Alveolar Gas Exchange Flashcards
Perfusion vs. ventilation
Perfusion is blood to lungs. Ventilation is gas to lungs.
Perfusion of the lungs
From the RV
5 L/min
Same CO as LV
Ventilation of the lungs
Volume of air reaching the alveoli.
V(dot)alv
4 L/min is average
Resistance equation
(8nL)/r^4
n = viscosity
L = length of tube
r = radius
Resistance is courtesy of:
Smooth muscles in the airways.
Why would one want to change airway resistance?
To send air to the right places.
Diffusion rate equation (J)
J = (SA x D x (P1-P2))/distance
P1-P2 is pressure gradient across alveolar membrane
D is diffusion coefficient
SA is surface area
Distance is thickness of alveolar barrier
Normal J for O2 and CO2 under resting conditions
250 ml/min for O2
200 ml/min for CO2
Diffusion of each gas is independent of one another.
Pulmonary capillaries can…
Vary in number due to necessity. If sitting, likely there are less open, etc. Play a big role in O2 exchange.
“Distance” of the barrier includes (4):
Fluid layer Alveolar epithelium Interstitial space Blood vessel wall Approx. 0.6 microns
As J increases:
Distance decreases, etc.
How can the distance increase?
Collagen can be deposited within the interstitial space increases the diffusion distance, but decreases the diffusion of gases across the barrier.
(i.e. interstitial lung disease)
D (diffusion coefficient for each gas) depends on:
The solubility of the gas in water (in the body) AND the moleculat weight of the gas.
O2 is less soluble in water than CO2.
CO2 weighs more than O2.
Dco2 vs Do2
Dco2 is 20x the Do2
The solubility more than counters the molecular weight difference.
P1-P2
Pressure gradient between Pao2 and Pvo2.
How does the RBC have a large “safety net” in the capillary?
RBCs spend about 0.75 seconds in a pulmonary capillary and only requires 0.25 seconds to reach equilibrium.
What happens to the “safety net” during exercise?
RBCs only spend about 0.25 seconds in the pulmonary capillary and still requires 0.25 seconds to reach equilibrium. The safety net is gone.
This causes problems to people with lung disease.
DLo2
Diffusion capacity of the lung for oxygen.
About 21 ml O2/min/mmHg at rest.
How is DLo2 measured?
Carbon monoxide.
CO binds to Hb avidly and does not allow it to dissolve in plasma.
Pt advised to inhale a single breathe of air with a small amount of CO added.
Correction factor for DLo2
DLo2 = 1.23 x DLco
Why is it unlikely for a patient to have problems with CO2 retention?
Because CO2 diffuses so easily and there is a significant safety margin. Pt would need to have very severe lung disease before they have problems with CO2 retention.
Equilibrium occurs almost immediately.
LaPlace’s law
P=(2T)/r P = pressure T = tension r = radius Therefore, as alveoli grow, the pressure decreases. Opposite for small alveoli.
Why is surfactant important?
It changes the surface tension. This is important because not all alveoli are the same size. Allows big and little alveoli to coexist.
Facts about surfactant
Produced via type II pneumocytes.
Composed of phospholipids and multiple proteins (mostly SPB).
Stored in intracellular lamellar bodies and secreted into alveolus.
How does surfactant affect surface tension?
It decreases T in proportion to the radius. This reduces the pressure in the small alveoli and removes the gradient, therefore, presenting the collapse.
