RESPIRATION. — This term denotes the exchange between the organism and the external environment of those substances 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 product of these oxidations.
Specific structures devoted to these particular exchanges do not always exist in animal organisms. In unicellular organisms, in the lowest invertebrates and, during the early stages of development, also in higher organisms, the external surface of the organism is the surface through which the passage of O₂ and CO₂ takes place. As one ascends in zoological organization 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 animal’s needs; such is the significance of the lungs and tracheae for aerial r., and of the gills for r. in an aquatic environment.
To facilitate gaseous exchange, the air (or the aquatic environment) is continually renewed over the respiratory surface by means of suitable movements. In mammals and in human beings, these movements consist of respiratory acts, each of which comprises an inspiration followed by an expiration; part of the pulmonary air is renewed with every respiratory act. The expansion of the lungs during inspiration and their collapse during expiration are determined by the increase and, respectively, the decrease in the capacity of the thoracic cage, resulting from the contraction of the inspiratory muscles and, respectively, from 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, through efferent motor pathways, cause 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. FISIOLOGIA) excited by mechanical stimuli arising in the lungs themselves and by afferent pathways originating from other peripheral organs.
The pressure of oxygen and carbon di-
(by C. H. Best and N. D. Taylor)
RESPIRAZIONE – Diagram illustrating the control of respiration; excitations originate in the cerebral cortex, the carotid sinus, the aortic vascular zone, the lungs, and the respiratory muscles (diaphragm, intercostal muscles, diaphragm), and, passing through the respiratory centre, regulate respiratory movements.
The pressure of oxygen and carbon dioxide prevailing in the blood is of great importance for the regulation of respiratory movements; these factors act both directly upon the respiratory centres themselves (through the bloodstream) and upon the peripheral chemoreceptors located at the carotid sinus, and through them, by way of afferent pathways, upon the respiratory centres (fig. in 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 carbon-dioxide pressure excites the respiratory centres both directly and indirectly. Pulmonary ventilation also increases during muscular work as a result of excitations originating in the contracting muscles and in 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 the partial pressure of these gases between the internal and external environments. In the blood, however, these gases are not found merely dissolved as gases, but are for the most part chemically combined: oxygen is combined with haemoglobin, the pigment of the red blood cells, and carbon dioxide is combined chiefly in the form of 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).