Ventilation and Diffusion Flashcards
Partial pressure equation
Pi Gas = Fgas (Patm - P H2O)
take into consideration dry air when doing calculation
FO2 - usually 0.21
tidal volume
normal breathing
vital capacity
maximum inspiration and expiration
residual volume
after maximum expiration some volume leftover
functional residual capacity
volume after a normal expiration
what cannot be measured spirometry?
residual or FRC
helium dilution method
use known [helium]
after some breaths, concentration eqilibriate
C1 x V1 = C2 x (V1 + V2) = C2 x (V1 + FRC)
body plethysmography
P1 x V1 = P2 x (V1 - deltaV)
with inspiratory effort against closed airway:
- increases volume of lung
- decreases airway pressure
- box pressure increases
- box volume decreases
anatomic dead space
volume of conducting airways
alveolar dead space
alveoli containing air but not participating in gas exchange
physiologic dead space
total dead space for system
space that does not eliminate CO2
alveolar ventilation
room air delivered to respiratory zone per minute
Va = (Vt - Vd) x f
Va = alveolar ventilation Vt = tidal volume Vd = dead space f = respiratory rate
Bohr’s method for alveolar ventilation
Vd / Vt = PaCO2 - PeCO2 / PaCO2
based around all expired CO2 derived from alveolar space and none from dead space
regional differences in ventilation?
lower lung - more ventilation (gravity)
lower zones - more compliant
upper zones - less compliant
changes in supine or laying on side
Fick’s Law of Diffusion
Vgas ~ A x D x (P1-P2) / T A = area D = sol / sqrt(MW) **diffusion constant** P1-P2 = pressure difference T = thickness
what diffuses faster?
CO2
because of greater solubility
what makes up respiratory membrane?
alveolar space surfactant water layer alveolar epithelial cells alveolar basement membrane alveolar interstitium capillary basement membrane capillary endothelium capillary lumen filled with fluid red blood cell
diffusion limited
molecule getting into blood dependent on diffusion properties of blood-gas barrier and not amount of blood available
ex/ carbon monoxide
perfusion limited
molecule getting into blood dependent on amount of blood available
-as it diffuses, sharp increase in partial pressure
ex/ nitrous oxide
O2 gas diffusion?
resting - perfusion limited
some conditions - diffusion limited
-exercise, thick blood-gass barrier, reduced PiO2
uptake of O2 in capillary?
normal conditions - PaO2 equals PAO2 when RBC 1/3 through capillary bed
decreasing PiO2 - increased time to equilbriate
CO2 transfer
diffusion of CO2 is 20x higher than O2
-however, concentration gradient is lower
hypercapnia is rare
-there is potential for elevated levels of CO2 due to thickening of blood-gas barrier
diffusing capacity
D-L
distance gas travels across membrane into blood and time it takes to react with hemoglobin
measured by uptake of CO in lung
-units mL / min x mmHg
normal ~ 25 mL / min x mmHg
pathology and D-L
pulmonary fibrosis - destruction of capillaries
COPD - loss of lung elastic and capillaries and Hb
loss functional lung tissue - decreased SA and Hb
anemia - fall in Hb
