A. Lungs: Adaptations for Terrestrial Vertebrates
Lungs are folded within the body and are restricted to one location. Since they are centrally located, a circulatory system must carry oxygen to all cells. Lungs contain a dense net of capillaries. Lungs consist of an inpocketing, branching tube that ends in a multitude of tiny air sacs called alveoli, where the blood and the air are separated by a thin moist membrane. By muscular control of its breathing apparatus and valves of its mouth and pharynx, an animal can control the amount of air that passes through the lungs.
1. First Lungs: Amphibians
Nearly all have lungs and exchange gases in two ways: through bag like lungs and through their moist, highly vascularized skin. These animals represent the transition from aquatic to terrestrial animals. The larval froms of amphibians breathe with gills, which are strictly aquatic structures. A metamorphosis occurs, the lungs expand, and gills are absorbed into their bodies. The adult amphibians then use the lungs and skin to obtain oxygen.
2. Reptiles
The reptile skin is impervious to air, so reptiles are strictly lung breathers. The trachea is subdivided into smaller passages that enter numerous membranous compartments. This increase the surface area of gas exchange membrane. The breathing movements fo pertiles occurs when the muscles around the entire body cavity contract and relax.
3. Birds
A one way, countercurrent circulation of air occurs in the lung. There is not much mixing of old air and new air. Birds breathe through mouth and nostrils but incoming and out going air are separated within the respiratory cavity. The repiratory system is the most complex of all vertebrates. Inhaled air passes in wide bronchi through the lungs and into air sacs, which can be surrounded by gonads, and into the cavities of some bones. When the bird is not flying, muscles on the thorax and abdomen respire for breathing movements. In flight, the rapide beating of wings is all that is required for air movements.