The human heart is a marvel of efficiency and function. It beats on average about 70 times per minute, every second of every day of your life. It never needs a rest and never pauses for a break - at least not if you are still alive.
Let's follow a blood cell as it travels toward your heart and through the four chambers.
Blood returns to the heart in a series of vessels known as veins. As the veins approach the heart, they begin to merge together, becoming larger and larger. Ultimately, only two major veins remain, called the inferior and superior vena cava. As a general rule, blood from the upper parts of your body drain in to the superior vena cava, and blood from the lower parts return to the heart in the inferior vena cava.
So our hypothetical little blood cell is cruising through one of the vena cavas, it doesn't matter which. Both vena cavas spill in to on of the top chambers of the heart, known as the right atrium. The right atrium is a thin walled chamber, just a little smaller than a golf ball. Interestingly, it is in the cells of the right atrium that the electrical signals which control the heart beat are generated.
When the right atrium contracts, it pushes the blood cells through a valve which leads to another chamber. This valve is called the tricuspid valve. It is named this due to it's having three leaflets. It is a one-way valve, normally only allowing blood to travel from the right atrium to the right ventricle. There are some conditions where the flow can reverse, most commonly something called tricuspid regurgitation, but this is not normal, and can cause a distinctive murmur if you were to listen to it flowing backwards.
Once the blood cells cross over the tricuspid valve, they enter the right ventricle. This is located roughly below the right atrium. The right ventricle has a much thicker wall and stronger muscles, as it is responsible for pushing blood further than the atrium. The right ventricle will soon push the blood cells toward the heart, where they will pick up oxygen that your body needs to live.
As the right ventricle contracts and pushes out the blood, the blood must pass through another valve. This valve is known as the pulmonary valve. When the right ventricle contracts, the pulmonary valve opens, allowing the blood to enter the pulmonary artery. Note that the tricuspid valve would be closed when the pulmonary valve is open, preventing tricuspid regurgitation as previously mentioned.
The pulmonary artery is the major blood vessels that leads to the lungs. It is also the only artery that contains deoxygenated blood. Once the blood flows through the lungs, it collects again in a group of vessel known as the pulmonary veins. These veins contain blood which is rich in oxygen and must be delivered to the tissues of the body. But first it must take one more trip through the heart.
The pulmonary veins drain in to the left atrium. The left atrium is on the upper part of the heart, next to the right atrium, but the two chambers do not normally allow their contents to mix - although there are some defects which allow this is happen.
The left atrium pushes blood through another valve, known as the mitral valve, in to the final chamber. The mitral valve connects the left atrium with the left ventricle. The left ventricle is the largest and strongest of the four chambers of the heart. It is responsible for pushing your blood around the entire body - no small task for sure.
As the left ventricle contracts, the mitral valve closes, preventing regurgitation in to the left atrium. The left ventricle pushes blood through one last valve, known as the aortic valve. Again, the aortic valve opens as the mitral valve closes. This synchronization keeps the blood flowing in the proper direction. Once in the aorta, the blood is distributed throughout your body in a series of arteries that are connected to every part of your body.
It is important to note one more aspect of the circulation in your heart. Being a large and very strong muscle, your heart demands a lot of oxygen to keep itself running properly. Interestingly, your heart cannot use the blood that is flowing through the pathway described above. Instead, there is a totally separate circulatory system built in to the muscular walls of the four chambers.
Just outside the aortic valve, in the very first inch of the aorta, are a series of very small openings. These openings lead to a series of arteries that line the walls of the heart. These arteries are called the coronary arteries. The working and flow dynamics of the coronary arteries are quite complex and are outside the scope of this article. It should be noted however, that blockage and damage to the coronary arteries are the ultimate cause of heart attacks.