Pulmonary Diffusion

Question 1

What is the partial pressure of water in the body?

Answer 1

47 mmHg

Question 2

What is the P_A O2 and P_A CO2 in steady state alveolar air?

Answer 2

100 mmHg for O2. 40 mmHg for CO2.

Question 3

What is steady state arterial blood P_a O2 and P_a CO2?

Answer 3

100 mmHg for O2. 40 mm Hg for CO2

Question 4

What is steady state venous blood P_V O2 and P_V CO2?

Answer 4

40 mmHg for O2 and 46 mmHg for CO2.

Question 5

What does Henry’s law of solubility state? What is the law?

Answer 5

That when a liquid and gas phase are in equilibrium at a given temperature the concentration of gas in solution is directly proportional to the partial pressure of the gas.
Ci = ai Pi
Alpha is the solubility coefficient and P is the partial pressure above the liquid.

Question 6

What does the solubility coefficient depend on?

Answer 6

The specific gas, the liquid the gas is dissolved in and the temperature.

Question 7

The dry gas fraction F is used for what only? The concentration of C is used only for what?

Answer 7

F is for the gas phase only. C is for the solution phase.

Question 8

How are Henry’s law and Fick’s law different?

Answer 8

Henry’s law describes the equilibrium between gaseous and liquid phases, while Fick’s law describes the rate of movement of gas between two compartments containing gases of differing partial pressures.

Question 9

What is the formula for Fick’s law?

Answer 9

V (dot) = DA/T (P_A - P_C). V is the flow of a gas. A is the alveolar surface area available for diffusion (~50-100m^2). T=the thickness of the alveolar membrane (<5 microns). (P_A - P_C) = the difference in partial pressure (mmHg) of the gas between the alveoli and the pulmonary capillary blood. D= the diffusion coefficient of the gas.
D_L = DA/T

Question 10

What is the diffusion coefficient D proportional to?

Answer 10

It is proportional to its solubility and inversely proportional to the square root of its molecular weight.

Question 11

What is the difference in the diffusion rates between oxygen and CO2? What does this manifest?

Answer 11

CO2 diffuses about 20 times more rapidly than oxygen across the alveolar-capillary membrane. When impairment of diffusion occurs, patients develop hypoxemia before arterial hypercapnea.

Question 12

What is the transit time equal to? What is a normal transit time?

Answer 12

t = Vol/Q. volume of blood in pulmonary caps is 75mL, cardiac output is 6L/min = 100ml/sec. So t=75/100 = .75 sec. Normal is about .75-1.2 seconds.

Question 13

What is a perfusion-limited uptake?

Answer 13

The diffusion of the gas across the alveolar-capillary membrane is rapid and thus the Hb is saturated well before it is moved out of the capillary. Increasing cardiac output will increase uptake of the substance (N2O and O2 are examples)

Question 14

What is a diffusion-limited uptake?

Answer 14

The gradient of partial pressure between the alveolus and capillary plasma is maintained during the entire time that the blood resides in the capillary, making this gas useful for the measurement of pulmonary diffusion capacity. (Example is CO)

Question 15

What happens to the transit time during exercise?

Answer 15

It is reduced. If the cardiac output (flow) increases by three fold it is reduce to .25 seconds from .75 with no change in volume being exchanged (blood volume in contact)

Question 16

What would happen to an individual with a diffusion problem when they exercise?

Answer 16

They could develop exercised-induced hypoxemia because the transit time is too quick for O2 to be loaded onto Hb completely.

Question 17

What happens to P_A O2 when breathing enriched oxygen air?

Answer 17

It will increase the partial pressure of oxygen in the alveoli.

Question 18

What does hypoxic vasoconstriction do to D_L CO?

Answer 18

It reduces it slowing the rate of equilibrium.

Question 19

What would happen to an individual at higher altitude?

Answer 19

A normal individual would still equilibrate with the environment but the lower partial pressure of O2 would reduce their diffusion rate and a shorter transit time could result in hypoxia that would not normally occur at a lower altitude (with a higher P_A O2).

Question 20

How does D_L O2 compare to D_L CO?

Answer 20

D_L O2 = 1.23 D_L CO. The diffusion capacity of O2 is faster than for CO which is why you multiply the CO diffusion capacity by 1.23. The area and thickness of which both are diffusing across is constant.

Question 21

How does body position affect D_L CO?

Answer 21

D_L CO is greater when supine than in the upright position because distension of the pulmonary vasculature by an increased pulmonary blood volume increases the area available for diffusion.

Question 22

How does exercise influence D_L CO?

Answer 22

It increases during exercise. This could be due to increased alveolar distension

Question 23

How does the loss of lung tissue from surgery affect D_L CO?

Answer 23

It decreases because of the decrease in surface area for gas exchange

Question 24

How does a mismatch of ventilation to perfusion influence D_L CO (ex. airway obstruction, or shunt, blood flow obstruction or alveolar dead space)?

Answer 24

They decrease D_L CO

Question 25

How does D_L CO change with pulmonary hypertension with edema?

Answer 25

D_L CO is decreased

Question 26

Why do diffusional barriers often cause hypoxemia without arterial hypercapnia?

Answer 26

CO2 is highly soluble and this high solubility permits equilibration with alveolar gas even when equilibration of O2 is impaired by diffusional barriers. It is true that hypercapnia is always associated with hypoxemia but not the other way around.