RESPIRATION PHYSIOLOGY: VENTILATION
Respiratory Muscle Contraction

RESPIRATORY MUSCLE CONTRACTION

A. Inspiration (inhalation)

1. active process (requires skeletal muscle contraction)

2. Diaphragm

a. creates pressure difference between intrapleural space and abdominal cavity (abdominal pressure usually assumed to be negligible)

b. Δ P ~ Tension in diaphragm  ( Laplace-Young equation: Δ P ~ T / r, where r = radius of curvature;  note inefficiency of flat diaphragm)

c. responsible for about 2/3 of inspired volume in quiet breathing   (diaphragm descends a few cm in quiet breathing, more in vigorous breathing;  ==> )

d. last respiratory activity to be effected by CNS depression ("abdominal breathing")

2. Intercostal muscle contraction, particularly external intercostals

a. raise rib cage, thus increasing chest anterior-posterior dimension

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B. Expiration (exhalation)

1. Quiet breathing

a. passive
b. due to elastic recoil upon relaxation of inspiration muscles
c. normal breathing at rest (but too slow for exercise)

2. Forced expiration

a. intercostal muscles, particularly internal intercostals (lower rib cage, decreasing anterior-posterior thoracic distance)

b. abdominal muscles (increase abdominal pressure, forcing diaphragm up)

C. Force Magnitude

1. Quiet breathing

a. inspiration: muscle generate the equivalent of about -5 cmH2O (= 3.5 mmHg, below atmospheric)

b. expiration: passive elastic forces generate about 5 cmH2O; decrease to 0 cmH2O as chest assumes relaxed position

Note:  Low pressures (compared to the cardiovascular system) are adeqate at rest because (1) air has low viscosity compared with blood; (2) airways are larger than blood vessels; (3) the normal respiratory system has high compliance)

2. Maximum respiratory effort

a. inspiration: approx. -100 cmH2O
b. expiration: approx. +100 cmH2O

Note: Maximum respiratory effort requires extreme work and interferes with cardiac output, particularly venous return (because of high intrathoracic pressure)

Valsalva maneuver: maximum expiratory effort while holding the glottis closed; develops maximum expiratory pressure