Alveolar Gas Exchange

Question 1

Perfusion vs. ventilation

Answer 1

Perfusion is blood to lungs. Ventilation is gas to lungs.

Question 2

Perfusion of the lungs

Answer 2

From the RV
5 L/min
Same CO as LV

Question 3

Ventilation of the lungs

Answer 3

Volume of air reaching the alveoli.
V(dot)alv
4 L/min is average

Question 4

Resistance equation

Answer 4

(8nL)/r^4
n = viscosity
L = length of tube
r = radius

Question 5

Resistance is courtesy of:

Answer 5

Smooth muscles in the airways.

Question 6

Why would one want to change airway resistance?

Answer 6

To send air to the right places.

Question 7

Diffusion rate equation (J)

Answer 7

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

Question 8

Normal J for O2 and CO2 under resting conditions

Answer 8

250 ml/min for O2
200 ml/min for CO2
Diffusion of each gas is independent of one another.

Question 9

Pulmonary capillaries can…

Answer 9

Vary in number due to necessity. If sitting, likely there are less open, etc. Play a big role in O2 exchange.

Question 10

“Distance” of the barrier includes (4):

Answer 10

Fluid layer
Alveolar epithelium
Interstitial space
Blood vessel wall
Approx. 0.6 microns

Question 11

As J increases:

Answer 11

Distance decreases, etc.

Question 12

How can the distance increase?

Answer 12

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)

Question 13

D (diffusion coefficient for each gas) depends on:

Answer 13

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.

Question 14

Dco2 vs Do2

Answer 14

Dco2 is 20x the Do2

The solubility more than counters the molecular weight difference.

Question 15

P1-P2

Answer 15

Pressure gradient between Pao2 and Pvo2.

Question 16

How does the RBC have a large “safety net” in the capillary?

Answer 16

RBCs spend about 0.75 seconds in a pulmonary capillary and only requires 0.25 seconds to reach equilibrium.

Question 17

What happens to the “safety net” during exercise?

Answer 17

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.

Question 18

DLo2

Answer 18

Diffusion capacity of the lung for oxygen.

About 21 ml O2/min/mmHg at rest.

Question 19

How is DLo2 measured?

Answer 19

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.

Question 20

Correction factor for DLo2

Answer 20

DLo2 = 1.23 x DLco

Question 21

Why is it unlikely for a patient to have problems with CO2 retention?

Answer 21

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.

Question 22

LaPlace’s law

Answer 22

P=(2T)/r
P = pressure
T = tension
r = radius
Therefore, as alveoli grow, the pressure decreases. Opposite for small alveoli.

Question 23

Why is surfactant important?

Answer 23

It changes the surface tension. This is important because not all alveoli are the same size. Allows big and little alveoli to coexist.

Question 24

Facts about surfactant

Answer 24

Produced via type II pneumocytes.
Composed of phospholipids and multiple proteins (mostly SPB).
Stored in intracellular lamellar bodies and secreted into alveolus.

Question 25

How does surfactant affect surface tension?

Answer 25

It decreases T in proportion to the radius. This reduces the pressure in the small alveoli and removes the gradient, therefore, presenting the collapse.