An Introduction to the Human Heart
In order to understand diseases that affect the human heart, it's important to know how the heart normally works. The human heart is a hollow, muscular organ about the size of a fist. Its job is to pump blood through a network of blood vessels. These vessels form a loop, which 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
- It picks up carbon dioxide and other waste products that your body produces so they can be disposed of.
The Chambers of the Heart
The inside of a normal human heart is divided into four chambers:
- The right atrium
- The left atrium
- The right ventricle
- The left ventricle.
In a healthy human heart, there are valves that keep blood flowing in one direction. When they open, they only let the right amount of blood through, and then they close to prevent blood from flowing backward in between beats.
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.
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, bringing oxygen and nutrients to your cells.
For the human heart to work right, each of the four chambers must contract, or squeeze, at just the right time. Your heart has an electrical system that helps coordinate this timing.
The arteries are major blood vessels connected to the human heart:
- The pulmonary artery carries blood pumped from the right side of the heart to the lungs to pick up a fresh supply of oxygen.
- The aorta is the main artery that carries oxygen-rich blood pumped from the left side of the heart out to the body.
- The coronary arteries are the other important arteries attached to the heart. They carry oxygen-rich blood to the heart muscle, which must have its own blood supply to function.
The veins are major blood vessels connected to the human heart:
- The pulmonary veins carry oxygen-rich blood from the lungs to the left side of the heart so it can be pumped out to the body
- The vena cavae are two large veins that carry oxygen-poor blood from the body back to the heart.
The Electrical Pathways in the Human Heart
There is a group of cells in the right atrium called the SA, or sinoatrial, node. It's also known as the "pacemaker." This pacemaker produces an electrical signal that spreads out over the muscles of both atria, causing the chambers to contract and push blood into the ventricles. The electrical signal travels so fast that it causes both chambers to contract uniformly.
But this signal can't continue on to the ventricles because they are insulated from the atria. The only way the signal can get there is through another group of cells in the right atrium called the AV (atrioventricular) node. Here, the electrical signal slows down and then continues on to the ventricles. This causes the ventricles to contract, too, but not until the atria do first. The slowing of the signal at the AV node gives the atria time to fill the ventricles with blood before the ventricles contract to push the blood out of the heart.
After the ventricles contract, the signal fades, and the SA node sends out another electrical impulse.
This sequence is what you are hearing when you hear the usual "lub-dup" beat of your heart. Normally, the SA node sends out a signal 60 to 100 times every minute. Notice how this is a smooth process that results in heartbeats that happen at a regular rate and rhythm. This continuous and controlled cycle is called a normal sinus rhythm.
