The heart is a hollow organ located approximately in the center of your chest and is about the size of your fist. It is made of special muscle tissue called myocardium, and it constantly pumps blood throughout your body. The heart is divided into four chambers. The two upper chambers (the right and left atria) receive and collect blood coming into the heart from the body. The two lower chambers (the right and left ventricles) pump blood out to the body. The four chambers are separated by one-way valves that keep the blood from flowing backwards. The atria and ventricles work together to circulate blood, delivering oxygen and nutrients to the entire body.
The heart contains four valves that open and close allowing blood to flow in one direction. The tricuspid and bicuspid valves separate the atria from the ventricles. When these valves are closed, the atria fill with blood. When the atria contract and the valves open, the blood moves from the atria to the ventricles below. When the ventricles contract, the semilunar valves, located at the beginning of the pulmonary arteries and the aorta, open and blood is pumped out.
The pulmonary arteries carry the blood that has just returned to your heart from your body to your lungs where the blood picks up a fresh supply of oxygen. The blood then travels back to your heart through the pulmonary veins to be pumped out to your body.
The aorta is the largest artery in your body and carries the freshly oxygenated blood from your heart to the rest of your body. The aorta travels from the left ventricle, over the top of your heart, curving down behind your heart toward your legs. At about the level of your belly button, the aorta splits into two vessels, the femoral arteries, that go into your legs. All along the length of the aorta, smaller arteries branch off and deliver blood to the various parts of your body.
The Coronary Arteries
Just like the rest of your body, the heart needs a continuous supply of oxygen. The coronary arteries carry oxygen-rich blood to your heart muscle.
As blood leaves the left ventricle, it begins its journey to your body through the aorta. At the very beginning of the aorta, near the top of your heart, two arteries branch off and go directly to your heart muscle. These are the “left” and “right” coronary arteries.
The first part of the left coronary artery is the left main artery. It is about as wide as a drinking straw and less than one-inch long. The left main artery divides into the left anterior descending artery, which travels down the side of your heart; and the left circumflex, which circles around the left side and then to the back of your heart.
The right coronary artery comes from the aorta, circles around the right side and then to the back of your heart.
The coronary arteries are on the surface of your heart and gradually divide into smaller branches. These penetrate deep into the heart muscle, carrying oxygen-rich blood to the cells.
Unlike the muscles in your arm or leg, you cannot voluntarily control the contraction of your heart muscle. Your heart contracts and relaxes on its own because of a specialized electrical system in your heart itself. This special tissue carries electrical signals along pathways through your heart every time it beats. This system ensures that all four chambers of your heart beat in the proper sequence.
In a healthy heart, the electrical signal begins in the sinoatrial node (also called the sinus node or SA node), and then travels throughout the heart. Located in the right atrium, the SA node is the natural pacemaker for the heart and typically initiates 60 to 80 heartbeats a minute in the average person at rest. If a person is active or exercising and the body needs more oxygen, the SA node increases the heart rate accordingly.
If the SA Node is creating a normal heartbeat, the heart is said to be in normal sinus rhythm. If the heart’s electrical system is interrupted, delayed or stopped, heart rhythm disturbances (arrhythmias) may result.
The right atrium receives blood coming to your heart from your body. This blood is low in oxygen and needs to be replenished before going back out to the rest of your body. When the atria contract, the blood from the right atrium goes through the now open tricuspid valve into the right ventricle. When the ventricles contract, the low-oxygenated blood in the right ventricle travels through a semilunar valve into the pulmonary arteries and to the lungs. Here, the blood picks up a fresh supply of oxygen and circulates back to the heart through the pulmonary veins. This oxygen-rich blood enters the left atrium. When the atria contract, the blood in the left atria passes through the bicuspid valve into the left ventricle. When the ventricles contract, the blood from the left ventricle moves through another semilunar valve into the aorta. From here it is delivered throughout your body, delivering its oxygen and nutrients. Eventually the blood returns to your heart and enters the right atrium to repeat the entire process.
A healthy heart beats approximately 100,000 times each day and pumps about five quarts of blood per minute, or 75 gallons per hour.