Basic Anatomy of the Heart

The heart is a 4-chambered muscle whose function is to pump blood throughout the body.

• The heart is really 2 "half hearts," the right heart and the left heart, which beat simultaneously.
  
  
• Each of these 2 sides has 2 chambers: a smaller upper chamber called the atrium (together, the 2 are called atria), and a larger lower chamber called the ventricle.
  
  
• Thus, the 4 chambers of the heart are called the right atrium, right ventricle, left atrium, and left ventricle.

This sequence also represents the direction of blood flow through the heart.

• The right atrium receives blood that has completed a tour around the body and is depleted of oxygen and other nutrients. This blood returns via 2 large veins: the superior vena cava returning blood from the head, neck, arms, and upper portions of the chest, and the inferior vena cava returning blood from the remainder of the body.
  
  
• The right atrium pumps this blood into the right ventricle, which, a fraction of a second later, pumps the blood into the blood vessels of the lungs.
  
  
• The lungs serve 2 functions: to oxygenate the blood by exposing it to the air you breathe in (which is 20% oxygen), and to eliminate the carbon dioxide that has accumulated in the blood as a result of the body's many metabolic functions.
  
  
• Having passed through the lungs, the blood enters the left atrium, which pumps it into the left ventricle.
  
  
• The left ventricle then pumps the blood back into the circulatory system of blood vessels (arteries and veins). The blood leaves the left ventricle via the aorta, the largest artery in the body. Because the left ventricle has to exert enough pressure to keep the blood moving throughout all the blood vessels of the body, it is a powerful pump. It is the pressure generated by the left ventricle that gets measured when you have your blood pressure checked.

The heart, like all tissues in the body, requires oxygen to function. Indeed, it is the only muscle in the body that never rests. Thus, the heart has reserved for itself its own blood supply.

• This blood flows to the heart muscle through a group of arteries that begins less than one-half inch from where the aorta begins. These are known as the coronary arteries. These arteries deliver oxygen to both the heart muscle and the nerves of the heart.
  
  
• When something happens so that the flow of blood through a coronary artery gets interrupted, then the part of the heart muscle supplied by that artery begins to die. This is called coronary heart disease, or coronary artery disease. If this condition is not stopped, the heart itself starts to lose its strength to pump blood, a condition known as heart failure.
  
  
• When the interruption of coronary blood flow lasts only a few minutes, the symptoms are called angina, and there is no permanent damage to the heart. When the interruption lasts longer, that part of the heart muscle dies. This is referred to as a heart attack (myocardial infarction).

Nerves of the heart: The heart's function is so important to the body that it has its own electrical system to keep it running independently of the rest of the body's nervous system.

• Even in cases of severe brain damage, the heart often beats normally.
  
  
• An extensive network of nerves runs throughout all 4 chambers of the heart. Electrical impulses course through these nerves to trigger the chambers to contract with perfectly synchronized timing much like the distributor and spark plugs of a car make sure that an engine's pistons fire in the right sequence.
  
  
• The ECG records this electrical activity and depicts it as a series of graph-like tracings, or waves. The shapes and frequencies of these tracings reveal abnormalities in the heart's anatomy or function.

Before describing the ECG itself, let's take a look at the heart's electrical system.