 As aquatic animals became adapted for land, their entire bodies underwent complex changes, which meant complex changes to their organ systems, their digestive, respiratory, circulation systems. The metabolic rates for amphibians or reptiles was more than that of fish and the metabolic rates for mammals is more than that of reptiles, which means their cells need a constant supply of nutrients and oxygen at a much different rate from each other. And that means the way that the blood who is carrying all these nutrients and oxygen, the way that the blood circulates through their bodies, even that needs to be changed, even that needs to be adapted. Finally, the pump or the heart who is pushing the blood through their bodies also underwent changes to meet these needs. And in this video, we will look at how the structure of the heart changes from fish to reptiles to mammals and also how the way that the blood is circulating through their bodies, how that changes as well. Let's start with the fish. This is how the circulation of blood in fish looks like. This is the heart in between and we can see that it has, this has two chambers. This has two chambers. Deoxygenated blood, which is blood that is poor in oxygen because all the oxygen has been used up by the cells and tissues that circulates to one of the chambers of the heart, which is called, which is called atrium. And then from atrium, the blood goes to ventricle. From ventricle, it goes to the gills where it gets oxygenated, where by diffusion it gets oxygenated and then the blood is circulated throughout the body. And then again, it returns and the cycle continues. Now if you start from gills and you, if you try to complete one circuit when you return back to gills, you will see that the blood flows through the heart only once and this type of circulation is called, this is called a single, a single circulation. This is when in one complete circuit or one complete cycle, the blood will flow past through the heart once. Well now we can, we can wonder if this, if this single circulation, will this work for amphibians or reptiles? A reptile or an amphibian is considered to be generally a more active organism than a fish. That is because it has a far more developed nervous system. It needs to, it needs to crawl, jump, sneak. It, it carries out many more activities than just a fish. So the cells, cells of the reptile or an amphibian, it, they need, they need energy at a much faster rate. Or we can say that the metabolic rate of a reptile or an amphibian is more than that of fish. And over here when, when ventricle pumps out the deoxygenated blood, it enters the capillaries where diffusion happens and it comes back to arteries. In this process, the blood loses a lot of velocity. Let's see, let's see how that happens. Deoxygenated blood, which leaves the heart, it enters the capillaries in the gills where it gets oxygenated and then it leaves, leaves the gills and travels to the rest of the body. But in this process, as the blood is passing through the capillaries, it loses a lot of velocity. That is because initially blood was, blood was, it was deoxygenated, so I'm just drawing it in blue. Initially it was traveling in a much wider vessel. So it was experiencing friction from the walls of the vessel, but only these two, these two layers, the boundary layers were experiencing friction. But as it enters the capillaries, which are far narrower vessels, they are, they are extremely tiny narrower vessels. Now more blood, more blood is in contact, is in contact with the walls of the vessel and in this case the capillary. So now more of the blood is experiencing friction because more of it is in contact with the walls of the vessel. So it loses a lot of velocity. And as the capillaries join together to form larger vessels, they do regain some velocity, but it has already lost considerable amount of velocity as it passed through the capillaries because of the friction experienced by the blood from the walls of the vessels. So when the blood leaves the gills, it has already lost a lot of velocity. And that is actually how the circulatory system of a fish works. Now we can ask ourselves, is this circulatory system, is this pathway good enough for a reptile or an amphibian? Turns out it doesn't work. The rate at which the blood is flowing is not enough to meet the requirements of the high metabolic activity of an amphibian or reptile. It's good enough for a fish because its swimming movements help to move the blood through the body, but not for an amphibian or a reptile. So there needs to be an evolutionary adaptation. From single circulation, we now go to double circulation. So let's see how that looks like. In double circulation, blood has to pass through the heart toys in one complete cycle. Over here the deoxygenated blood, it enters this chamber of the heart and this is called the right atrium. Now it's called right instead of left because think about it from the perspective of the organism itself. For the organism, this would be their right. From their