 If you have ever noticed a frog very closely, you must have seen that the floor of the mouth goes up and down continuously. Even though their mouth is closed, the floor of the mouth goes up and down. It seems like they are chewing something, right? But from our previous video, we learned that frogs never chew their food. They only use their teeth to grasp the prey they catch, right? So what are they actually doing? They are breathing, breathing through their mouth. But the mouth is closed, right? So how are they even breathing through their mouth? Well frogs have the most amazing respiratory system. They can breathe through their mouth without even opening the mouth. They can also breathe through their skin and they have lungs just like us. And that's not it. They also have gills just like fishes. Amazing, right? Well amphibians, amphibians, simply means dual life. They can live both on land and in water. And their life starts in water. And the initial stage of the tadpole or the larval stage of frog looks more like fish and less like a frog, right? So in that stage, they respire or breathe through gills exactly like fishes do. And then as they transition to land, they develop all other means of respiratory mechanisms. So in this video, let's have a quick overview of all different type of respiratory systems of frog. And we will begin with the tadpole or the larval stage of frog, okay? So as we just discussed, they have gills on their sides. So how do gills work? So let me draw them here outside. Let's say these are the gills or the openings, okay? And these gills are comb-like in structure and are highly vascularized. They have a very close network of blood capillaries around them, a lot of blood capillaries around the gills. Now, as the tadpole opens its mouth, water rushes in and gets out through the gills. Excess water gets out through the gills. So as water passes through this highly vascularized gills, what happens? The dissolved oxygen is taken into the blood and the excess carbon dioxide from the blood is sent out. And this exchange of gases could take place because we know gases always moves from region of higher pressure to regions of lower pressure. That is why oxygen could move into the blood, carbon dioxide from the blood could move out into the water. So this tadpole as they grow and develop into an adult frog, they lose a bunch of stuff like the tail and the gills, okay? They lose their gills and they develop other parts of the body like the limbs instead, okay? Now, even without gills, you must have seen frog happily surviving underwater. How are they able to do that? Well, that is possible because they can respire with the help of their skin. And you know what's even more interesting? They can respire with the help of their skin, not just under water but also while on land. So why are we not able to respire with the help of our skin? Well, that's because our skin is dry and it has a layer of keratin on top, keratin protein which makes our skin impermeable. But when it comes to frog, the skin is moist. It is very thin, okay? And it has a layer of mucus on top. And again, it is highly, highly vascularized, okay? It has very good supply of blood network underneath the skin. So the oxygen from the air dissolves first into the mucus lining or the wet surface on top of the skin and from there it gets into the blood, okay? And carbon dioxide from the blood moves out first into this mucus layer, into the wet layer above the surface of the skin and then diffuses out into the environment. That is how frogs can breathe with the help of their skin. And this type of respiration is called cutaneous respiration, okay? Now another reason why this kind of respiration works for frog is because they don't need much energy. In our previous video, we learned that frogs are cold-blooded, right? They don't need to maintain their internal body temperature, which is not true in case of humans. We need to put in a lot of energy to maintain our own internal body temperature, right? And our energy demand is much higher than that of a frog. So as per the requirement of the frog, skin works just fine. The oxygen attained from its skin is sufficient for its survival, especially during the months when they go into a phase of very less physical and physiological activity, which is very popularly known as hibernation or estivation. During those days, their energy demand reduces even further. So cutaneous respiration becomes the perfect way to respire because it needs less energy and their oxygen demand is also very less at that time. Now moving ahead as we discussed, frogs have two more options through which they can breathe or respire, okay? And one such way is through their mouth, the one we saw at the beginning of the video. Remember? Now to understand how this kind of respiration works, we need to look at its internal structure a little bit, okay? So here we have the nostrils and opening to the outside just like we have ours, okay? Then this is the buckle cavity, this area here, and then they have a pair of lungs just like we do, okay? Now try to recall how humans breathe. We increase the volume of our lungs so that the pressure inside decreases, air pressure inside decreases and the air from outside could rush in, right? The frog does the same. We saw how the buckle cavity floor moves up and down, right? Like this. What are they actually doing? They are lowering the floor of the buckle cavity so that the volume inside increases so that the air pressure inside will decrease and that is when the air from outside will rush into the buckle cavity. Now guess the next step. What do you think? The air will be moved to the lungs, right? No. Do you see there is a closing here? This is the glottis. It stops the entry of air into the lungs. So where is the exchange of oxygen and carbon dioxide happening in this case? What do you think? Well, here it is happening in the floor of the buckle cavity itself. The floor of the buckle cavity is very thin, is lined with mucus and is highly vascularized, just how we discussed in case of skin or frogs. The oxygen could easily diffuse in and carbon dioxide diffuses out into the buckle cavity. The frog then lifts the floor of the buckle cavity and the air is rushed out through the nostrils and this kind of respiration is called buccal pharyngeal respiration. Interesting, right? Now let's move on to the fourth way of respiration of frogs, that is through their lungs. Let's see how they do that. So the first step the frogs do is to lower their buckle cavity floor. This is number one so that the air pressure inside decreases and the air could rush in through the nostrils into the buckle cavity. The next step is to close the nostrils so that air cannot rush out. Then the frog lifts the buckle cavity floor. So now carefully look at the next step. The frog lifts the buckle cavity floor. Look again. It lifts the buckle cavity floor and the air rushes into the lungs. The glottis opens, the air rushes into the lungs. Now the lungs have alveoli which increases the surface area for absorption of oxygen. But the lungs are not as well developed as in case of humans and they also have fewer alveoli than we humans possess and as you may already know these alveoli are highly vascularized. They are very thin walled and oxygen and carbon dioxide can easily exchange through the walls of this alveoli and blood capillaries. Now as the exchange of gases takes place the next step is exhalation. So how do the frogs exhale now? They don't have a diaphragm like us. Well frogs have other muscles responsible for exhalation. Those muscles push as the lungs so that the air pressure inside increases and the air could rush out into the buckle cavity. The buckle cavity floor lowers, air is filled here now, the nostrils open. So look carefully the glottis is closed now, the only opening is the nostril now and when the buckle cavity floor will be raised the air will move out through the nostrils. That is how they exhale. This was the respiration through their lungs which we also call the pulmonary respiration. So these were the four different types of respiration. Four different mechanisms, one through gills in the tadpole stage in the larval stage then we saw cutaneous respiration that is respiration through skin in adult frog then we saw how bucopharyngeal respiration works and then we looked at the pulmonary respiration in frogs. Now the natural question would be how does the frog decide which mode of respiration to opt for? Well it depends upon the amount of energy the frog requires at the moment. Okay the cutaneous respiration goes on all the time when it is at rest, when it is hibernating, estivating, I mean all the time the cutaneous respiration goes on but the bucopharyngeal respiration starts when the frog needs a little more energy. Okay and then it shifts to pulmonary respiration when it needs the most energy maybe when chased by a predator or when it is mating. So just like we change gears in our vehicle to increase the speed these frogs change their mode of respiration to meet their energy need.