Chapter 39 - Principles of Gas exchange, diffusion of Oxygen and Carbon Dioxide through the Respiratory Membrane

Question 1

Chemical composition of air

Answer 1

• 78% N2, 21% O2, 1% other gases.

Question 2

Describe Henry’s law.

Answer 2

• Partial Pressure = Concentration of dissolved gas / Solubility coefficient

Question 3

How much is 1 atmosphere in mm Hg?

Answer 3

• 760 mm Hg

Question 4

What is the solubility coefficient of carbon dioxide compared to oxygen?

Answer 4

• CO2 = 0.57, H2O= 0.024.

Carbon Dioxide is 20x more soluble than oxygen.

Question 5

How much vapor pressure exerts on incoming air?

Answer 5

• 47 mm Hg

Question 6

What is the most important thing to determine direction of diffusion?

Answer 6

• The pressure difference.

Question 7

What are the factors that affect the rate of gas diffusion in a fluid?

Answer 7

• 1. Pressure difference.
     
     2. Solubility of the gas in the liquid.
     3. The cross-sectional area of the fluid.
     4. The distance thru which the gas must diffuse.
     5. The molecular weight of the gas.
     6. The temperature of the fluid.

Question 8

Describe the diffusion of gases thru tissue.

Answer 8

• The gases that are of respiratory importance are all highly soluble in lipids; can cross the cell membrane easily. The major limitation is the rate at which the gases can diffuse thru the tissue water.

Question 9

What does the water vapor pressure do to incoming dry air?

Answer 9

• Dilutes it with vapor pressure (b.c it can’t raise it above atmospheric pressure). At sea level. Changes the average of 159 mm Hg to 149 mm Hg.

Question 10

How much air is actually brought into the alveoli with each normal inspiration?

Answer 10

• 350 milliliters of new air

Question 11

What is the importance of the slow replacement of alveolar air?

Answer 11

• To prevent sudden changes in gas concentrations in the blood.

Question 12

How many alveoli are in the two lungs combined and the average diameter of one?

Answer 12

• About 300 million. About 0.2 millimeter

Question 13

What is the pulmonary membrane (Respiratory membrane)?

Answer 13

• Alveoli and the membranes of the terminal portions of the lungs where gas exchange also takes place.

Question 14

What is the total surface area of the respiratory membrane?

Answer 14

• About 70 square meters

Question 15

What is the total quantity of blood in the capillaries of the lungs at any given instant?

Answer 15

• 60 to 140 milliliters.

Question 16

What is the average diameter of the pulmonary capillaries?

Answer 16

• About 5 micrometers.

Question 17

What is the respiratory membrane’s diffusing capacity?

Answer 17

• The volume of gas that will diffuse thru the membrane each minute for a partial pressure difference of 1 mm Hg.

Question 18

What is the diffusing capacity for oxygen under resting conditions?

Answer 18

• 21 ml/min/mm Hg.

Question 19

What is the mean oxygen pressure difference across the respiratory membrane during normal, quite breathing?

Answer 19

• About 11 mm Hg.

Question 20

How many milliliters of oxygen diffuse thru the respiratory membrane each minute?

Answer 20

• Multiplication of mean oxygen pressure difference across the respiratory membrane by diffusing capacity (11 x 21) gives us 230 milliliters of oxygen diffusing thru the respiratory membrane each minute; about equal to the rate at which the resting body uses oxygen.

Question 21

How much does diffusion capacity for oxygen change during strenuous exercise or other conditions that greatly increase pulmonary blood flow and alveolar ventilation?

Answer 21

• About 3x (65ml/min/mm Hg)

Question 22

What are the 2 factors that determine the PO2 and PCO2 in the alveoli?

Answer 22

• 1. The rate of alveolar ventilation

2. The rate of transfer of oxygen and carbon dioxide thru the respiratory membrane.

Question 23

What is the ventilation/perfusion (V/Q)?

Answer 23

• Ventilation of alveoli over ventilated alveoli must be perfused with pulmonary blood to allow air-to-liquid gas transfer.

Question 24

What happens to the ventilation/perfusion ratio when there is adequate ventilation but zero perfusion?

Answer 24

• The ratio is infinity. The air in the alveolus comes to equilibrium with the air inspired (PO2 = 149 mm Hg and a PCO2 of 0 mm Hg)

Question 25

What happens to the ventilation/perfusion ratio when there is adequate perfusion but zero ventilation?

Answer 25

• The ratio is zero. The air in the alveolus comes to equilibrium with the blood oxygen and carbon dioxide content of venous blood (PO2 = 40 mm Hg, PCO2 = 45 mm Hg)

Question 26

What are the PO2 and PCO2 of alveolar air under normal conditions (normal ventilation/perfusion ratio)?

Answer 26

• PO2 = 104 mm Hg, PCO2 = 40 mm Hg.

Question 27

What is shunted blood?

Answer 27

• Certain fraction of the venous blood passing thru the pulmonary capillaries that does not get oxygenated.

Question 28

Define physiologic shunt.

Answer 28

• The total quantitative amount of shunted blood per minute.

Question 29

• The total quantitative amount of shunted blood per minute.

Answer 29

• The volume of the lungs that does not participate in gas exchange ( anatomical dead space + non-functioning respiratory membrane).