BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.5. Gas exchange in mammals

9.5.5. Control of breathing

Normally, a person is unaware of their breathing because the process is regulated independently of conscious will. To some extent, however, breathing can be controlled voluntarily, as discussed below.

Involuntary regulation of breathing is carried out by the respiratory centre, located in the Medulla Oblongata (part of the Hindbrain) (Fig. 9.28). The ventral (lower) part of the respiratory centre is responsible for stimulating inspiration and is called the inspiratory centre. Stimulation of this centre increases the rate and depth of inspiration. The dorsal (upper) part and both lateral parts inhibit inspiration and stimulate expiration; they are collectively termed the expiratory centre. The respiratory centre connects to the intercostal Muscles via the intercostal nerves, and to the Diaphragm via the phrenic nerves. The bronchial tree (the Bronchi and bronchioles combined) is innervated by the Vagus nerve (Fig. 9.28). Rhythmically recurring nerve impulses directed to the diaphragm and intercostal muscles drive ventilatory movements.

The expansion of the Lungs during inspiration stimulates stretch receptors (proprioceptors) located within the bronchial tree, which then send an increasing number of impulses via the vagus nerve to the expiratory centre. This temporarily suppresses the inspiratory centre and inspiration itself. The external intercostal muscles subsequently relax, the stretched lung tissue recoils elastically, and expiration occurs. Following expiration, the stretch receptors in the bronchial tree are no longer stimulated. As a result, the expiratory centre switches off, allowing inspiration to begin anew. This entire cycle repeats continuously and rhythmically throughout the Organism's life. Forced breathing involves the action of the internal intercostal muscles.

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Fig. 9.28. Regulation of breathing.

The basic rhythm of breathing is maintained by the respiratory centre in the medulla oblongata, even if all incoming nerves to it are severed. Under normal conditions, however, various influences modify this baseline rhythm. The primary factor controlling breathing rate is not the oxygen concentration in the Blood, but rather the concentration of CO₂. When CO₂ levels rise (for example, during physical exertion), chemoreceptors in the carotid and aortic bodies (Fig. 9.28) within the Circulatory system send nerve impulses to the inspiratory centre. Chemoreceptors are also located within the medulla oblongata itself. From the inspiratory centre, impulses travel via the phrenic and intercostal nerves to the diaphragm and external intercostal muscles, leading to more frequent contractions and, consequently, an increased breathing rate. The accumulation of CO₂ in the body can be highly detrimental. When CO₂ combines with Water, it forms an acid capable of denaturing Enzymes and other Proteins. Consequently, organisms have evolved a rapid physiological response to any increase in CO₂ concentration. If atmospheric CO₂ concentration increases by just 0.25%, pulmonary ventilation doubles. To achieve a similar effect, oxygen concentration in the air would need to drop from 20% to 5%. Oxygen concentration also affects breathing; however, under normal conditions, oxygen is always in adequate supply, making its regulatory impact relatively minor. Chemoreceptors sensitive to oxygen concentration are located in the medulla oblongata, carotid bodies, and aortic bodies, just like the CO₂ receptors.

Within certain limits, the rate and depth of breathing can be controlled voluntarily, as demonstrated by our ability to "hold our breath." We resort to voluntary breathing control during heavy exertion, speaking, singing, sneezing, and coughing. In these cases, impulses generated in the cerebral hemispheres are transmitted to the respiratory centre, which executes the corresponding actions.

The regulation of inspiration via stretch receptors and chemoreceptors is a classic example of negative feedback. The voluntary activity of the cerebral hemispheres can override this mechanism.