RESPIRATION. – By this term is meant the exchange between an organism and the external environment of those substances which are found in the environment in a gaseous state, namely oxygen (O₂) and carbon dioxide (CO₂).
Oxygen is the indispensable element for the oxidative degradation of organic substances, and carbon dioxide is, together with water, the final end-product of these oxidations.
Not always do animals possess specifically differentiated structures adapted to these particular exchanges. In unicellular organisms, in the lower invertebrates, and in the early stages of development even in higher organisms, the external surface of the body is the surface through which the passage of O₂ and CO₂ occurs. As one ascends in the zoological scale and in the size of organisms, the external surface becomes increasingly inadequate to ensure respiratory exchanges, and respiratory organs develop with respiratory surfaces suited to the needs of the animal; such are the lungs and tracheae for aerial respiration, and the gills for respiration in an aqueous environment.
To facilitate gaseous exchanges, the air (or the aqueous environment) is continuously changed over the respiratory surface by means of appropriate movements. In mammals and in man, such movements consist in respiratory acts, each of which consists of an inspiration followed by an expiration; part of the pulmonary air is renewed with each respiratory act. The expansion of the lungs during inspiration and their collapse during expiration are determined by the increase and, respectively, the decrease of the capacity of the thoracic cavity as a result of the contraction of the inspiratory muscles, and, respectively, their elastic relaxation.
The succession of respiratory acts is due to the automatic rhythmic activity of respiratory centres located in the medulla oblongata; these centres provoke, through efferent motor pathways, the contraction of the respiratory muscles. This activity, however, and consequently the frequency and depth of the respiratory acts, is regulated by afferent pathways (v. PHYSIOLOGY) which are stimulated by mechanical stimuli arising in the lungs themselves and by afferent pathways coming from other peripheral organs.
Also of great importance for the regulation of respiratory movements is the pressure of oxygen and carbon dioxide in the blood; these factors influence the respiratory movements both directly on the respiratory centres themselves (via the bloodstream) and on the peripheral chemoreceptors located at the carotid sinus, and through them, via afferent pathways, on the respiratory centres (fig. at col. 800).
For example, during muscular work respiratory activity increases, and this increase is due in part to the rise in the pressure of carbon dioxide in the blood caused by the intensification of oxidative processes; the higher pressure of carbon dioxide stimulates both directly and indirectly the respiratory centres. Pulmonary ventilation also increases during muscular work as a result of stimuli originating from the contracting muscles and from the cerebral cortex which reach the respiratory centres.
Gases diffuse from the lungs into the blood (oxygen) and from the blood into the lungs (carbon dioxide) because of the difference in partial pressure of these gases between the internal and external environments. In the blood, however, these gases are not found only dissolved as gases, but for the most part are chemically combined, namely oxygen combined with hemoglobin, the pigment of the red blood cells, and carbon dioxide combined especially as bicarbonate. These combinations give the blood a much greater capacity for these gases and greatly facilitate the transport of oxygen from the lungs to the tissues where it is utilized, and of carbon dioxide from the tissues where it is formed to the lungs where it is eliminated (cf. CIRCULATION).