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 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 spirometer and are of clinical significance.

Exchange of O2 and CO2 at the alveoli and tissues occur by diffusion. 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 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 (HCO3 – ) 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 brain. A pneumotaxic centre in the pons region of the brain and a chemosensitive area in the medulla can alter respiratory mechanism.

EXERCISES

1. Define vital capacity. What is its significance?

2. State the volume of air remaining in the lungs after a normal breathing.

3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?

4. What are the major transport mechanisms for CO2 ? Explain.

5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air ?

(i) pO2 lesser, pCO2 higher (ii) pO2 higher, pCO2 lesser (iii) pO2 higher, pCO2 higher (iv) pO2 lesser, pCO2 lesser

6. Explain the process of inspiration under normal conditions.

7. How is respiration regulated?

8. What is the effect of pCO2 on oxygen transport?

9. What happens to the respiratory process in a man going up a hill?

10. What is the site of gaseous exchange in an insect?

11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.

13. Distinguish between (a) IRV and ERV (b) Inspiratory capacity and Expiratory capacity. (c) Vital capacity and Total lung capacity.

14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.