Blood itself is mostly water. Its characteristic properties and color are more related to the many different types of cells, salts, and proteins that reside in the fluid. In terms of oxygen delivery, the red blood cells are the most important. These cells contain oxygen (O₂) trapping molecules called hemoglobin that will uptake oxygen in the lungs and deliver it to where the circulation leads. All living tissues require oxygen, especially the brain and heart. Without this O₂ delivery, tissues weaken and die. In cases of a heart attack (or myocardial infarction) this occurs because of an occlusion, or blockage, of a coronary artery feeding the ventricles or main pumping chambers of the heart.

In this example, a fatty deposit or “plaque” has reduced the diameter that blood can flow through. This occurs in a condition called atherosclerosis, the thickening and hardening of arterial walls. The plaques are not simply accumulations of fats – current thinking involves inflamed smooth muscle tissues inside the vessel intermingling with cholesterol and mineral deposits. Atherosclerosis is the major cause of Coronary Artery Disease (CAD). In the above example, a partial blockage would likely cause angina, chest pain or pressure caused by inadequate oxygen delivery to heart tissue. However, as this plaque enhances blood clot (thrombus) formation (with a partial blockage progressing rapidly to a complete occlusion), a myocardial infarction (heart attack) becomes more probable. CAD is responsible for 75% of all deaths from heart disease in the United States! CAD is not simply a matter of diet though – its occurrence has a strong link to genetics and advancing age. While lifestyle can impact its onset greatly (e.g. smoking, lack of exercise, stress) other disease states are also responsible (for example, diabetes and hyper-cholesterolemia). It is largely treated by medical regimens, angioplasty, and bypass surgery along with changes in lifestyle. The injuries caused to the heart by a blocked coronary artery vary with the location and severity of the occlusion. Dead heart tissue will cease to contract properly and will interfere with conduction of signals that cause the rest of the heart to contract. Arrhythmias such as ventricular fibrillation may be fatal unless promptly converted with electrical paddles. This area of dead or dying heart tissue is called a myocardial infarction, often causing or referred to as a heart attack. This may cause severe chest pain or pressure that radiates to the left arm, neck and jaw. Some individuals with diabetes may experience a heart attack with no awareness of chest pain. Blockages that are severe, but not completely closed, may also hamper the heart’s ability to function as a pump. In either way, coronary artery disease can ultimately lead to a decrease in the reserve of heart output and start a decline into heart failure.