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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
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. Click here to learn more
about CO2 measurement and acid-base
balance. Copyrighted by Behavioral Physiology Institute, Santa
Fe, New Mexico USA |