RESPIRATION. - This term refers to the exchange between the organism and the external environment of those substances that are found in the environment in the 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.
Not always do specifically differentiated structures devoted to these particular exchanges exist in animal organisms. In unicellular organisms, in the lowest invertebrates and, in 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 needs of the animal; 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 exchanges, the air (or the aquatic environment) is continuously changed over the respiratory surface, by means of appropriate movements. In mammals and in man such movements consist of the respiratory acts, each of which consists of an inspiration followed by an expiration; part of the pulmonary air is renewed at each respiratory act. The dilation of the lungs during inspiration and their deflation during expiration are determined by the increase and respectively the decrease in the capacity of the thoracic cage as a result of the contraction of the inspiratory muscles, and respectively of their elastic relaxation.
The succession of respiratory acts is due to the automatic rhythmic activity of respiratory centers located in the medulla oblongata; it is they that bring about, through efferent motor pathways, the contraction of the respiratory muscles. Such 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 coming from other peripheral organs.
Also the pressure of oxygen and of carbon diox-
(from C. H. Best and N. D. Taylor)
RESPIRATION - Diagram to illustrate the control of respiration; from the cerebral cortex, the carotid sinus, the aortic vascular zone, the lungs, the respiratory muscles (diaphragm, intercostal muscles, diaphragm) impulses depart which through the respiratory center regulate the respiratory movements.
carbon dioxide that prevails in the blood is of great importance for the regulation of the respiratory movements; these latter factors act both directly on the respiratory centers themselves (by way of the blood) and on the peripheral chemoreceptors at the carotid sinus and through them, by way of afferent pathways, on the respiratory centers (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 greater pressure of carbon dioxide excites the respiratory centers both directly and indirectly. Pulmonary ventilation increases in muscular work also as an effect of impulses arising from the contracting muscles and from the cerebral cortex and reaching the respiratory centers.
The 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 the aforesaid gases between the internal environment and the external environment. In the blood, however, these gases are not found only dissolved as gases, but for the most part chemically combined, and precisely oxygen combined with hemoglobin, the pigment of the red blood cells, and carbon dioxide combined above all as bicarbonate. These combinations confer on 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 used, and of carbon dioxide from the tissues, where it is formed, to the lungs where it is eliminated (cf. CIRCOLAZIONE).