→ Cells utilise oxygen for metabolism and produce energy along with substances like carbon dioxide which is harmful.

→ Animals have evolved different mechanisms for the transport of oxygen to the cells and for the removal of carbon dioxide from there. We have a well developed respiratory’ system comprising two lungs and as-sociated air passages to perform this function.

→ The first step in respiration is breathing by which atmospheric air is taken in (inspiration) and the alveolar air is released out (expiration).

→ Exchange of O₂ and CO₂ between deoxygenated blood and alveoli, transport of these gases throughout the body by blood, exchange of O₂ and CO₂ between the oxygenated blood and tissues and utilisation of O₂ by the cells (cellular respiration) are the other steps involved.

→ Inspiration and expiration are carried out by creating pressure gradients between the atmosphere and the alveoli with the help of specialised muscles intercostals and diaphragm. Volumes of air involved in these activities can be estimated with the help of a spirometer and are of clinical significance.

→ Exchange of O₂ and CO₂ at the alveoli and tissues occurs by diffusion.

→ The rate of diffusion is dependent on the partial pressure gradients of O₂ (pO₂) and CO₂ (pCO₂), their solubility as well as the thickness of the diffusion surface. These factors in our body facilitate the diffusion of O₂ from the alveoli to the (deoxygenated blood as well as from the oxygenated blood to the tissues.

→ The factors are favourable for the diffusion of CO₂ in the opposite direction i.e. from tissues to alveoli.

→ Oxygen is transported mainly as oxyhaemoglobin. In the alveoli where pO₂ is higher, O₂ gets bound to haemoglobin which is easily dissociated at the tissues where pO₂ is low and pCO₂ and H+ concentration are high.

→ Nearly 70 per cent of carbon dioxide is transported as bicarbonate (HCO₂) with the help of the enzyme carbonic anhydrase. 20-25 per cent of carbon dioxide is carried by haemoglobin as carbamino-haemoglobin.

→ In the tissues where pCO₂ is high, it gets bound to blood whereas, in the alveoli where pCO₂ is low and pO₂ is high, it gets removed from the blood.

→ Respiratory rhythm is maintained by the respiratory centre in the medulla region of the brain.

→ A pneumatic centre in the pons regions of the brain and a chemosensitive area in the medulla can alter the respiratory mechanism

→ Breathing/ Respiration: O₂ has to be 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.

→ Gills/Lungs: Special vascularised structures called gills are used for respiration by most of the aquatic arthropods and molluscs whereas vascularised bags called lungs are used by the terrestrial forms.

→ Alveoli: Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.

→ Partial Pressure: Pressure contributed by an individual gas in a mixture of gases is called partial pressure.

→ Respiratory rhythm centre: A specialised centre present in the medulla region of the brain called the respiratory rhythm centre is primarily responsible for this regulation.