How It Works
The heart has four chambers that pump, on average, about 2,000 gallons of blood per day. The upper chambers are called the atria, and the lower chambers, the ventricles.

The left atrium (1) receives oxygenated blood from the lungs via the pulmonary vein.

The blood then passes through the mitral valve (2) into the left ventricle (3). The left ventricle is the largest and strongest chamber, with walls about half an inch thick.

When the left ventricle is full, it contracts. Simultaneously, the mitral valve snaps closed like a trapdoor (preventing backflow) and the aortic valve opens, allowing the blood to flow into the aorta (4)—the body's largest blood vessel. It routes oxygen-rich blood through the body.

The right atrium (5) receives oxygen-depleted blood via two veins: the superior vena cava, from the upper body, and the inferior vena cava, from the lower body.

The tricuspid valve (6) opens to allow the depleted blood to flow into the right ventricle (7).

As the right ventricle contracts, the tricuspid valve closes and the pulmonary valve opens, allowing blood to flow into the pulmonary artery and then to the lungs.

Meanwhile, the heart itself is a muscle that needs oxygen-rich blood, and this is delivered through coronary arteries that branch off of the aorta.

The heart's left side is larger because it pumps blood to the entire body.

How It Fails
Big MI coming! That's what doctors, at least the ones that play them on TV, like to say when a heart-attack patient comes wheeling into the ER. It stands for myocardial infarction, which means your heart's blood supply is cut off, usually the result of coronary-artery disease. CAD is a hardening of the arteries due to atherosclerosis, a fancy way of saying there's gunk in the plumbing—that is, your arteries are clogged with plaque, a by-product of cholesterol. In its milder form, CAD results in reduced blood flow to the heart and a feeling of tightness in the chest or outright pain (called angina). If an artery becomes completely blocked—usually when a bit of plaque bursts and a blood clot forms—lack of oxygen means heart tissue will start dying within half an hour.