Normally, your heart pumps blood to blood vessels called coronary arteries. The blood, which is carrying oxygen, travels through the arteries. The oxygen then leaves the blood and enters your heart cells.
But coronary arteries can become blocked, usually by a buildup of cells, fat, and cholesterol, which is called "plaque." As the arteries become blocked, they can't send as much blood and oxygen to the heart muscle. This is called coronary heart disease, or just heart disease.
The heart needs blood--and the oxygen and nutrients it carries--to function properly. When you are active or under stress, the heart needs even more blood. But if the coronary arteries become blocked, there might not be enough blood flow to the heart muscle. This lack of blood and oxygen, which is called "ischemia," might cause symptoms like an irregular heart rhythm, shortness of breath, and angina--which is another name for pain, tightness, or pressure in your chest. These symptoms may only last for a few minutes, depending on how much of the heart isn't receiving enough blood.
The longer the heart goes without blood and oxygen, the worse the problems might be. Over time, the parts of the heart that don't get enough oxygen can be permanently damaged and die. This is what happens during a heart attack. Longer periods of ischemia can also cause serious irregular heart rhythms and heart failure, which can be fatal.