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Breathing and Exchange of Gases

Oxygen (O2) is utilised by the organisms to indirectly break down nutrient molecules like glucose and to derive energy for performing various activities.

Carbon dioxide (CO2) which is harmful is also released during catabolic reactions. It is, therefore, evident that O2 has to the continuously provided to the cells and CO2 produced by the cells have to be released out. This process of exchange of O2 from the atmosphere with CO2 produced by the cells is called breathing, commonly known as respiration. It takes place in three basic steps

Pulmonary ventilation: The first process, pulmonary (pulmo = lung) ventilation, or breathing, is the inspiration (inflow) and expiration (outflow) of air between the atmosphere and the lungs.

External (pulmonary) respiration: This is the exchange of gases between the air spaces of the lungs and blood in pulmonary capillaries. The blood gains O2 and loses CO2.

Internal (tissue) respiration: The exchange of gases between blood in systemic capillaries and tissue cells is known as internal (tissue) respiration. The blood loses O2 and gains CO2. Within cells, the metabolic reactions that consume O2 and give off CO2 and give off CO2 during production of ATP are termed cellular respiration.

Types of respiration: It is of two types –

(1) Aerobic respiration: It occurs in the presence of molecular oxygen. The oxygen completely oxidizes the food to carbon dioxide and water, releasing large amount of energy. The organisms showing aerobic respiration are called aerobes. It is found in most of animals and plants. Aerobic respiration is of two main type direct and indirect. 

C6H12O6 +6H2O → 6CO2+6H2O+ 2830 KJ

(i) Direct respiration: It is the exchange of environmental oxygen with the carbon dioxide of the body cells without special respiratory organs and without the aid of blood. It is found in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most arthropods.

(ii) Indirect resptration: It involves special respiratory organs, such as skin, buccopharyngeal lining, gills and lungs, and needs the help of blood. The respiration in the skin, buccopharyngeal lining, gills and lungs is respectively called cutaneous buccopharyngeal, bronchial and pulmonary respiration. Cutaneous respiration takes place in annelids, some crustaceans, eel fish, amphibians and marine snakes. It occurs both in water and in air. Buccopharyngeal respiration is found m certain amphibians such as frog and toad. It occurs in the air. Branchial  respiration is found in many annelids, most crustaceans and mollusks, some insect larvae, echinoderms, all fishes and some amphibians. It occurs in water only. Pulmonary respiration is found in snails, pila, some amphibians and in all reptiles, birds and mammals, it takes place in air only.

(2) Anaerobic respiration: It occurs in the absence of molecular-oxygen and is also called fermentation. In this, the food is only partially oxidised so only a part of energy (5%) is released and some of energy remains trapped in the intermediate compounds. It is found. in lower organisms like bacteria and yeast. It is also found in certain parasitic worms (Ascaris, Taenia) which live in deficient medium. 


Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O2 with CO2 by simple diffusion over their entire body surface.

Earthworms use their moist cuticle. Insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body. In cockroaches, three pairs of longitudinal tracheal trunks are present all along the length of body which are further connected with each other with the help of transverse branches. The main tracheae give of smaller tracheae whose branch repeatedly form a network of trachioles throughout the body Trachea internally lined by chitinous cuticle called intima, which spirally thickened to form taenidae Trachioles without taenidae, trachioles lined b) trachein protein. From each tracheal trunk three branches come out. The dorsal branch is suppliec to the dorsal muscles where as ventral one to nerve cord and ventral muscles and middle one to the alimentary canal.

Special vascularised structures called gills are used by most of the aquatic arthropods and molluscs whereas vascularised bags called lungs are used by the terrestrial forms for the exchange of gases

Among vertebrates, fishes use gills whereas reptiles birds and mammals respire through lungs Amphibians like frogs can respire through their moist skin also. Mammals have a well developer respiratory system. 

  • Respiratory organs of animals 


Respiratory organs

Protists, Bacteria

Direct respiration through plasma membrane


Plasma membrane of each cell


General body surface

Platyhelminthes (Fasciola hepatica, tapeworm)


Nematodes (Ascaris)


Annelids (Earthworm and Leeches)










Spider and Scorpion, ticks, mites

Book lungs

Marine king crab

Book gills

Prawns, Unio and Pila



Dermal branchiae, Tube feet, Respiratory tree, Bursae

Fishes, Tadpoles


Frogs, Toads

Buccopharyngeal living, Lungs, Skin

Reptiles, Mammals



Air sacs/Lungs

Lung fish

Air bladder

Urochordata (Herdmania)


Marine turtle

Cloacal respiration

Mollusca (Unio)


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