Respiration is under both nervous and chemical regulation.
Nervous Regulation of Respiration :
Normal quiet breathing occurs in voluntarily. Adult human beings breathe 12 to 14 times per minute, but human infants breathe about 44 times per minute. In each breath in human beings, inspiration accounts for about two and expiration for about three seconds.
The ‘respiratory centre’ is composed of groups of neurons located in the medulla oblongata and pons varolii. The respiratory centre regulates the rate and the depth of the breathing. The respiratory centre is divided into three major collections of neurons.
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1. Dorsal Respiratory Group: It is located in dorsal portion of the medulla oblongata. Nerve impulses from the dorsal respiratory group stimulate the muscles of the diaphragm (primary inspiratory muscle) to flatten the latter and the external intercostal muscles to raise the ribs. This brings about inspiration. Thus the dorsal respiratory group mainly causes inspiration.
2. Ventral Respiratory Group: It is located in the ventroleteral part of the medulla oblongata. It issues signals for both inspiration (to diaphragm and external intercostal muscles) and expiration (to internal intercostal muscles and muscles. Of abdominal wall. Thus the ventral respiratory group can cause either inspiration or expiration, depending upon which neurons in the group are stimulated.
3. Pneumotoxic Centre: It is located in the dorsal part of ponsvarolii. It issues impulses to all the neurons of the dorsal respiratory group and only to the inspiratory neurons of ventral respiratory group. These impulses regulate the time of inspiration in both normal and abnormal breathing. The primary effect of these is to control the ‘switch off point of inspiratory signal. Therefore, the function of the pneumatic centre is primarily to limit inspiration.
There is another strange centre called the apneustic centre, located in the lower part of the ponsvarolii. It is thought that it operates in association with the pneumotaxic centre to control the depth of inspiration.
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Chemical Regulation of Respiration :
The largest numbers of chemoreceptors are located in the corotid bodies. However, a sizable number of chemoreceptors are in the aortic bodies. The carotid bodies are located bilaterally in the bifurcation of the common carotid arteries and their afferent nerve fibres pass through glossopharynageal cranial nerves and thence to the dorsal respiratory area of the medulla oblongata. The aortic bodies are located along the arch of the aorta and their afferent nerve fibres pass through the vagi (Sing, vagus) cranial nerves and hence to the dorsal respiratory area.
These chemoreceptors of carotid and aortic bodies are stimulated by an increase in carbon dioxide concentration and by an increase in hydrogen ion concentration (pH) in the arterial blood. Increased CO2 lowers the pH resulting acidosis. These chemoreceptors send signals to the in spiratory and expiratory centres. Thus rate of breathing is increased.