Cells continually use oxygen (O2) for the metabolic reactions that release energy from nutrient molecules and produce ATP. At the same time, these reactions release carbon dioxide. Since an excessive amount of CO2 produces acidity that is toxic to cells, the excess CO2 must be eliminated quickly and efficiently. The two systems that cooperate to supply O2 and eliminate CO2 are the cardiovascular system and the respiratory system. The respiratory system provides for gas exchange, intake of O2 and elimination of CO2, whereas the cardiovascular system transports the gases in the blood between the lungs and body cells. Failure of either system has the same effect on the body: disruption of homeostasis and rapid death of cells from oxygen starvation and buildup of waste products. In addition to functioning in gas exchange, the respiratory system also contains receptors for the sense of smell, filters inspired air, produces sounds, and helps eliminate wastes.

Respiration : Respiration is the exchange of gases between the atmosphere, blood and cells. It takes place in three basic steps :
(1) 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.
(2) 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.
(3) 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.

Respiration is a process which involves intake of oxygen from environment and to deliver it to the cells. It include stepwise oxidation of food in cells with incoming oxygen, elimination of produced in oxidation, release of energy during oxidation and storing it in the form of ATP.
(i) Respiratory surface : The surface at which extend of gases (CO2 and O2) takes place is called respiratory surface. Respiratory surface must be vascular and have enough area for gas exchange. For example – plasma membrane in protozoa, body wall (skin) in annelids, alveocapillary membrane in men.

(ii) Respiratory medium : Oxygen is dissolved in air and water. Thus water and air are source of oxygen for animals and called respiratory medium. Water and air are external respiratory medium. Respiratory medium comes in contact with respiratory surface and gaseous exchange takes place between respiratory medium and blood or any other transport medium through respiratory surface by simple diffusion. Inside the body an internal respiratory medium is also found. This internal respiratory medium is tissue fluid. Cells exchange their gases with tissue fluid through plasma membrane.

(iii) Types of respiration : It is of two types
(a) Aerobic respiration : It occurs in the presence of molecular oxygen. The oxygen completely oxidises 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 types direct and indirect.

# (1) 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.
- Protists : Amoeba proteus is about 0.25 mm. Wide and has a large surface area to volume ratio. Diffusion of gases occurs over the entire surface via cell membrane, and is enough to fulfill its metabolic requirements.
- Coelenterates : In Hydra and Obelia, practically all cells are in contact with the surrounding water. Each cell can exchange gases sufficient for its own needs through the cell membrane adjacent to water.
- Flatworms : Planaria can also exchange gases sufficient for its needs by diffusion over its body surface. This is facilitated by its very thin body which increases the surface area to volume ratio.

# (2) Indirect respiration : 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 in 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.

(b) 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 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. The organism showing anaerobic respiration, are called anaerobes. These involve one of following reactions.

- Certain body tissues of even aerobes also show anaerobic metabolism e.g., during the vigorous contraction of skeletal muscle fibres. In this, the glucose is metabolised into the lactic acid in anaerobic conditions. The rapid formation and accumulation of lactic acid are responsible for muscle-fatigue. The mammalian RBCs shows anaerobic respiration as these lack the mitochondria. In lens of eye and cornea of eye respiration is anaerobic because these structures are a non vascular. Anaerobic respiration appeared first in primitive organisms because there was absence of O2 in primitive atmosphere.