14.7 Summary

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 associated 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 \(\mathrm{O}_2\) and \(\mathrm{CO}_2\) between deoxygenated blood and alveoli, transport of these gases throughout the body by blood, exchange of \(\mathrm{O}_2\) and \(\mathrm{CO}_2\) between the oxygenated blood and tissues and utilisation of \(\mathrm{O}_2\) 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 spirometer and are of clinical significance.

Exchange of \(\mathrm{O}_2\) and \(\mathrm{CO}_2\) at the alveoli and tissues occur by diffusion. Rate of diffusion is dependent on the partial pressure gradients of \(\mathrm{O}_2\left(\mathrm{pO}_2\right)\) and \(\mathrm{CO}_2\left(\mathrm{pCO}_2\right)\), their solubility as well as the thickness of the diffusion surface. These factors in our body facilitate diffusion of \(\mathrm{O}_2\) 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 \(\mathrm{CO}_2\) in the opposite direction, i.e., from tissues to alveoli.

Oxygen is transported mainly as oxyhaemoglobin. In the alveoli where \(\mathrm{pO}_2\) is higher, \(\mathrm{O}_2\) gets bound to haemoglobin which is easily dissociated at the tissues where \(\mathrm{pO}_2\) is low and \(\mathrm{pCO}_2\) and \(\mathrm{H}^{+}\)concentration are high. Nearly 70 per cent of carbon dioxide is transported as bicarbonate \(\left(\mathrm{HCO}_3^{-}\right)\)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 \(\mathrm{pCO}_2\) is high, it gets bound to blood whereas in the alveoli where \(\mathrm{pCO}_2\) is low and \(\mathrm{pO}_2\) is high, it gets removed from the blood.

Respiratory rhythm is maintained by the respiratory centre in the medulla region of brain. A pneumotaxic centre in the pons region of the brain and a chemosensitive area in the medulla can alter respiratory mechanism.