During inspiration the pressure inside the lungs (the
intrapulmonary pressure) decreases to -1 to -3 mmHg compared
to the atmosphere. The variation is related to the forcefulness and
depth of inspiration. During expiration the intrapulmonary pressure
increases to +1 to +3 mmHg compared to the atmosphere.
The pressure oscillates around zero or atmospheric pressure.
The intrapleural pressure is always negative compared to the
atmosphere. This is necessary in order to exert a pulling action on
the lungs. The pressure varies from about -4 mmHg at the end of
expiration, to -8 mmHg and the end of inspiration.
Respiratory output is determined by the minute volume, calculated
by multiplying the respiratory rate time the tidal volume.
Minute Volume = Rate (breaths per minute) X Tidal
Volume (ml/breath)
Rate of respiration at rest varies from about 12 to 15 bpm. Tidal
volume averages 500 ml
•Assuming a rate of 12 breaths per minute and a tidal volume of
500, the restful minute volume is 6000 ml. Rates can, with strenuous
exercise, increase to 30 to 40 bpm and volumes can increase to
around half the vital capacity.
Not all of this ventilates the alveoli, even under maximal conditions.
The volume air in conducting zone is about 150 ml (dead space) and
does not extend into the respiratory zone.
•The Alveolar Ventilation Rate, AVR, is the volume per
minute ventilating the alveoli and is calculated by multiplying
the rate times the (tidal volume-less the conducting zone
volume).
AVR = Rate X (Tidal Volume - 150 ml)
For a calculation using the same restful rate and volume as
above this yields 4200 ml.
•The tendency of the lungs to expand, called compliance
or distensibility, is due to the pulling action exerted by
the pleural membranes. Expansion is also facilitated by
the action of surfactant in preventing the collapse of the
alveoli.
