EXTERNAL
RESPIRATION
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,