 Hey everybody, Dr. O. Welcome to our circulatory system, so I'm going to start by just comparing arteries and veins. So as you can see they look quite a bit different, that's because they have different structures and different functions. So real big picture. Arteries are designed to withstand the pounding, right? They have a lot of elastic tissue, thick muscular walls, they have these nice big round lumens. It's because the blood's going to be flowing through them with really high pressure especially at the aorta and the beginning of your circulatory system. So their job is to withstand this stretching without rupturing and causing serious problems. Vains are the opposite. The pressure in the blood flowing through your veins is very low, so the veins primary job is to offer very little resistance or else the blood couldn't get back to your heart. So veins are going to have this nice wide open lumen, they're going to be thinner, they're not going to be near as tough because they're not, you know, it wouldn't matter, right? If you flip the two around and a vein looked like an artery, blood couldn't flow through it back to your heart. The blood would all be trapped in your ankles and if arteries look like veins, they would just pop and rupture, so they are different and that's because their structure is different because their function is different and then the other key thing to remember is that veins have valves and then we'll come back to that. Let's see, just a couple of things to note, you'll see on the left hand side your arteries there are going to have a lot more muscle. When those muscles contract, that's vasoconstriction. That's going to increase the blood pressure in an area and then vasodilation when they relax is going to decrease the blood pressure to the area. So that's going to impact blood pressure but also blood flow. If blood needs to flow to certain areas then it's going to be inhibited in other areas so that's why these smooth muscles are so important. With veins, you'll see that the veins actually are going to have some smooth muscle right in the outside of the veins and the reason that's important is because veins are like a reservoir. About 21% of your blood is in what's called a venous reserve or in this reservoir inside your venous system. There's actually more blood in your veins than in your arteries. That's going to be primarily in your bone marrow, your skin, and your liver. Reason that's important is if you're losing blood then these veins can contract, squeeze out this excess blood over into the arterial system. It's about 21% of your blood which is why if you're losing blood you have to lose about 20% of it before you start to see signs of hypovolemic shock because you do have this reservoir. That's kind of an important little factoid there. Just a comparison, remember that by definition the only true definition of arteries is they carry blood away from the heart and veins carry blood back. The reason for that is your pulmonary arteries are carrying blood away from the heart but it's not oxygen rich blood. It's carrying it to the lungs and then vice versa. The pulmonary veins are carrying oxygen rich blood so we can't say that arteries carry oxygen rich blood and veins don't. That's not true. This is the only definition that's really acceptable. Then the rest of this here is just saying what they're designed for. The pressure in arteries is high so they're designed to withstand that pressure. The pressure in the veins is low so their job is to offer as little resistance as possible so blood can trickle back through them and get back to the heart. Different types of arteries so your largest arteries are called elastic arteries. They have extra elastic tissue because they're going to be stretched to the max when your heart beats. You can feel this right if you take your pulse in different places. You feel waves of blood traveling through these arteries so blood vessels have to stretch as that wave travels through it and then when the blood vessels recoil because they're elastic that helps propel the blood forward. Your larger vessels are going to be the most elastic. Your muscular arteries they're primarily muscular. They're the ones that play a big role in vasoconstriction and vasodilation. Then your arterials which means kind of little or baby arteries they have a really thin lumen. They're very small. The pressure and blood flow is going to greatly drop as you reach arterials and that's good because you want to slow that blood down before it gets into a capillary bed which we'll cover in probably the next video. Then your veins on the way back here we go so then you have so venules are going to be your small little baby veins and then they're going to get larger and larger and you're going to have medium and large veins. Large veins like your superior and inferior vena cava their job is to coax that blood forward into the heart because the blood pressure leaving your aorta can easily be 120 millimeters of mercury or more. The blood pressure in those vena cava is usually 2 millimeters of mercury so you see a huge difference in pressure. The other key thing to note here is the valves. I have another picture of valves here so valves like everywhere else in the body their job is to prevent the back flow of blood. So what happens is so valves are going to keep your blood from flowing backwards so it causes your venous blood to move back to your heart primarily as muscle contraction. So you see here that when the muscles contract they coax or milk the blood up towards your heart but the valves keep the blood from flowing backwards so that's why valves are so important. Your blood is so movement and muscular contract contraction is primarily what carries blood back towards your heart and the valves just make sure it doesn't go the wrong direction. If your valves quit working you could have varicose veins or all sorts of other problems. The second thing is a minor deal but there is what's called a respiratory pump so the change in pressure in your thoracic cavity as you inhale and exhale also does play a role in the movement of blood but I want you to primarily know that muscular contraction is what moves venous blood back to your heart. That's why if you're going to be standing for a long time you don't want to lock your legs. You want to make sure your muscles are still tensing and contracting or you might pass out. All right so this is just a reminder I already mentioned that there's more blood in your veins than in your arteries in your circulation and that's what creates that venous reserve that can be called upon if you're losing blood somewhere. Okay so that's a quick introduction into arteries versus veins, their structures and their functions. I hope this helps. Have a wonderful day. Be blessed.