In this lesson you will learn about the main blood vessels that travel between the heart and lungs. You will also discover that these arteries and veins follow a somewhat unusual pattern.
Major Blood Vessels
Pulmonary arteries carry deoxygenated blood to the lungs
Mr. Smith runs a very busy widget factory, and because his trucks have to make so many deliveries, his truck drivers are constantly heading to the gas station to refuel. Like Mr. Smith's delivery trucks, your blood supply needs constant refueling. But your body doesn't run on gasoline; instead, it must refuel with oxygen from your lungs. In this lesson, you will learn about the major blood vessels that travel to and from the lungs.
We previously learned that one of the main purposes of the heart is to pump blood through your body. What we don't always think about is the fact that the heart also pumps blood to the lungs so it can pick up oxygen. This circuit from the heart to the lungs and back to the heart is called the pulmonary circuit. You can recall this by remembering that 'pulmonary' is the anatomical term that we use when we're referring to the lungs. We also remember that the true pumps of the heart are the ventricles and that the heart has two ventricles, one on the right and one on the left. As we look at this picture of the heart, we are looking at it from an anatomical perspective, and that means that we're looking at the organ as if it's in a person who's facing us. So we see that the right ventricle is right here.
Carbon dioxide is removed and oxygen is retrieved in the capillary beds
When this right ventricle contracts, it forces the pulmonary semilunar valve to open, allowing a rush of blood into the pulmonary trunk, which is the large artery that leaves the right ventricle. This trunk is very short, and it only spans about two inches (or about five centimeters) before it branches off into the right and left pulmonary arteries, which are the arteries that carry deoxygenated blood from the heart to the lungs. If you can imagine, it's not a long trip to the lungs from the heart. So even though these arteries are leaving the heart (which is a strong pump), they are pumped under relatively low pressure. Otherwise, blood would be pumped too forcefully into your lungs and it would overload your system. Because there is not as much force needed to travel that short distance to the lungs and because blood going to the lungs comes from the right ventricle, we see that the wall of the right ventricle is not as muscular as that of the left ventricle, which has a much bigger job, because its job is to pump blood throughout your entire body.
Now let's go back and visit Mr. Smith's drivers as they show up at the gas station. What we notice is that their trucks don't miraculously fill up with gas as soon as they pull into the station. Instead, they must pull up right next to the gas pump to refuel. The same goes for the blood entering your lungs. Your blood doesn't just flow into your lungs like an ocean wave; instead, it travels through capillaries within capillary beds. The walls of these capillaries are very thin, and capillaries are very plentiful. They lie right next to the microscopic air sacs of your lungs. So it's here in these capillary beds of the lungs that carbon dioxide is removed and oxygen is picked up from these tiny air sacs. Now that your blood is freshly oxygenated by the lungs, it's ready to return to the heart to be pumped around your body.
Oxygen levels in pulmonary arteries and veins are different than others in the body
To get there, it travels through pulmonary veins, which are the large veins that carry oxygenated blood from the lungs to the heart. There are four pulmonary veins, two coming from each of your lungs. These pulmonary veins deliver the oxygenated blood to the left atrium. But wait! Does something seem unusual? We previously learned that arteries carry blood away from the heart and that veins carry blood toward the heart. This idea holds true here because we see that the pulmonary arteries are carrying blood away from the heart and the pulmonary veins are carrying blood to the heart. The interesting thing is that in most areas of the body, veins carry blood that is low in oxygen and arteries usually carry blood that's rich in oxygen. In the case of the pulmonary blood vessels, this fact is reversed. The pulmonary veins carry the richest oxygenated blood because they just came from the lungs, and the pulmonary arteries carry oxygen-poor blood because they're heading to the lungs. This is a really unusual pattern in the body, and it's something worth noting when we talk about the pulmonary circulation.
Let's review. Your blood travels to the lungs to pick up oxygen. It's propelled toward your lungs by the pumping action of the right ventricle of the heart. When the ventricle contracts, blood is pushed into the short pulmonary trunk before branching toward each lung through the right and left pulmonary arteries. The pulmonary arteries carry deoxygenated blood from the heart to the lungs. This is that unusual pattern that we talked about, because we usually see arteries carrying oxygenated blood. After picking up oxygen from the capillary beds of the lungs, freshly-oxygenated blood returns to the left atrium of the heart through the four pulmonary veins, where it gets ready for its trip around the body.
After watching this lesson, you should be able to explain the oxygenation of the blood through the pulmonary arteries and veins.