Veins – Blood vessels and blood circulation

00:01 Let’s move on then and talk about the veins.

00:03 So once the blood has been through the capillaries giving up its oxygen, taking up carbon dioxide and waste products, it’s collected into tiny venules – little tiny veins – that then coalesce into larger and larger veins. And then these large veins carry the blood back to the heart for the entire circulatory process to start again.

00:27 The veins are blood reservoirs. Interestingly enough, they hold about 60% of the body’s blood at rest. They of course drain the capillaries as I’ve said and return the blood to the heart.

00:40 These systemic veins are carrying deoxygenated blood – that is blood that’s given up its oxygen. And they carry it eventually back to the collecting chamber – the right atrium of the heart. They’re usually depicted in diagrams in blue because deoxygenated blood is darker in colour than arterial blood which is more brightly red. And then, as we talked about before, the blood passes through the tricuspid valve into the right ventricle and is pumped out to the lungs where it picks up oxygen and gives off carbon dioxide. And then, eventually of course, it returns through the pulmonary veins to the left atrium and then across the mitral valve into the left ventricle and is pumped out of the aorta and the whole circular process starts again.

01:29 Let’s says a few more things about the veins. The veins feed the blood back to the heart and they receive the blood from the very tiny capillaries. The venules then merge to form full-sized veins. The walls of the veins are relatively thin compared to the arteries and we’ll see some actual microscopic pictures later in this lecture that show you how much more muscle there is in the arterial system than in the venous system.

02:04 A very important component of the venous system are valves. Just like in the heart, the valves ensure that the blood keeps travelling in the right direction. And as we’ll talk about in a later lecture, when the valves are dysfunctional in the veins, we can develop varicose veins and even a situation where blood clots can form in the veins.

02:28 Here we see diagrams of the veins in the arm. You’ll notice that there are many veins and that they’re connected by anastomoses so that, if one of the branches is closed, the blood will still be able to leave the arm. And you can see there’s extensive veins in the hand and in the forearm and all the way up into the shoulder. And these veins coalesce into larger and larger veins and, just below the shoulder, there’s very large veins – the basilic vein – that eventually feeds into the superior vena cava.

03:06 And here we see the veins of the hand. There are many, many veins in the hand because there are many small muscles in the hands. The hand’s a very complex muscular structure and every muscle has to have a blood supply and then they have to have veins to drain the blood from this. And you can see also multiple connections, multiple anastomoses between all of the veins of the hand.

03:31 Here we see veins in the leg. There are two very large veins: a greater saphenous and a smaller saphenous. The greater saphenous runs along the medial – or inner – aspect of the leg and the small saphenous a little more on the outside of the leg. And you can see again many anastomoses, many smaller veins that connect and, eventually, feed the blood into these two saphenous veins that go up and give their blood to the femoral vein and then, eventually, the iliac vein and, eventually, the inferior vena cava and carry it back to the right side of the heart.

04:18 Veins have a very important function. And that is they have to overcome gravity, particularly the veins in the leg. The veins, as you can see, are quite distant from the heart because the blood has already gone through the arterial system and the capillaries. The blood pressure is much lower by the time the blood reaches the veins. And this lower blood pressure in the veins is considerably lower than in the arteries. The veins also have to withstand not only the pressure from the blood coming to it but also gravity increases the pressure within the veins. And gravity pulls the blood down. So that theoretically if there weren’t valves in the veins, the blood would just pool in the veins and would pool in the limbs, particularly the lower limbs. So the veins are very, very important in that they enable the blood to continue to move back to the heart.

05:23 As we’re going to see in the next slide, veins with their valves enable the body to overcome the effect of gravity. And here you see two means by which veins are able to move the blood back up against gravity and into the right atrium where it, of course, begins to be recycled through the heart.

05:49 The first component that keeps the blood moving in the right direction is the skeletal-muscle pump. So in fact as I’m standing here talking, I’m intermittently contracting the muscles in my legs and this squeezes the veins so that the blood goes in one direction. And you can see the valves help the blood to move only in one direction. If the blood tried to move back down going with gravity, the valve closes and the blood can’t go that way. So the valve keeps the blood moving in one direction and the muscles squeeze and push the blood forward in the veins back to the right atrium. It’s very, very important that the valves in the veins function well otherwise blood will pool in the legs and, eventually, would result in fluid escaping from the small vessels and the result would be swelling and oedema of the leg.

06:58 So, again, let’s take the next step and talk about how blood vessels are structured.