perspective, this is their right. And from here the blood goes into the ventricle. This is the ventricle. From the ventricle, it goes to the lungs where it gets oxygenated. And because we're talking about amphibians, some of the oxygen also comes from the skin. The oxygenated blood is then passed on to the left, the left atrium. This right here is a left atrium. This right here is a ventricle. From the atrium, the blood is then passed to the ventricle and from the ventricle, then it goes to the body. Now to make up for the lost speed of blood when it passes through capillaries, blood again passes through the heart to the ventricle where it is pushed again and then it goes to every cell of the body. So here we can see that in one cycle, if you start from lungs and you come back to lungs, in one cycle the blood is passing through the heart toys. It passes through the heart once. Then it goes to the body. Then it again passes through the heart. Then it goes to the lungs. This heart has three chambers. So this is called a three-chambered heart. However, over here we see that this right atrium is receiving deoxygenated blood. This left atrium is receiving oxygenated blood from the lungs. However, in the ventricle, they get mixed up in the single ventricle which pumps out the mixed blood to the lungs and to the body. In the three-chambered heart of turtles or snakes or lizards, the ventricle is partially divided. So it kind of looks like this. We see that the ventricle is partially divided like this because of this less mixing of blood duggers. There is an exception to this, which are crocodiles. Ventricles are completely divided in them. So they have a heart with four chambers. Now because of the mixing of the blood, this is really called an incomplete double circulation. Incomplete because the blood is really mixing the oxygenated and the deoxygenated blood and double because the blood is passing through the heart toys in one complete cycle. Now we can again wonder if this circulatory pathway, if this makes sense for mammals like birds, elephants, people, we know that mammals differ from amphibian centripetals in one crucial way and that is that we are warm-blooded animals. We regulate our own body temperature and in order to do that, we need extra energy. The metabolic rate of ourselves is even higher than that of amphibian centripetals because they are cold-blooded animals. They don't regulate their body temperature. It depends on the external environment but their cells do not need this extra energy to maintain a constant body temperature. And in fact, a warm-blooded animal uses 10 times as much energy as an equally-sized cold-blooded animal. So this setup where the body is receiving mixed blood, not purely oxygenated blood, does not prove to be too efficient. And an evolutionary adaptation to meet this to deal with this is that the ventricles are also separated, they are separated by a wall. And this is how the heart looks like in mammals. So this is really, this is a four-chambered heart. Now over here, deoxygenated blood, it enters the right atrium and then it goes to the right ventricle. This right here is the right ventricle. From there, it is pushed to the lungs where it gets oxygenated. Then it is brought to the left atrium. From there, it goes to the left ventricle. And from there, it is pushed through the arteries to each and every cell of the body. Now here, purely oxygenated blood is being circulated, which proves to be more efficient to meet the metabolic needs of mammals. Now this type of circulation is called a double circulation. It's not an incomplete double circulation because there is no mixing of blood happening over here. And double circulation again, because the blood is passing through the heart twice in one complete cycle. All right, now to summarize, we went from two-chambered hearts to three-chambered hearts to four-chambered hearts. And each jump was made to meet a need and there were adaptations in how the heart is structured, also how the blood flows through the circulatory system for each of these organisms. For fish, we have a two-chambered heart with just one atrium, one ventricle. But the blood loses a lot of velocity in this pathway, not fast enough to meet the high metabolic needs of an amphibian or reptile. So therefore, the blood was again sent back to the heart where then it is pushed so that it can gain some velocity and then it reaches the cells and tissues of an amphibian or reptile. So here we have a three-chambered heart. We have right atrium, left atrium, and one ventricle. And finally, for mammals who regulate their own body temperature, so they need more energy, 10 times more energy as that of an equally sized amphibian or reptile. So they can't really work with a mixed blood. They need pure, purely oxygenated blood, blood carrying enough oxygen to meet the high metabolic needs. So therefore, you have a separation in the ventricle itself and now the heart became four-chambered. You have a right atrium, a left atrium, left ventricle, and a right ventricle.