Oxygen is one of the inputs and carbon dioxide is one of the outputs of respiration. Therefore, oxygen has to be supplied to all cells of the body and carbon dioxide produced in cells has to be taken away. There is a particular part of organ where oxygen from atmosphere or hydrosphere is given to the blood and carbon dioxide of the blood is given off. This organ is the main organ of respiration i.e., the organ involved in the exchange of gases. We shall see, in the succeeding paragraphs, how this occurs in plants and animals. We shall also study different types of respiratory organs in animals arising out of necessity and adaptation to different lifestyles.
Exchange of Gases in Plants
In plants, the gases diffuse in though the stomata, lenticels, and intercellular spaces. Oxygen enters through stomata and diffuses into the air spaces of spongy mesophyll cells. In woody plants, the stem is covered with impermeable layer of bark (cork). There are small openings called lenticels in the pits of the bark through which there is an exchange of gases.
Exchange of Gases in Animals
In small organisms (e.g., Amoeba) the exchange of gases is not a big problem. Whatever oxygen is needed for respiration can penetrate from the surface to all parts of the organisms. But in larger animals (e.g., birds, mammals) there is a high rate of oxygen consumption. Sufficient oxygen cannot be supplied to them by diffusion through the general body surface. Therefore, they have special respiratory organs having greatly enlarged surface area through which oxygen can diffuse. The surface area of human lungs is forty times greater than the body surface area. In animals, there are three types of respiratory organs-(a) gills (b) tracheae and (c) lungs.
If the respiratory surface is enlarged and turned out as an appendage, it is called gill. We find this organ in winter-dwelling animals. You may have seen the gills of a fish. Insect have a fine system of air tubes reaching all parts of the body. These tubes are called tracheae which pipe the oxygen directly to the tissues. If the respiratory surface of an animal is turned in to form a cavity, it is called lung.
In unicellular organisms such as Amoeba and paramecium there is diffusion of gases through the entire surface of cell body. In sponges, the dissolved oxygen of water enters the body along with the water current which passes through a system of canals. During the course of passage of water current, there is an exchange of gases in which oxygen is taken up by the cells of the sponge body and carbon dioxide is given off. In hydra and planaria too, there is an exchange of gases earthworm is used for exchange of gases.
Higher Invertebrates In higher groups of invertebrates for example, crayfish and cuttle fish, we find gills as the respiratory organs. In higher invertebrates, which are terrestrial, we find lungs (e.g., in pila), book lung (e.g., in scorpions), and a system of air tubes called tracheae (e.g., in insects, centipedes).
Among the lower aquatic vartebrates are fishes. In the pharynx of the fish there are a series of paired, laterally-placed openings called gill slits. These openings lead into tunnels running across the body. The tunnels open to the exterior to the lateral surfaces. Gills are present in these tunnels. As the water enters the pharynx through the mouth, the food contained in the water passes down the oesophagus, but water enters through the gill slits and runs along the tunnels. In this movement the gills are bathed in, and the oxygen of water is taken in and carbon dioxide is given off as the water sweeps over the gills.
Fishes that have bony skeleton also have their gills covered by an operculum. In some amphibians, gills may be projected from the anterior part of the body and maybe weakly supported by skeleton. In some fish there is a swim bladder connected to the oesophagus. In lung fish, particularly, swim bladder acts as breathing organ. In frog exchange of gases occurs in three different organs: skin (cutaneous respiration), buccal cavity (buccopharyngeal respiration), and lungs (pulmonary respiration). Reptiles (snakes, lizards, turtles, crocodiles) have only lungs as the organs for exchange of gases.
Higher Vertebrates: Higher vertebrates include birds and mammals. In birds, there are somewhat compact lungs, but associated with them are air sacs. As an example of mammalian system, we shall study the respiratory system of man in detail.