Lecture Eight- Wednesday, January 20th, 2010
I. Spirometry: Measuring lung volumes
-Spirometer: upside down canister floating in water
-All the air in the lungs=total lung capacity
-Spirometer can measure only what you can breathe in and out. However, total lung
capacity cannot be measured because you’re not breathing out residual volume
-Clinically, it is useful to be able to measure the volume of air inhaled during
inspiration under a number of different circumstances. Subdivisions of the lung
volumes can be determined by means of a spirometer (Figure 8): a spirometer
measures volumes of inhaled or exhaled gas, so can be used to measure tidal
volume, vital capacity, inspiratory capacity, expiratory reserve volume, and
inspiratory reserve volume (Figures 8 & 9). It cannot be used to measure functional
residual capacity, total lung capacity, or residual volume.
see pictures in slides
J. Measurement of FRC
-Helium dilution technique: subject hooked up to a spirometer, helium is in the tube
(dissolved in air). Subject breathes out to FRC, that’s when valve is opened up and
subject breathes helium. Can use mass spectrometer to measure new [helium] which
is lower, because it is now over a larger volume (C2). V2=V1+FRC.
FRC can be measured by helium dilution (Figure 10). Let C1 be the helium
concentration in a spirometer of volume V1 and let the subject breath out to FRC.
Then, open the valve and ask the subject to breath in and out from the spirometer.
After equilibration with the subject's lungs, the concentration in the spirometer is C2.
Since the total amount of helium is conserved, we have:
C1 x V1 = C2 x (V1 + FRC)
FRC = (C1 x V1 / C2) - V1
*Note: Think of the lung as a balloon not as a jar.
see picture in slides
A. Minute ventilation vs Alveolar ventilation
The amount of air inspired into the lungs over some period of time is called
ventilation. Usually, it is measured for one minute, and therefore we call it minute
ventilation (VE). Therefore, VE is the amount of air inspired (or expired) during one
dot on top of VE-> VE = VT x f (tidal volume x frequency<-# of breaths/minute)
where VT is the tidal volume, and f is the number of breaths per minute. (A dot
above a symbol means a change with respect to time).
In a normal adult male, VT = 500ml, and f =12 breaths/minute. Therefore, VE =
6000ml/min. However, not all the air inhaled into the lungs reaches the gas
exchanging area (the respiratory zone). Some of the air remains in the conducting
airways, i.e. in the anatomical dead space (VD) (Figure 12). The volume of the