Regulation of Respiration
Whenever the need for oxygen in the body increases, ventilation of the respiratory organs also increases. In the same way whenever the level of oxygen in the medium falls, the ventilation must increase or the body should be able to extract more oxygen from the respired air or both these processes must take place. In lungs of warm-blooded animals i.e. birds and mammals the ventilation is regulated primarily by the amount of carbon dioxide in the lung air. If we add more carbon dioxide to the inhaled air, there is a rapid increase in ventilation, even if carbon dioxide content in inhaled air is increased to that found normally in the lungs (5%), respiratory ventilation volume increases several folds. In higher concentrations, carbon dioxide becomes dangerous. Oxygen on the other hand, has a much smaller effect on ventilation, if we reduce the oxygen concentration from 21% to 18.5% there is virtually no effect.
The rhythmic contractions of the diaphragm and of the intercostal muscles are controlled by a respiratory centre which is located in the area of the medulla - oblongata and pons of the brain. There are separate neurons for inspiration and expiration that work alternatively.