The heart is a hollow, muscular organ that's about the size of a fist. It's surrounded by a thin layer of tissue called the pericardium, or "heart sac." The main job of the heart is to pump blood through a network of blood vessels. These vessels form a loop that starts at the heart, goes out through your body, and then ends back at the heart again. Together, the heart, blood, and blood vessels are part of the circulatory system, which does two of the most important jobs in the body:
* it carries oxygen and nutrients to all of your cells, and
* it picks up carbon dioxide and other waste products that your body produces, so they can be disposed of.
The inside of a normal heart is divided into 4 chambers:
* the right atrium,
* left atrium,
* right ventricle, and
* left ventricle.
The circulatory loop begins with blood entering the right atrium of the heart. When the heart beats, blood flows from the right atrium into the right ventricle through a valve.
In a healthy heart, the valves keep blood flowing in a one-way direction. When they open, they only let the right amount of blood through, and then they close to keep blood from flowing backwards in between beats.
From the right ventricle, blood flows through another valve and then to the lungs, where it picks up oxygen. From the lungs, it flows back into the left atrium of the heart, and through the mitral valve into the left ventricle. From the left ventricle, blood is pumped through the aortic valve and into the aorta, where it goes out to the rest of the body, and transfers oxygen and nutrients to your cells. In exchange, the wastes from your cells, including carbon dioxide, are transferred into the blood. As veins carry the deoxygenated blood back to the right atrium of the heart, other organs filter out and dispose of the waste products. After entering the heart, the deoxygenated blood is then pumped through the lungs, where it is loaded back up with oxygen, and the cycle is repeated.
If you could see what your heart looks like when it beats, it would look like this.
Like all the other tissues in your body, the heart muscle needs blood, oxygen, and nutrients to stay healthy. But the heart DOESN'T get oxygen from the blood that flows through its chambers. Instead, it gets blood and oxygen from the coronary arteries, which wrap around the outside of the heart. The coronary arteries branch several times, so that they can reach your entire heart. Since the heart muscle never rests, it needs a CONSTANT supply of blood and oxygen. So it's important that blood can easily flow through all the branches of the coronary arteries.