The organs which are concerned With the gaseous exchange, e.g., intake of O2 and output of CO2 and haw; greater rate of gas exchange per unit area than the general body surface are referred to as respiratory organs.
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Thus, respiratory organ may be a part of special region of the body or may be an organ specifically meant for this purpose such as lung.
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In animals various types of respiratory organs are found. These may differ in their structure but all have a large surface of contact with the surrounding environment and are richly supplied with blood vessels and capillaries which ensure rapid gaseous exchange between the external environment (either air or water) and the blood.
The respiratory organs may be classified by their physiology or by their morphology. If they work in water they are gills, if in air, lungs.
They may project outwards or they may be invaginations. The most important respiratory organs of the animals are as follows:
1. Integument:
There are many animals such as protozoans, sponges, coelenterates, helminthes and amphibians in which integument or skin functions as a respiratory organ.
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The integument of these animals is richly vascularized and remains moist all the time so that oxygen from the surrounding environment can pass into the blood through simple diffusion.
In aquatic animals a circulatory mechanism is present to ensure free flow of water over the respiratory surface. This is effected by cilia present (in most of the cases) over the respiratory surface or integument.
In Chaetopterus and Nereis specialized parapodia are found which move in a fan-like manner, thereby maintaining a constant stream of water over the integument.
2. Gills:
Gills are the respiratory surfaces of a number of aquatic animals including the chordates which all at one time or another had gills or gill-slits during their development.
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Gills are found in lamellibranchs, molluscs among others, also in many crustacean and fishes.
Typically gills are filamentous structures richly supplied with blood capillaries. They are located inside the body when derived from the anterior part of the alimentary canal as in fishes or outside the body when they are outgrowths of the body surface as in amphibian larvae, Polypterus and other invertebrate larvae.
The gills found inside the body are referred to as internal gills where as the gills found outside the body are referred as to external gills. Both are meant for respiration.
In crustaceans such as the crayfish, lobster and crabs, etc., the gills are filamentous outgrowths from certain of the segmental appendages and are enclosed in a chamber of chitin carapace.
These gills are usually ventilated by the paddle-like movements of special appendages such as the scaphognathites.
In Malacostraca specialize branches of appendages (epipodites) are found which function for respiration.
Numerous aquatic insects have gills but these are usually abdominal or caudal. Tracheal gills occur in nymphs of Odonata, Trichoptera and in larvae of some beetles but these gills are supplied with fine tracheae instead of blood capillaries.
Molluscs possess a variety of gills. The lamellibranchs possess two pairs of ctenidia which are variously modified.
In Chiton there are 6 to 80 gills in each paliial groove. In gastropods the gills or ctenidia are relatively simple and are located in the mantle cavity but in cephalopods the gills are large and are richly vascularized.
In echinoderms finger-like evaginations of the coelomic cavity, the so called dermal papillae or dermal branchiae, serve as respiratory organs.
Protochordates also possess gills as their respiratory organs. In Amphioxus as many as 90 pairs of gills are present in series along the pharynx.
In fishes true gills are found which are usually covered. They are ventilated by breathing movements of mouth and operculum.
In certain fishes accessory respiratory organs are also found which are respiratory in function like gills.
In Clarias these are the branch extensions of the gill arches bearing numerous papillae. In Anabas these are labyrinthiform organs, in Ophiocephalus supra-branchial chamber while in Saccobranchus pharyngeal lung.
As soon as water comes in contact with gills, simultaneously gaseous exchange takes place at the gill surface, e.g., oxygen is absorbed and carbon dioxide is released into the environment. Thus external respiration is affected.
3. Lungs:
Lungs are referred as the chief respiratory organs of all the land living vertebrates such as reptiles, birds and mammals.
Virtually all lungs are parts of or outgrowths from the alimentary canal and are richly vascularized.
Most lungs are aerial, a few are water filled. Ventilation and diffusion lungs are distinguished according to the presence or absence of mechanisms for air renewal.