 blood, a life-giving liquid. It circulates throughout your body. It goes to every corner of the body to do what? To bring in food and oxygen and take away the waste products. So, what does blood flow through? Through blood vessels, right? These long tubes through which the blood flows. Now let me ask you a question. Does our blood ever leave the blood vessels? No, right? That's ridiculous. If my blood left my blood vessels, I would die. So that means we have a system called the closed circulatory system. The blood vessels never open up. They're always closed. The blood always stays within. That is why it's called a closed circulatory system. Let's take a closer look at this closed circulatory system. So what I'm going to show you is a schematic of our circulatory system. It's not going to show you all the nitty-gritties of it, but just to give you an idea about how a closed circulatory system works. So first of all, we have the heart. You know, it's a muscular organ which pumps blood. Pumps blood, that means it releases blood under pressure so that the blood can go to all the parts of the body. So we have the heart and that pumps blood to some blood vessels. So the heart pumps blood into the blood vessels and the blood flows through these blood vessels and then the blood vessels subdivide to form very fine small blood vessels called capillaries. So these are blood capillaries. So it is these capillaries since they are so small and fine that they can reach all the parts of the body, namely the tissues. So here we have the tissues. So the capillaries supply blood to all the tissues of the body. Now we know that the blood does not leave the blood vessels, even the capillaries. So what do we mean by supplying blood? So the capillaries go very close to the tissues and exchanges some material with the tissues. So the tissues themselves, they are surrounded by this fluid called the interstitial fluid. So what happens is between capillaries and this interstitial fluid that bathes the tissues, there is this exchange of materials. So the capillaries provide oxygen and nutrients to the interstitial fluid which surrounds the tissues. And then the tissues give back to the capillaries the waste products, wastes and also carbon dioxide. So this is how blood supplies material to all the tissues of the body through the diffusion that is taking place between the interstitial fluid and the blood in the capillaries. Now once this exchange has happened, where does the blood go? It goes back to the heart through some other blood vessels. So the capillaries, they join again to form these bigger blood vessels which are actually the veins. So the blood vessels coming from the heart for the arteries and then these are the veins which carry blood back to the heart. This is how blood circulates throughout the human body, not just in the human body. This is how it circulates throughout all vertebrates through the closed circulatory system. It is an incredibly efficient system. Imagine 5 to 6 litres of blood going through the entire body in less than 20 seconds. So the blood in the human body in a day covers 19,000 kilometres. That's an incredible number of kilometres. That is how efficient it is and how is it so efficient because of the astonishingly powerful heart. It is this muscular organ which pumps blood at a huge force and that is what pushes the blood throughout the body so fast. So that's nice. It's a very efficient system but the drawback is there's a huge amount of energy expended and that is important for us. We need to expend this much of energy because we are highly active animals. Yes, believe it or not, even the laziest of the human beings are highly active. Their metabolic rates are high as compared to some other animals. In some animals like insects there is something called the open circulatory system. Let's take a look at that. So they do have a heart just like we do which is a muscular organ which pumps the blood so we have the heart and then there are blood vessels which carry the blood from the heart towards different organs and tissues and then what happens is the blood leaves this blood vessels. Wow! If the blood leaves the blood vessels shouldn't they die? Well, they don't because their metabolic rates are much lower than ours. For example, you know how we are warm-blooded animals and insects are cold-blooded animals so they don't have to keep their body temperature so high all the time like we do so they don't really need as many nutrients as we do and not as much oxygen as we do because it's a metabolic reactions of the body that keep the temperatures high so they don't have to perform their metabolic reactions at a high rate like we do and hence they don't need a very efficient circulatory system. Another thing is the animals that have an open circulatory system like insects and other arthropods, some mollusks, they're small animals and the blood can easily diffuse from one part of the body to another but imagine the blood diffusing in our bodies from one part to another it's a very difficult feed because our bodies are so much bigger than theirs so that's another reason open circulatory system works very well with them so let's look at the reasons again why they have an open circulatory system metabolic rate is low and the animals are much smaller let's see what happens to the blood after it leaves the blood vessels well it goes to the tissues and bathes it directly we saw in the closed circulatory system that the blood does not directly touch the tissues it is the interstitial fluid that touches tissues here it's the same thing it is the interstitial fluid that is touching the tissues but in this case the blood and the interstitial fluid is one and the same thing so blood is the same as the interstitial fluid which is the same as which is also called hemolymph now what happens to the blood after it bathes tissues well it does go back to the heart so it goes back to the heart through these these are tiny pores that are there in the heart which are called ostea and so the blood enters re enters the heart through this ostea of course you can imagine right the blood is scattered throughout the body of the animal and it is finding its way back into the heart it's obviously not very efficient and very very slow because there is no one directed path like we have in our blood vessels they don't have that so it takes much longer but it is less energy consuming less energy consuming but it is also less efficient but that suffices for these small and simple animals so we have seen that there are two types of circulatory systems closed and open in the close circulatory system the blood does not leave the blood vessels and it is incredibly efficient whereas in the open circulatory system the blood does leave the blood vessels but it is less energy consuming