And the semilunar valves open and close at the same time. That's why you hear only two thumps ("lubb-dubb") per heartbeat, not four.

  WHAT DO HEART CHAMBERS DO?
The inside of your heart is divided into four sections, or chambers. The chambers are like separate rooms that hold the blood before pumping it out to the body. Each chamber has doors (valves) that let blood pass in and out.

THE ATRIA RECEIVE BLOOD FROM THE BODY
The two upper chambers of your heart are called the atria. (Just one of these chambers is called an atrium). When blood flows into your heart from the body or lungs, it always flows into either the right or left atrium. When blood flows into your heart from the lungs, it always flows into the left atrium. Blood flows into both atria at the same time. When the atria are full with blood, they contract and push blood down into the ventricles at the same time.

THE VENTRICLES PUMP BLOOD OUT TO THE BODY
The two lower chambers in your heart are called ventricles. The ventricles are known as the pumping chambers of your heart. When blood leaves your heart to go to your lungs, it is always pumped out from the right ventricle. When blood leaves your heart to go to the rest of your body, it is always pumped out from the left ventricle. The ventricles are larger than the atria. The ventricles are also very strong because they have to pump hard enough to push blood throughout your entire body.

THE RIGHT AND LEFT SIDES OF YOUR HEART
Sometimes the right and left sides of your heart are called your right heart and left heart. The right atrium and right ventricle are, of course, on the right side of your heart. (It's the same side as your right arm.) The left atrium and left ventricle are on the left side of your heart. However, when you look at a picture of the heart, the right heart is on the left.

A wall called the septum separates the right and left sides of your heart. The septum also separates the oxygen-rich blood from the oxygen-poor blood in your heart. Blood that hasn't yet been to the lungs (oxygen-poor blood) stays on the right side of the septum. Blood returning from the lungs (oxygen-rich blood) stays on the left side of the septum.

WHAT DOES THE CIRCULATORY SYSTEM DO?
Your circulatory system continuously delivers blood to all parts of your body. It also returns oxygen-poor blood to the lungs. The circulatory system consists of:

The heart which pumps blood into the vessels.
Blood vessels (tubes) that carry blood to your entire body and back to the heart.
The lungs which supply oxygen to the blood.

You can compare your circulatory system to a figure eight. One loop routes blood from the heart to the lungs and back to the heart. On this loop carbon dioxide is romoved from the blood, and oxygen is put into the blood. You fill your lungs with oxygen when you breathe in. Carbon dioxide is removed when you breathe out.

The second loop delivers blood - along with oxygen and nutrients - to every other part of your body. The blood vessels then take away waste products (for example, carbon dioxide). Finally, the oxygen-poor blood returns to the heart for more oxygen. The cycle then repeats continuously. This second loop is large and very complex. To give you an idea, an adult has about 60,000 miles (96,560 kilometers) of blood vessels throughout the body.

Your circulatory system does more than carry oxygen and carbon dioxide, however. It carries nutrients from your intestines to your body's tissues. It carries hormones to the appropriate parts of your body from your glands. The circulatory system also carries waste products to your liver and kidneys to be removed.

  WHAT DOES THE ELECTRICAL SYSTEM DO?
Just like your home, your heart needs electricity to work. The good news is that the heart creates its own electrical signals. The electrical system in your heart is what actually causes your heart to beat (contract and relax). The electrical system also controls the speed of your heartbeat.

Your heart's electrical system includes a network of pathways, similar to the electrical wiring in your home. The pathways carry electrical signals through your heart. The movement of the signals makes your heart contract and relax. Your heart's electrical system is also called the cardiac conduction system.

When working properly, your heart's electrical system automatically respondsto your body's changing need for oxygen. It slows down your heart rate when you sleep, for example. And it speeds up your heart rate as you climb stairs. When your heart rate speeds up, your heart pumps faster and your body gets more oxygen-rich blood.