GASEOUS EXCHANGE IN ANIMALS
The gaseous exchange in different animals takes place by different methods and organs. In unicellular aquatic animals like amoeba, the dissolved oxygen in water diffuses directly through their cell surface into the interior of the animal and the carbon dioxide similarly diffuses out from their bodies into the external water. This is the simplest way of gaseous exchange and it can occur only in small animals with a diameter of less than one millimeter. These animals have greater surface area of volume ratio and have low rate of metabolism.
During evolution, as the animals became complex and complex and grew in their size, their skin or external body surface become impervious to water. Thus the gaseous exchange became impossible through diffusion. In large animals certain organs were developed for exchange of gases w.g. the moist vascular skin, gills, lungs and tracheoles. These large animals have developed blood vascular system which transports oxygen from the respiratory surface to the deep cells and tissues in all parts of the body. The blood in all animals has some respiratory pigments like hemoglobin which carry large amount of oxygen efficiently from respiratory surface to the interior cells.
Properties of a Respiratory Surface
1. Respiratory surface should have large surface area.
2. Respiratory surface should be moist.
3. Respiratory surface should be thin walled.
4. Respiratory surface should have blood supply.
Gaseous Exchange Through Skin:
For the exchange of gases through the skin the skin must be moist and richly supplied with blood. The oxygen is diffused from the external water to the blood and the carbon dioxide is diffused from the blood to exterior water. In amphibia and fishes the gaseous exchange occurs through the skin besides through the gills or lungs. The frogs and tortoises breath through the skin during their hibernation period.
Gaseous Exchange by Gills:
The gills are very effective for gaseous exchange in aquatic animals. Gills are of two types:
(a) External Gills
(b) Internal Gills
· External Gills:
Some animals have external gills which project out of body of animals. These gills have very thin and highly vascularized surfaces e.g. the dermal papillae of star fish and arthropods.
· Internal Gills:
These are present inside the body inner to skin e.g. in fishes and arthropods. Have you ever examined a fish closely? How ill you know that the fish is fresh or not? If the color of gills is red then it is fresh but if the color of gills is changed, it is definitely not fresh. The red color of the fish gills shows the presence of oxygenated blood.