External respiration is about the mechanics of breathing, getting oxygen into the lungs and regulating it in a way that ensures its diffusion into the blood. It is also about ensuring proper diffusion of CO2 from the blood into the lungs, and its subsequent excretion into the atmosphere. Breathing mechanics include breathing rate, breathing depth (volume of air in a single breath), breathing rhythmicity (holding, gasping, sighing), locus of breathing (“chest” and diaphragm), breathing resistance (nose and mouth), and accessory muscle activity (muscles other than the diaphragm).
Breathing mechanics, regardless of their potential acrobatic configurations that may serve other behavioral objectives (e.g., talking), must be coordinated in a way that ensures, from moment to moment, proper distribution of CO2 for continued regulation of acid-base balance. Some CO2 is excreted, and some of it is returned to systemic circulation. At rest, only about 12 to 14 percent of CO2 arriving in the lungs is actually excreted, while during exercise the percentage excreted is radically increased. In both cases, however, the amount reallocated to systemic circulation is the same: arterial PCO2 (PaCO2) is maintained between 35 and 45 mmHg, keeping pH within its normal range (7.35 to 7.45). In the case of lactic acidosis as a result of anaerobic glycolysis during severe exercise, however, compensatory overbreathing may take place (PCO2 levels below 35 mmHg).
Click here to learn more about compensatory overbreathing.
The diaphragm is the primary inspiratory muscle. Inspiration, at rest, typically includes only the diaphragm, and the external intercostal muscles. As the diaphragm contracts, the viscera are moved aside, and the lungs are drawn downward into the abdominal cavity, creating the negative pressure necessary for inhalation. The ease with which lung tissue can be expanded into the thoracic cavity is about lung compliance. Expiration, at rest, is passive; no muscle contractions need be involved, only the relaxation of the diaphragm and the external intercostals.
Accessory breathing muscles, used to assist external breathing, include abdominal, chest, back, and neck muscles useful during exercise, talking, singing, coughing, and so on, e.g., abdominal, internal intercostal, trapezius, pectoral, scalene, and sternocleidomastoid muscles. “Chest breathing” has reference to the use of accessory muscles, and “diaphragmatic breathing” has reference to breathing dominated by the diaphragm and external intercostal muscles. When the use of accessory muscles is counterproductive, “chest breathing” may become problematic.
Chest breathing, at rest, may mean (1) using accessory muscles when they are not required, (2) using accessory muscles to do the work of the diaphragm, and worst of all, (3) using accessory muscles at the expense the diaphragm, e.g., “reverse” breathing. During reverse breathing the diaphragm contracts and pulls the lungs downward to inflate the lungs, while the abdominals contract and push upwards against the diaphragm (an action normally reserved for forced exhalation during exercise or talking).
Inefficient and unnecessary use of accessory muscles usually constitutes “effortful breathing,” which for psychological reasons increases the likelihood of overbreathing, leading to misallocation of carbon dioxide and disturbed acid-base balance. “Effort” typically translates into worry and anxiety about “getting enough air,” vicious-circle breathing behaviors (mechanics), self-defeating solutions (e.g., taking deep breaths), and misinterpretations about personal physiology. Unfortunately the solutions are usually counterintuitive: small breaths, slow breaths, and long transition times between breaths.
Behavioral Physiology Institute,