(3) ALVEOLAR GAS EXCHANGE notes

Question 1

Diffusion - Review of Fick’s Law of Diffusion.

Answer 1

Dco2 is 20 times Do2

Question 2

Diffusion and Perfusion Limitations In rest situation average time of blood cell transit through

Answer 2

capillary is about 0.75 sec. (Less in exercise).

Question 3

Fig. 1 shows uptake of CO, N2O and O2 along pulmonary capillary.

Answer 3

a. CO binds Hb tightly therefore transfer of CO is diffusion limited.
b. Nitrous oxide does not bind hemoglobin so transfer of N2O is perfusion limited.
c. Oxygen transfer is normally perfusion limited but can be diffusion limited.

Question 4

Oxygen uptake time courses with normal and abnormal diffusion at rest and during exercise and with low alveolar pO2 (Fig. 2, 3 & 4).

Answer 4

Question 5

Measurement of diffusion capacity

A. Recall that Fick’s Equation states.

Answer 5

Question 6

Therefore, transforming the equation allows for the calculation of DL

Answer 6

DL = Vgas / (P1 -P2)

Question 7

Use CO for measurement - since the partial pressure of CO in capillary blood is so small, it can be generally neglected.

Answer 7

a. Thus DL = VCO/ PACO or the volume of CO transferred in mL/min per mmHg of alveolar partial pressure.
b. Normal values for DLco are about 25 mL/min/mmHg at rest; For DLo2 = 21 mL/min/mmHg. c. DL is 2-3 times greater with exercise.

Question 8

CO2 transfer is about 20 times faster than O2, thus CO2 transfer

Answer 8

should not be affected by diffusion difficulties.

Question 9

ig. 5, shows effect of thickening of pulmonary membrane on CO2 equilibration

Answer 9

Question 10

Time similar to that seen with O2. Why?

Answer 10

Since there is so small a difference between venous and arterial levels, PCO2 is not significantly increased.

Question 11

hypercapnias cannot result from

Answer 11

CO2 diffusion blocks whereas hypoxemias may result from diffusion blocks.

Question 12

The 3 main causes of hypoxemia are

Answer 12

a. Diffusion blocks
b. Anatomical shunting
c. V/Qinequalities.