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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 behavioural
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 behaviours (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. Click here to learn more about CO2 measurement and acid-base balance.
Copyrighted by Behavioral
Physiology Institute, |