Mechanism of Respiration :
The term respiration includes a complete process of taking in oxygen and giving out carbon dioxide. It includes breathing, external respiration, internal respiration and cellular respiration.
Breathing : The process by which air comes in and goes out of the lungs is called breathing. It is the process that speeds up the rate of gaseous exchange.
The terms respiration and breathing are not synonymous as breathing is a part of respiration.
Breathing includes inspiration and expiration. During inspiration air containing oxygen is taken into the lungs while while during expiration air containing more carbon-dioxide and water vapour is forced out of the lungs.
Inspiration : It occurs in the following steps :
It is an active process brought about by ribs, intercostal muscles, sternum and diaphragm. The intercostal muscles contract, pulling the ribs outward and increase the space in the thoracic cavity. The lower part of the breast bone (sternum) is also raised.
The diaphragm contracts and becomes almost flat. Volume of the thoracic cavity is further increased.
Pressure on the lungs decreases. The lungs expand and their volume increases.
Atmospheric air rushes into the lungs through the respiratory passage to make the pressure equal. Thus the air enters the lungs.
Expiration : It takes place in following steps :-
It is a passive process.
The inter-costal muscles relax pulling the ribs inwards.
The diaphragm relaxes and again becomes dome-shaped. Thus collective contraction of intercoastal muscle and diaphragm reduces the volume of the thoracic cavity.
The pressure on the lungs increases. The lungs get compressed and the air in the lungs, rushes out through the external nares.
Alternate inspiration and expiration together form the respiratory cycle. It occurs 16-20 times per minute in man. Breathing is under the control of the medulla oblongata of the brain.
Regulation of breathing : Contraction of the diaphragm and intercostal muscles is under the control of medulla oblangata.
Breathing is controlled by rhythm centre of medulla and pneumotaxic centre of pons. Receptor cells in aorta carotid artery detect changes in CO2 and H+ concentration. Increases in their concentration stimulates pneumotaxis centre of pons, which activates rhythm centre. It brings about necessary changes in the rate of breathing.