Arteries vs. veins-what's the difference? | Circulatory system physiology | NCLEX-RN | Khan Academy

Let's talk a little bit about arteries and veins and the roles they play in the circulatory system. So, I want you to pause this video and first think to yourself, do you have a sense of what arteries and veins are? Well, one idea behind arteries and veins is that, well, in most of these drawings, arteries are drawn in red, and I even made the word "artery" here in red, and veins are drawn in blue. So maybe that represents how much oxygen they have.

One possible explanation is that arteries carry oxygenated blood while veins carry deoxygenated blood. So, blood that has less oxygen. Now, this is actually incorrect. It is many times the case that arteries are the ones carrying oxygenated blood and veins are carrying the deoxygenated blood. But as we will see, this is not always the case.

Since we're already talking about oxygenated blood and deoxygenated blood and the colors red and blue, it's worth addressing another misconception. Many times, it is said that deoxygenated blood looks blue. The reason why people believe that is if you look at your wrist and you're able to see some of the vessels in there, you will see some blue vessels, and those, or they at least look blue when you're looking from the outside of your skin. Those indeed are veins. This is where the misconception has come from: that veins, which when you're arm are carrying deoxygenated blood, that that deoxygenated blood is blue.

It turns out that it is not blue; it is just a deeper red. The reason why the veins look blue is because of the optics of light going through your skin and then seeing the outside of the veins and then reflecting back. That is not the color of the actual blood. So, so far I have not given you a clear definition of what arteries versus veins are.

A better definition: arteries carry blood away from the heart, and veins carry blood towards the heart. I can get a zoomed-in image of the heart right here, and that will make it a little bit clearer. You can also see—or we're about to see—why this first definition or this first distinction between arteries and veins does not always hold.

Let's just imagine some blood that is being pumped away from the heart. Right when it gets pumped away from the heart, it'll be right over here. It gets pumped through the aorta, and you can see the aorta branches. Some blood can go up towards your head, and if it didn't, you would pass out and die. A lot of the blood goes down towards the rest of your body, and that indeed is the most oxygenated blood.

So it'll flow through your body, and these arteries will keep branching and branching into smaller vessels, all the way into—they form these very small branches. It's that place, especially, where they will lose a lot of their oxygen to the fluid and the cells around them. Then the blood is less oxygenated.

Even though deoxygenated blood is not blue, it often gets depicted as blue in a lot of diagrams, so I will do the same. Then these vessels start building into your veins. These really small vessels that bridge between arteries and veins, where a lot of the gas and nutrient exchange occurs, these are called capillaries.

After going through the capillaries, the blood will then come back to the heart. Now it's coming towards the heart through the veins. It comes into the right atrium, then the right ventricle, then that gets pumped towards the lungs. This is the exception to the first incorrect definition of arteries and veins that we looked at.

This right over here is an artery, even though it's carrying less oxygenated or deoxygenated blood. It's an artery because it's carrying blood away from the heart, but in this case, it's not carrying it to the rest of the body; it is carrying it to the lungs. That is why it is called the pulmonary artery, even though it's carrying less oxygenated blood.

Then it goes to the lungs, and in the lungs, there's more gas exchange that occurs. The blood gets oxygenated, and then it comes back to the heart. It comes back to the heart in these vessels right over here, and even though these are carrying highly oxygenated blood, these are considered veins because they're carrying blood towards the heart.

So these are pulmonary veins. Then the cycle starts again. The pulmonary veins bring that oxygenated blood into the left atrium, then the left ventricle, and then that pumps it to the rest of the body, to the aorta for your systemic circulation. You have your pulmonary circulation, which circulates the blood to and from the lungs, and you have your systemic circulation, which takes the blood to and from the rest of the body.

Now that we have this main distinction between arteries and veins, what are some other interesting things that we know about it? Well, one thing to keep in mind is that since arteries are being pumped directly by the heart towards the rest of the body, they have high pressure. I'll write that in caps: HIGH PRESSURE.

If you were to have an accident of some type, which you do not want to have, and you were to accidentally cut an artery, because of that high pressure, it would actually spurt blood a lot more than if you were to cut a vein. Most of the time, where you get a cut, you're really just cutting capillaries. If you were to prick your finger, it's usually a series of capillaries that get cut, and that's why the blood would come out very, very slowly.

Now, if arteries are high pressure, veins are low pressure. LOW PRESSURE. One way to think about it is that in arteries, the blood is being pumped directly by the heart, but then once it goes through the capillaries and comes back through the veins, it's kind of sluggishly making its way back to the heart.

It's not being directly pumped, and that's why in veins, because you don't have that high pressure to bring everything back to the heart, you have these valves that make sure that, for the most part, the blood is going in one direction. I'm going to draw the blood in red in the veins just so that we don't keep going with that misconception that blood in the veins is blue somehow.

Now, related to the fact that the blood in the arteries is under higher pressure, in order to transport a fixed volume of blood in a certain amount of time, you need less volume. That’s why arteries are low volume, and, on the other hand, veins are high volume.

To appreciate the difference: the blood volume in arteries is only approximately 15 percent of the entire blood volume in your body, while the blood volume in veins is closer to approximately 65 percent. If you're wondering where the rest of the blood is: about 5 percent is in capillaries, 5 percent is in your heart, and about 10 percent is in your lungs.

So, I will leave you there. The big takeaway: arteries are the vessels that take blood away from the heart, and veins are the vessels that take blood towards the heart.