In the vertebrate world, there are two major types of respiration: unidirectional and bidirectional (or tidal).

In unidirectional ventilation, the medium (air or water) moves across tissues in one direction. This method is efficient because the medium is always fresh. Fish and birds have unidirectional respiration.

The second type is bidirectional respiration, which implies that the medium enters and exits through the same channel. In this case, the medium (air) contains more waste and is not as efficient.

Cutaneous respiration is also possible and occurs via the skin. Cutaneous respiration is unique in that it can occur in air or water. Amphibians utilize this form. Each type of respiration requires modified organs and methods of obtaining oxygen.

Unidirectional Respiration: Fish

Fish have two important organs for respiration: gills and gas bladders.

Gas bladders have two possible functions. They can help the fish stay buoyant, in which case they are called swim bladders, or aid in respiration (lungs).

The function of the gills is to draw water into the lungs, and fish have two methods of using their lungs: bucal pump (opercular) and ram (passive) ventilation.

In opercular ventilation, the fish takes in water through the mouth (bucal chamber) and it exits through the gills (opercular chamber). Pressure in the mouth is kept higher than pressure at the gills to ensure a supply of fresh water and oxygen. Unidirectional flow is advantageous because it allows for greater oxygen concentration in the bloodstream.

Bidirectional Respiration: Mammals and Reptiles

This type of respiration is not as foreign to us because this is the type of respiration humans have. Bidirectional is not as efficient as unidirectional because there is extra waste and the oxygen concentration is much lower. The amount of waste is greater because air that is inhaled and air that is exhaled travel through the same channel.

Mammals and reptiles have very similar respiratory systems, but there are a few key differences. Reptiles have alveoli and bronchioles, but they are fewer in number. The lungs of reptiles also have less surface area as they are ectothermic and do not require as much oxygen for metabolism.

Respiration is slightly variable among the reptiles, but the overall method is like that found in mammals.

Cutaneous Respiration: Amphibians

Amphibians have two forms of respiration: bidirectional and cutaneous.

Cutaneous respiration allows organisms to breathe through their skin. As a result, they must live near the water and have moist skin. This type of respiration requires moist skin and diffusion of oxygen takes much longer than in lungs. The epidermis (top layer of skin) is much thinner than the layers beneath it. The deeper layers of skin are also vascularized so the oxygen can get into the bloodstream.

Cutaneous respiration allows amphibians to live in water and on land, and it also makes the amphibians an unique group of organisms.

Summary:

Among the vertebrates, there is a great deal of variety in the types of respiration and the modifications involved. In summary, fish and birds have unidirectional flow, mammals and reptiles have bidirectional while amphibians have bidirectional as well as cutaneous respiration.

The important organs for fish include the gills and swim bladder, while the lungs are important for mammals, reptiles and amphibians. It is also important to remember that amphibians are special because they can use their skin for respiration.