 In this lecture, we are going to talk about the circulatory system and human beings and its specific parts, the heart and the vessels. Circulatory system in human beings serves all the requirements for the transport of materials within body and sometimes exchange with the environment. Circulatory system in human beings consists of three major constituents, the blood, circulatory fluid, the vessels, the vascular system in which the blood, the liquid tissue can flow and the heart, the pumping arc, blood consists of plasma, the watery liquid, fluid part of the circulatory system that actually transport the materials from one part of the body to another part of the body and from heart to the body and from body to the heart, back to the heart. Blood consists of plasma, the liquid part and cell or cell-like materials. Plasma is about 92% of water and it have many dissolved proteins, enzymes, hormones, metabolic waste products, oxygen, carbon dioxide, the gases, not many other things. Blood also have cellular contents, different kinds of blood cells, the red blood cells, the white blood cells, the platelets which are cell-like bodies. Red blood cells have the main function in transport of oxygen and carbon dioxide. They have hemoglobin molecule which help them to carry oxygen from the blood to the tissues and get the carbon dioxide back from the tissues. The white blood cells serve for immunity for different types of immune reactions. The white blood cells help the body in handling or dealing with the invaders, inflammation, parasites and related problems. The platelets help the blood in clotting. Whenever clotting is required, platelets come in action and causes the clotting of the blood. If platelets are deficient, then clotting could not occur and which may result in seeping out of the blood from the vessels. That is blood may seep out of the vessel, that is go out of the vessels and may be going to the tissues or may be going out of the body. Then comes the vessels, the vascular system. The vessels are the tubes or we can say the system of tubes or canals which allows the flow of blood from heart to the body and from body back to the heart. There are three types of vessels, arteries, veins and capillaries. Arteries are those vessels which carry blood from the heart and distribute that blood inside the all of the body organ. Veins are the vessels which takes blood back from the body and return it to the heart. Capillaries are present in between the arteries and the vein. Actually capillaries connect arteries with the veins. Capillaries are those sites. These are very small vessels. These are those sites where actually exchange of materials takes place between the tissues and the blood. So the arteries because they are going from the heart towards the body have the oxygenated blood. Blood which is saturated with oxygen. The veins have deoxygenated blood because it is coming back from the body and it have given away its oxygen content to the tissues, to the cells and now it is going to return that deoxygenated blood to the heart. So that it can send it to the heart, can send it to the lungs to oxygenate that blood because we know that lungs are the part of respiratory system which have air inside which is coming in and going back through the process of inhalation or ventilation, inhalation and exhalation which makes actually the ventilation and in the lungs blood is oxygenated again and is returned back to heart to distribute inside the body. Now we look at the vessels and the heart one by one. Heart is the pumping organ. Heart in human beings as all the mammals do have four chambers. It is a four-chambered heart. It is a highly muscular organ because it have to contract and send blood towards the lungs and also towards the whole of the body. Lungs are closer to the heart. The other body parts are too far away. Muscles of the heart have to be very strong because they have to send blood or they have to push blood with a very high pressure. For that heart have highly muscular balls and lumen. The vessels, blood vessels are of three types. Artaries which takes blood from the heart and distribute it towards the body. That is, arteries are the vessels which are from heart to the body. Then comes capillaries. Capillaries are the continuation of the arteries at tissue level. When the arteries reaches the organs they are divided into smaller arteries called arterioles. Arterioles when reaches the tissues actually from the organ to the tissues they are further divided and become very small and makes the capillaries. Capillaries exchange materials with the tissues. Oxygen, nutrients, metabolic base, everything required. Then the capillaries join towards other side to make the vanuels. Vanuels join together to make veins. Vains are those vessels which return blood back to the heart. So, arteries have an oxygenated blood, veins have a deoxygenated blood. Artaries have more pressure, veins have less pressure. Now we look at them one by one. They are different characteristics, their structure, how the vessels they look like and what are their functions. Structure and the functions, the blood vessels. Blood vessels, all of them except for capillaries consist of three layers, an external most layer, a middle layer and an inner layer. The external most layer is a layer that consists of connective tissues. The middle layer consists of smooth muscles, may be different layers of smooth muscles. The inner layer is an epithelial layer because this is towards inside. We call it an endothelium because this is present inside. Then there is a lumen for blood flow. Let us have a look on the diagram of arteries, veins and capillaries. Here in the diagram you can see on the left side there is an artery in red colour because arteries have oxygenated blood. They are represented by red colour. Then you can see that artery is divided into smaller arteries. We call them arterioles. These arterioles are further divided to make a network of very small vessels. These are the capillaries. This is the level where actually the exchange occur between the tissues and between these capillaries because these capillaries are very small and the blood cells passes one by one from the lumen of these capillaries so that the exchange of material can easily take place. Then on the other side you can see that this network is joining back to make a small vein. This is called a vanuule. Vanuules join to make a large vein. This system which you can actually see here exists in all of the tissues. Artaries, arterioles, capillaries, vanuules and veins. Coming from the heart, distributing the things and back to the heart. Now we compare the structure of arteries, veins and capillaries. Artaries are actually coming out of the heart and they are actually branches of a major vessel, aorta. Aorta is a major vessel that is coming out of heart and heart pumped the blood into aorta and aorta then have to distribute that blood into the all parts of body. When heart is pumping, pumping action is very strong because it have to send blood to all parts of body which are far away from the heart. To the legs, to the feet, to the hands, to the head, everywhere. So arteries have to