 Dear students, in this topic we shall discuss about the vertebrate lungs. The terrestrial vertebrates use a pair of lungs to exchange oxygen and carbon dioxide with the environment. Lungs are found in most amphibians except few ones. They are found in all reptiles, all birds, and all mammals. We shall discuss about the basic plan of vertebrate lung. Although there are considerable variations among the four groups of vertebrates, the generalized structure of lung consists of a complex network of tubes and airspaces are sex. The number of airspaces per unit volume becomes progressively greater in the evolutionary order. Amphibians, reptiles, birds, and mammals. The number of airspaces per unit volume is the lowest among the amphibians, the most among reptiles, and the most among mammals is found in the gas exchange. The size of these terminal airspaces becomes smaller in the same order. The size of the airspaces in the amphibians is greater, while the size of the mammals is smaller. These terminal airspaces make the respiratory epithelium of the lung thin and vascularized. These airspaces make the respiratory epithelium of the lung thin and vascularized. Dear students, we shall discuss the lungs of all these four classes of vertebrates one by one. We are going to discuss amphibian lungs first. The lungs of amphibians vary in complexity. The lungs of the urodeals are just like a smooth walled pouch. This is a pouch with a smooth walled pouch and a bag-like and empty pouch. There is only air sex on the wall with the gas exchange. The frogs and toads in this pouch have balloon-like lungs. There are septa and folds in these lungs that subdivide these lungs into interconnected air sex. Through this, the gas exchange is comparatively better than a bag-like pouch. Dear students, the lungs of reptiles are sponge-like. They have sponge-like texture. If we take out any reptile's lung and touch it, we will feel it as if we have caught a sponge. Each reptilian lung has a single bronchus which runs down the centre of the lung. Usually, the bronchus is divided into lungs and mammals after entering the lung. However, they have single bronchus that comes to the centre of the lung, to the middle of the lung, or goes further than this. Each bronchus makes branches and from these branches, individual pockets are present that go to the lungs. These pockets have a gas exchange. These are the surfaces where the gas exchange occurs. These pockets are similar to the alveoli in mammals. However, these pockets of reptiles are much larger and fewer in number as compared to those of mammals. Now, we shall discuss the mammalian lungs. Mammalian lungs are more subdivided to increase the respiratory surface area. They branch repeatedly to terminal bronchioles and these terminal bronchioles divide to finally form the respiratory bronchioles. Each respiratory bronchiole is attached to an alveolar duct with a cluster of alveolar sacs which make respiratory epithelium and from which the gas exchange occurs. Dear students, the lungs of birds are relatively smaller in size and they are connected to seven or nine air sacs which extend much of the body. The size of the lungs is small but depending on the species of birds, the seven or nine air sacs are attached which extend to the body in various parts and even bones. Air sacs may be connected to air spaces within the bones. The lungs of birds are slightly different. They do not have alveoli but they contain millions of parabronchi. These parabronchi are tiny, parallel, tube-like passages. These are the sites of gas exchange in birds. This system in birds enables them to remove 90% of oxygen from the air with each breath. They can remove only 25% of oxygen from the air once breathed.