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Physiology 3120 Lecture Notes - Functional Residual Capacity, Breathing, Tidal Volume

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Tom Stavraky

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Human Physiology
Wednesday, February 3, 2010
“Respiratory III”
Lung Volumes & Deadspace
Measure volumes by spirometry
Hook person up to tube
Every turn person inhales, bell moves down and pen moves up; vice-versa for exhalation
Volume definitions (using spirometry as basis)
Tidal volume: volume inhaled with each breath; usually around 500mL
Vital capacity: volume of air that can be forcibly exhaled after maximal inspiration
Residual volume: volume of air remaining in the lungs after maximal expiration
Cannot be measured with a spirometer (only measures movement of pen in response to
Functional residual capacity: volume of air remaining in lungs after normal expiration
Inspiratory reserve volume: volume of air that can be forcibly inhaled following normal
inspiration; becomes smaller as you exercise
Total lung capacity: volume of air in the lungs at the end of a maximal inspiration
Minute volume/pulmonary ventilation: volume inhaled per minute; calculated by TV x
frequency of respiration
Forced expiratory volume-1 second (FEV-1sec): fraction of vital capacity expired in one
About 80% in 1 sec normally
In asthmatic, this may decrease (<60%); airways are constricted
Maximum voluntary ventilation (MVV); a.k.a. maximal breathing capacity: volume that
can be moved into and out of the lung in one minute by voluntary effort (about 125-170L per
TLC, RV, and FRC cannot be measured using spirometry, but can use the helium dilution method
Doesn’t diffuse into blood, so doesn’t cross blood-gas barrier (stays in the lung)
By opening up valve, helium distributes itself in the lungs
Using the concentration/volume relationship, we can calculate these values
Airways don’t participate in gas exchange; referred to as anatomic deadspace; physiological
deadspace is the volume of air entering the lung that doesn’t participate in gas exchange
In normal subjects, there is no difference
In some diseases, there may be increased deadspace
Impact on alveolar ventilation
Alveolar ventilation is pulmonary ventilation minus deadspace ventilation
With slower, larger breaths, alveolar ventilation goes up
Bohr equation
Assumes that all exhaled CO2 comes from gas exchange
Partial pressure of CO2 in deadspace is 0mmHg, so can use proportion of partial
pressures in deadspace and inhaled air to obtain deadspace volume
oCan also assume that PCO2 is the same in the alveoli as in the arteries
Vd/Vt = (PACO2 – PECO2)/PACO2
On average
Deadspace (anatomic) = 150mL
Alveolar gas = 3000mL
Minute volume = 7500mL/min
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