Structure of Blood Vessels – Blood vessels and blood circulation

00:00 So, again, let’s take the next step and talk about how blood vessels are structured.

00:07 I promised you before we would get some look at them through the microscope so we can actually see what the walls of these blood vessels look like.

00:17 Each of the blood vessels, that is the veins and the arteries – not the capillaries but the arteries and the veins – have three layers. Latin words are used as I’m sure you know in anatomy. So the innermost layer is called the tunica intima. That is the most intimate layer with the blood. Then there’s the middle layer, the tunica media. And then there’s the outer layer, the tunica adventitia. Tunica means coat.

00:48 Let’s talk first about the tunica intima.

00:51 It’s the innermost layer. It’s composed of endothelial very simple squamous epithelial cells and it’s very, very thin. It is actually also an area for production of a number of hormones that keep the underlying parts of the vein and the artery functioning and healthy.

01:17 We may talk a little bit about that later.

01:19 The tunica media is the muscle layer of the vessel wall, again both for the artery and the vein. They are composed of circularly-arranged, smooth-muscle cells and they are innervated by the sympathetic nervous system, part of the autonomic nervous system. When the sympathetic nervous system signals and releases some noradrenaline or norepinephrine from the endings of the nerves, this contracts these smooth-muscle cells so the vein contracts and the channel narrows. The same thing happens on the arterial side. When the sympathetic nervous system sends stimulation, the arterioles contract, resistance goes up, as we’ve talked about before, and the amount of blood flowing into the capillaries goes down. And, by the way, blood pressure behind the arterioles rises when they constrict. They are the resistance vessels as we’ve talked about. Any time that the smooth muscle contracts in the vein or the artery, they constrict the channel of either the artery or the vein. And any time they relax, they increase the channel of the artery or the vein.

02:37 The tunica adventitia is the outermost layer. And you can see here in this diagram all three layers are labelled. Level 1 – the first arrow – is the tunica intima, the thin layer of endothelium that touches on the lumen and actually touches the blood. The second and biggest layer – you can see all these wavy fibres – they are the smooth muscle that can contract or relax. And then number 3 is the adventitial layer – the tunica adventitia – that really just holds the vein or the artery in place within the tissues so that it doesn’t move around.

03:19 Now in this little diagram you can see a comparison of the various layers, comparing artery and vein and actually also capillary. Let’s look first at the artery.

03:32 You’ll notice that in each of these three vessels, the artery, the vein and the capillary, the inner layer – the endothelium, the tunica intima – is the same. It’s only one cell, sometimes two cells thick and it’s about the same in all three vessels. The difference occurs in the amount of smooth-muscle tissue. You’ll notice that there’s a lot of smooth muscle – the tunica media – in the artery, some but much less in the vein and none in the capillary. So the capillary cannot contract or dilate but both the artery and the vein can contract and dilate. But you can see there’s much more muscle in the artery so the artery is much more capable of contraction because it has much more smooth muscle compared to the vein.

04:24 And here we see an actual histological section. You can see what is depicted here is an artery right next to a vein. What the arrows show is the thickness of the smooth-muscle layer.

04:38 Notice how much thicker the smooth-muscle layer is in the artery compared to the vein.

04:45 And this is an actual human specimen and it really reflects what we just showed you in the diagram in the last slide.

04:54 And here we see another human specimen. Again you’ll see the artery with its much-thicker media with the smooth muscle, the vein with its much-smaller smooth muscle. You’ll notice that the arterial lumen or the channel is even a little bit smaller than the vein. Remember, 60% of the blood is in the veins. They are also the reservoir for the body. And by the way, you can see two sympathetic nerves up at the top of this histologic section. And also you can see fat tissue – adipose tissue – and adventitia, which are holding the nerve, the artery and the vein in place.

05:37 And once more the diagram showing you a comparison of the artery to the vein, showing you the three layers: the very, very thin tunica intima – one cell or at most two cells thick – the thicker tunica media will all the smooth muscle and you’ll notice, on the left, the artery has a much thicker muscle layer than the vein on the right. And then finally the tunica adventitia which holds these vessels in place. And you can see again the major difference here is the thickness of the smooth muscle layer.

06:14 Finally, let’s talk about which physical principles influence the blood circulation.