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Anatomy of the circulatory system

The anatomy of the circulatory system explained in a video from the free online course Heart Health: A Beginner's Guide to Cardiovascular disease.
Now I’m going to talk about the anatomy of the circulatory system. The cardiovascular system consists of the blood vessels and the heart as pump, and this is similar to the central heating system in your homes. The blood vessels are represented by the water pipes and the heart by the pump that drives the water around your house. Most people have heard of arteries and veins, but we’ll also be learning about the capillaries and the lymphatics. Blood vessels are hollow tubes, and many of them consist of three layers, or tunicae. The innermost layer, or tunica intima, is made up of a layer of endothelium. The endothelium has several important roles. It lines the vessel, regulating the fluid that can leave the blood.
It also provides a protective barrier between the blood and the rest of the vessel wall. Finally, it also secretes factors which are involved in regulating blood pressure. The middle layer, or tunica media, is made up of smooth muscle cells. These have a role in regulating the diameter of the blood vessel as they can contract and relax. And this is able to direct the flow of blood from different parts of the body. For example, after eating, we relax the blood vessels in the gut, dilating them. Or perhaps when we exercise, we contract the blood vessels in the gut, and we relax the blood vessels in the legs so we divert the blood to the legs for exercise.
The smooth muscle layer also has an important role in regulation of blood pressure, and this responds to the factors secreted by the endothelium. The outer layer, or tunica externa, is a layer of connective tissue, and this wraps around the blood vessel, protecting it and anchoring it to the surrounding tissues. Each of these three layers is different sizes in the different types of blood vessel, and each is appropriate for the function of that vessel. When the heart contracts, blood is forced out of the heart under high pressure, and so the vessels that receive this blood have to have a structure appropriate to be able to deal with these forces. The vessels that receive the blood are known as arteries.
Arteries typically carry oxygenated blood away from the heart and to the body’s tissues. They have a thicker smooth muscle layer in order to be able to cope with the forces. And we can feel these forces if we feel our pulse. You can locate your radial pulse by feeling on the side of your wrist near your thumb, and you can feel the beating there with each contraction of the heart. When blood exits the heart, the first vessel it reaches has to cope with the highest forces. So this is the biggest artery in the body, and it’s known as the aorta. Blood travels through the aorta and branches come off.
It branches again and again, and the vessels get smaller, and these arteries become arterioles. Arteries and arterioles both have this smooth muscle layer that enables them to change their diameter, contracting and relaxing to direct blood to different parts of the body. As the blood travels deeper and deeper into the tissues, the vessels get smaller and smaller, and these become known as capillaries. The capillaries are tiny blood vessels. They don’t have a tunica media or externa and instead just have a single layer of endothelium. It’s in the capillaries that we have the transfer of oxygen from the blood into the tissues and carbon dioxide from the tissues back into the blood again.
Once the blood has given up its cargo of oxygen, it’s known as deoxygenated, and it travels back to the heart through the veins. The smallest of the veins are known as venules, and these gather together, becoming the larger veins. The veins gather together to become the largest of the veins, the vena cava. And this drains back into the heart and then travels to the lungs to be oxygenated again. Veins have a thinner tunica media, so thinner smooth muscle layer, but they have a valve system to prevent the backflow of blood. Blood in veins is running at a lower pressure than at arteries.
It’s not squeezed or pumped so much by the heart but instead by the movement of the muscles in our arms and legs and around the body. This is why walking is supposed to be good for our circulation. Every time we take a step, the muscles in our legs contract and squeeze the blood vessel, forcing the blood uphill. The valves then prevent that backflow. So with each step, it goes up, and it can’t go back. It goes up, and it can’t go back. The final type of blood vessel is known as the lymphatic system. Arteries and veins were discovered hundreds of years ago because, of course, they transport a bright red liquid.
The lymphatic vessel, however, wasn’t discovered until many, many years later because it transports a clear liquid known as lymph.
Along with giving up its cargo of oxygen in the capillaries, some of the fluid leaves the blood and enters the tissues. This fluid doesn’t drain back via the venous system but instead drains back via the lymphatic system. Lymph is a clear fluid because it no longer contains the blood cells. It drains back into the lymphatic vessels, and these have a similar structure to veins. They have a smaller or thinner tunica media, and they have the valve system in which to prevent the backflow. The lymphatic vessels converge at sites known as lymph nodes. Lymph nodes are where the lymph, the fluid, gets filtered, looking for signs of infection.
Lymph nodes are found in various sites around the body, for example, underneath the jaw and down the sides of the neck. And anyone who’s had a head cold or a sore throat is probably familiar with these. We usually think that our glands are swollen when what we actually mean is the lymph nodes are enlarged as they’re fighting the infection. We also have other sites of lymph nodes, for example, in the armpits and in the groin. After being filtered by the lymph nodes, the lymph travels back to a major vessel and that then drains into one of the veins near the heart where it can join back in with the general circulation.
Of course, the final part of the cardiovascular system is the heart, the pump, and we’ll be learning about that later on.

Join Dr Natasha Barrett as she explains the structure of the circulatory system, including the different types of blood vessels and their construction.

The network of blood vessels in the body is vast. There are a number of different types of vessels that make up your circulatory system. The wall of the vessels consist of three layers (tunica):

  • Tunica intima: The innermost layer or tunica intima is a layer of endothelial cells
  • Tunica media: The middle layer or tunica media is made of smooth muscle cells
  • Tunica externa: The outer layer or tunica externa consists of connective tissue and wraps around the vessel to protect it and to anchor it to surrounding structures

The different types of vessels are:

  • Arteries: Arteries have a thicker smooth muscle layer to enable them to cope with the force of blood
  • Capillaries: Capillaries are tiny blood vessels. They no longer have a tunica media or externa and consist of a single layer of endothelial cells
  • Veins: After the capillaries the blood travels in the smallest veins known as venules, then in the veins before finally travelling in the largest of the veins, the vena cava to be returned into the heart
  • Lymphatics: Carries hormonal messages, immune responses and bathes the cells of the body

You can download the Week 1 supplement, which contains additional images and descriptions to help you understand the topics covered in this video.

Have you heard of the house plumbing system analogy used to describe the circulatory system before? Which parts of the circulatory system were you already aware of?

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Heart Health: A Beginner's Guide to Cardiovascular Disease

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