Review Lung Volumes
Tidal Volume (Vt)
 volume moved during either an inspiratory
or expiratory phase of each breath (L)
Inspiratory Reserve Volume (IRV)
 Reserve ability for inspiration (L)
 Volume of extra air that can be inhaled after
a normal inhalation (L)
Expiratory Reserve Volume (ERV)
 Volume of extra air that can be exhaled after
a normal exhalation (L)
Forced Vital Capacity (FVC or VC)
 Maximal volume of air that can be moved in
one breath, from full inspiration to full
expiration (L)
 SVC may be greater due to air trapping
Residual Volume (RV)
 Volume of air remaining in lungs following a
maximal exhalation (L)
 Usually increases with age
 Allows for uninterrupted exchange of gases
Functional Residual Capacity (FRC)
 Volume of air in the lungs at the end of
a normal tidal exhalation (end tidal) (L)
 Important for maintaining gas pressures
in the alveoli
Total Lung Capacity (TLC)
 Maximal amount of air in the lungs
 RV + VC = TLC (L)
Maximal Ventilatory Volume (MVV or
MBC)
 Maximal amount of air that can be moved in
one minute (L/min)
Pulmonary Ventilation
 @ rest, usually ~ 6 l/min
 Increase due to increases in rate and depth
 Rate: inc. 35-45 breaths/min, elite athletes:
60-70 breaths/min, max. ex.
 Vt 2 lit, Ve > 100 lit/min
 Vt may reach 2 lit, still 55-65% if VC (Tr and
UNTr)
Anatomic Dead Space
 Volume of air that is in conducting
airways, not in alveoli, not involved in
gas exchange
 Nose, mouth, trachea, other nondiffusible conducting portions of the
respiratory tract
 Air is identical to ambient air, but
warmed, fully saturated with water
vapor
 350 ml of 500 ml tidal volume will enter into
and mix with existing alveolar air
– 500 ml will enter alveoli, but only 350 ml is fresh
air
– 350 ml is about 1/7 of air in alveoli
– This allows for maintenance of composition of
alveolar air (concentration of gases)
Dead space versus tidal volume
 Anatomic dead space increases with
increases in tidal volume
 Increase in dead space is still less than
increase in tidal volume
 Therefore, deeper breathing allows for more
effective alveolar ventilation, rather than an
increase in breathing rate
Physiologic Dead Space
 Gas exchange between the alveoli and
blood requires ventilation and perfusion
matching: V/Q
 @ rest, 4.2 l of air for 5 l of blood each
minute in alveoli, ratio ~.8
 With light exercise, V/Q ratio is
maintained
 Heavy exercise: disproportionate
increase in alveolar ventilation
 When alveoli do not work adequately during
gas exchange, it is due to
– Under perfusion of blood
– Inadequate ventilation relative to the size of the
alveoli
 This portion of alveolar volume with poor V/Q
ratio is physiologic dead space
 Small in healthy lung
 If physiologic dead space >60% of lung
volume, adequate gas exchange is
impossible
Techniques of assessing lung
volumes:
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Spirometry (cannot determine RV and FRC)
Helium dilution
Oxygen washout
Plethysmograph (what we have)
– based on Boyle’s Law: PV = P1V1
Alveolar Ventilation
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> 300 million alveoli
elastic, thin-walled membranous sacs
surface for gas exchange
blood supply to alveolar tissue is greatest to
any organ in body
 are connect to each other via small pores
 capillaries and alveoli are side by side
 at rest, 250 ml of O2 leave alveoli to blood,
and 200 ml of CO2 diffuse into alveoli
 during heavy exercise, (TR athletes) 25X
increase in quantity of O2 transfer
Gas exchange in the lungs
 molecules of gas exert their own partial
pressure
 total pressure = mixture of the sum of the
partial pressures
 Partial pressure = % concentration X total
pressure of the gas mixture
Ambient Air @ sea level
 Oxygen: 20.93% X 760 mm Hg = 159 mm
Hg
 Carbon Dioxide: 0.03% X 760 mm Hg = 0.2
mm Hg
 Nitrogen: 79.04% X 760 mm Hg = 600 mm
Hg
 Partial pressure is noted by P in front, e.g.,
PO2 = 159
Tracheal Air
 as air enters respiratory tract, it is
completely saturated with water vapor
 water vapor will dilute the inspired air
mixture
 @ 37 degrees C, water exerts 47 mm
Hg
 760 - 47 = 713
 Recalculate pressures, PO2 = 149
Alveolar Air
 different composition than tracheal air
 b/c of CO2 entering alveoli from blood
and O2 leaving alveoli
 average PO2 in alveoli ~103 mm Hg
 PCO2 = 39
 these are average pressures, it varies
with the ventilatory cycle, and the
ventilation of a portion of the lung
 FRC is present so that incoming breath has
minimal influence on composition of alveolar
air
 therefore, partial pressures in alveoli
remains stable
Gas Transfer in lungs
 PO2 is about 60 mm Hg higher in alveoli
than capillaries
 b/c of diffusion gradient, oxygen will
dissolve and diffuse through alveolar
membrane into capillary
 CO2 pressure gradient is smaller, ~ 6
mm Hg
 adequate exchange still occurs b/c of
high solubility of CO2
 Nitrogen is not used nor produced, PN
is relatively unchanged
 Equilibrium is rapid, ~ 1 sec, the
midpoint of blood’s transit through the
lungs
 during exercise, transit time decreases
~ 1/2 of that seen at rest
 during exercise, pulmonary capillaries
can increase in blood volume 3X resting
 this maintains the pressures of oxygen
and carbon dioxide
Gas Transfer in the Tissues
 Partial pressures can be very different than
those seen in the lung
 @ rest, PO2 in fluid outside a muscle cell
are rarely less than 40 mm Hg
 PCO2 is about 46 mm Hg
 During exercise, PO2 may drop to 3 mm Hg,
and PCO2 rise to 90 mm Hg
 O2 and CO2 diffuse into capillaries, carried to
heart and lungs, where exchange occurs
 body does not try to completely eliminate CO2
 blood leaves lungs with PO2 of 40 mm Hg,
this is about 50 ml of carbon dioxide/100ml of
blood
 PCO2 is critical for chemical input for control
of breathing (respiratory center in brain)
 By adjusting alveolar ventilation to metabolic
demands, the composition of alveolar gas will
stay constant, even during strenuous
exercise (which can increase VO2 and CO2
production by 25X)