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Mechanics and Regulation of Respiration

Breathing, or respiration, consists of two complementary processes: inhalation or inspiration, during which air enters the lungs, and exhalation or expiration, during which air leaves the lungs. Inspiration occurs when the brain sends a signal to the diaphragm, a sheet-like muscle at the base of the thoracic cavity, and the intercostal muscles, located between the ribs. When the diaphragm contracts, the overall volume of the thoracic cavity increases, causing the air pressure in the lungs to decrease. Air automatically flows into the lungs due to the external air pressure. When the diaphragm relaxes, the thoracic cavity returns to its initial size, and air is forced out of the lungs. Thus we can see that inspiration is active while expiration is passive.


The respiratory rate is controlled by centers in the brainstem, which normally send out rhythmic impulses to the diaphragm causing a regular cycle of inspiration and expiration to occur involuntarily. (To some extent, breathing can be brought under conscious control.) The respiratory center neurons are also sensitive to information about the chemical composition of the blood, and this allows them to alter the rate of breathing when conditions dictate, as during exercise or physical/emotional stress. The respiratory neurons are mainly sensitive to the pH of the blood (the concentration of H+ ions, which as we shall soon see, is intimately related to the concentration of carbon dioxide). As metabolic activity increases, the levels of carbon dioxide and hydrogen ions in the blood rise, triggering an increase in the rate of breathing to restore normal levels of these substances. Whenever carbon dioxide concentration increases, oxygen concentration decreases, so there is really no need for the brain to monitor both parameters. There are receptors called the aortic and carotid bodies, however, which are sensitive to the levels of oxygen in the blood; they transmit this information to the respiratory centers of the brain, enabling “fine-tuning” of the system.

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