When looking at the human heart, especially in an exterior view, it can be difficult to get a sense of the chambers of the heart, and where and what they are. All mammals have a four-chambered heart, unless defects are present, and each chamber serves an important purpose in keeping the body alive. It might be best to think of the heart's chambers as four connected boxes within a box, though this image doesn’t exactly fit from an anatomical perspective.
There are two types chambers of the heart, called the ventricles and the atria (singular is atrium). Each side of the heart has one ventricle and one atrium, and most people make a common mistake of assuming that the ventricles sit atop the atria. In fact the reverse is true. Each ventricle is located below an atrium. The common designation splits the heart into right and left sides, so that right atrium and right ventricle are located on one side, and left atrium and ventricle are located on the other. In between these is a separating wall, which keeps the mechanisms of left and right side apart, which is called the septum.
Within the four chambers of the heart it would be fair to call the atria the receivers of blood. The right atrium receives blood that has traveled through the body and been primarily stripped of oxygen. It then passes through a small valve called the tricuspid valve and reaches the right ventricle. Similarly, the left atrium receives oxygenated blood that has returned from the lungs, makes it way through the mitral valve and ends up in the left ventricle.
The ventricles could be called the main force or “pumpers” of the blood. The right ventricle contracts and sends blood back to the lungs so that it can pick up oxygen. Meanwhile the left ventricle, which is the largest and strongest of the heart's chambers contracts and sends oxygenated blood to the body where it nourishes the tissues and provides them with the needed oxygen that sustains life. Any interruption of this system can quickly cause tissue death. For instance, a blood clot in the brain could result in brain damage because even with the left ventricle’s strength, it can’t push the oxygenated blood past the clot in order to nourish the tissues beyond it.
In rare circumstances, about 8 in 1000 births, defects exist in the heart, which can interfere with the way the chambers of the heart work. Holes in the septum can create issues where oxygenated and unoxygenated blood get mixed. Some children are born with insufficient mitral or tricuspid valves, making it difficult for blood to pass smoothly from atrium to ventricle. Another congenital anomaly can occur when children are born with no or very small right or left ventricles. Fortunately, though these defects are serious, there are now many surgeries that can reroute blood circulation to make up for insufficient valves, mixed blood or small chambers of the heart. These innovations have made defects in the cardiac system much more survivable, and many children who are born with insufficient size or strength in a single chamber have excellent survival outlooks.