face maximum of the pressure that is pressure of the blood. So arteries are thick vaults. Their walls are thick. These walls due to their thickness can withstand that pressure which is exerted on their walls by the blood. If they are thin, they could rupture or break down. The veins on the other hand have to face minimum pressure because heart pushes the blood into the aorta. Aorta when goes down to different organs, its pressure is slowly reduced. At the capillary level, there is a less pressure. When it makes venues, more less pressure because you know that pressure were exerted only by the heart and then it is only going to slow down. Then in the large veins, there is also almost negligible pressure. Veins are comparatively thin wall. They have to face less pressure. And second thing, that the lumen of vein is more large. The lumen of capillary, the lumen of artery is small. This is also to manage pressure because heart pumps blood with the pressure. But the lumen of capillary also help it out because if lumen is larger, then the pressure will be reduced. As we know that normally if we open a tap and we attach a pipe to it, normal water pipe to it, we just simply open the tap at a specific level and we attach a small pipe. That is a pipe with a small diameter. Water will go with more pressure. It will make maybe like a fountain or so. Fountainous throw. And if we attach to the same tap at the same level of water, a larger pipe, you will see that water will not go so far. So the arteries also help in maintaining the pressure of the blood because their lumen are small. Pumping action of the heart, the blood goes with more pressure and will go more far, just like as in the pipe. But the veins just have to return the blood to the heart. So their lumen are large. They are thin vault, large lumen. Second thing is this, that they have walls. We will talk about the importance of walls in the lumen of the veins later. Third is capillaries. Capillaries are actually very small vessels which have capacity of only blood cells passing one by one through them. Blood cells can pass through them only one by one. I even squeezed at some places. Their membrane is just one cell thick. It consists of only one cell on all the sides. That is it's a single cellular membrane and othelium only because capillaries are meant for exchanging materials with the surroundings. This is the reason that they are so thin walled. Their walls are thin. Exchange of material can easily take place. If they are thick walled this will be more difficult to exchange materials between them and the tissues. Let's have a look of on two diagrams to compare the structure of an artery and the vein. There is an artery on the left and there is a vein on the right. You can see that lumen of the artery is smaller. The lumen of the vein is larger. Inside the artery there are no walls. Inside the vein there are walls. Walls are actually the flaps of membrane, flaps of endothelium membrane which can close that vessel sometimes. You can see two walls on a side. Then you can see the second layer. Second layer is the middle layer. That is the inner one we were talking about was the endothelium layer or the inner layer. The second layer is the layer of muscles, middle layer. You can see that in an artery the muscle layer is thick and in the vein that muscle layer is comparatively thinner because artery need to handle more pressure changes. So the elastic layer and muscles are required in more quantities to contract, to relax, to press and to handle the pressure and so on. Let's look at the external most layer. External most layer is also thicker in the case of an artery but it is thinner in case of a vein because again arteries have to face higher pressures and they have to withstand those pressures. Due to this reason they have a thicker connective tissue layer to protect them from bursting out. In veins because there is no such problem they are more thin in comparison. Now we look at the capillaries. This diagram shows again a capillary bed. When the capillaries are making actually a network we call it a capillary bed. This diagram shows that how capillaries are making a network and the capillaries are actually the continuation of arteries entering in an organ then dividing into arterioles which is the organ level then dividing into the capillary when they reaches different tissues at the organ and then they are going back to make venues and then they are going further back to make veins and then returning to the large. Capillaries are single cell thick. There is a reason they have to actually exchange materials with the surroundings. Let us look at the next diagram. In this diagram you can see that a tissue is also shown. There are tissue cells on the base and on the top there is a capillary bed. On the on one side you can see in red color there is an artery on the other side there is a vein because at this level the cells of the tissue are so close to the capillary that the blood which is passing by blood cells and its contents which are very near to it can transport materials from the tissue towards the capillaries and from capillaries to the tissue by simple diffusion due to just pressure of the fluid the tissue fluid and due to pressure in the arteries the materials are exchanged. Oxygen is released towards the tissues tissues absorb it. Carbon dioxide which was produced by the tissues is taken up by the blood cells because they are so close they are very close they are just there is just one cell thick layer in between them. There is one important thing here you can observe that there are small groups of muscles surrounding the base of each capillary. These muscles are called sphincters. These muscles can increase or decrease or we can say control the blood flow towards the tissue. Sometimes few tissues need more blood for example if we are exercising our exercising muscles need more blood. If we have eaten food just now then our digestive system needs more blood that is for exchange of materials. What happened that these sphincters are present on each capillary bed and when more blood is required for exchange of transport at a specific in a specific organ system then the availability of blood or we can say transport of blood in other tissues is reduced. How it happens these sphincters simply closes when they are closed blood cannot flow towards this tissue and the blood is distributed towards those tissues which need them more. Maybe exercising muscles maybe the muscles of our digestive tract their their the asophagus maybe the stomach maybe the intestines if they need more blood then blood is diverted towards them. It happens normally in our body. So these sphincters are actually to control the blood flow towards different tissues and divert that blood towards some other tissues which need maybe more oxygen maybe they need to transport their carbon dioxide because they are metabolizing fast maybe they need to release their toxic waste materials and so on. So this is how through diffusion at tissue level exchange of materials takes place with the capillaries and then the blood is returned towards the venues and then to the veins and then back to the heart.