Gas Exchange Lecture

Question 1

What is the molecular basis of gas exchange?

Answer 1

Free moving molecules in lungs via diffusion (also true of gases dissolved in fluids and tissues of the body)

Question 2

Define pressure

Answer 2

Caused by multiple impacts of moving molecules against a surface
- Directly proportional to the concentration of gas molecules

Question 3

What is two things are proportional?

Answer 3

Pressure of a gas on surfaces of the respiratory passages and alveoli is proportional to summated force of impact of all gas molecules striking the surface at any given instant

Question 4

Define partial pressure

Answer 4

Rate of diffusion of each gas is directly proportional to the pressure caused by each gas alone

Question 5

Solubility of the gas in the fluid?

Answer 5

Increase and diffusion increases

Question 6

Cross-sectional area of the fluid?

Answer 6

Increase and diffusion increases

Question 7

Distance through which the gas must diffuse?

Answer 7

Increase and diffusion decreases

Question 8

Molecular weight of the gas?

Answer 8

Increase and diffusion decreases

Question 9

Which two factors determine the diffusion coefficient of the gas?

Answer 9

Solubility of the gas

MW of the gas

Question 10

Diffusion coefficient is proportional to?

Answer 10

Relative rates at which different gases at eh same partial pressure levels will diffuse are proportional to their diffusion coefficients

Question 11

Pressures of gases dissolved in water?

Answer 11

depends on the solubility of the gas because some molecules are not physically or chemically attracted to water leading to high partial pressure developing with fewer dissolved molecules

Question 12

Henry’s Law

Answer 12

Partial pressure = concentration of dissolved gas/ solubility coefficient

Question 13

Diffusion of Gases Through Tissues

Answer 13

Gases that are of respiratory importance are all highly soluble in lipids= highly soluble in cell membranes –> major limitations to movement of gases in tissues is the rate at which the gases can diffuse through the tissue water instead of through the cell membranes –> diffusion of gases through the tissues = diffusion of gases in water

Question 14

Why are the alveolar air and atmospheric air different?

Answer 14

Alveolar air partially replaced by atmospheric air with each breath
Oxygen constantly absorbed into pulmonary blood from alveolar air
CO2 constantly diffusing from pulmonary blood into alveoli
Dry atmospheric air that enters respiratory passage is humidified even before it reaches the alveoli

Question 15

Rate at which alveolar air renewed by atmospheric air?

Answer 15

Functional residual capacity of lungs = 2300 mililiters (volume of air remaining in lungs at end of normal expiration
New air brought into the alveoli with each normal inspiration = 350 mL (amount of expired old air = 350 mL)

Question 16

What is the rate of removal of excess gas from alveoli?

Answer 16

Normal alveolar ventilation (1/2 gas is removed in 17 seconds)
Rate of alveolar ventilation is 1/2 normal (1/2 gas is removed in 34 seconds)
Twice normal ventilation (1/2 gas is removed in 8 seconds)

Question 17

Oxygen?

Answer 17

Continually absorbed from the alveoli into the blood of lungs; new oxygen is continually being breathed into alveoli from atmosphere

• Increase absorption in blood, decrease alveoli conncentration
• Increase new oxygen being breathed in, increase alveoli concentration

Question 18

Oxygen concentration is controlled by what?

Answer 18

Rate of absorption of oxygen into the blood rate of entry of new oxygen into the lungs by the ventilatory process

Question 19

CO2?

Answer 19

Continually being produced in body and then carried in blood to alveoli
Continually being removed from alveoli by ventilation

Question 20

As alveolar PCO2 increases? decreases?

Answer 20

Increase: directly proportion to the rate of CO2 excretion
Decrease: inverse proportion to alveolar ventilation

Question 21

Concentration and partial pressures of both O2 and CO2 in alveoli are determined by?

Answer 21

Rates of absorption or excretion of the two gases and by the amount of alveolar ventilation

Question 22

What is a part of the respiratory unit or lobule

Answer 22

Respiratory bronchioles
Alveolar ducts
Atria
Alveoli

Question 23

Define alveoli

Answer 23

Wall is very thin

Between alveoli is an almost solid network of interconnecting capillaries – “sheet”

Question 24

What are the layers of the respiratory membrane?

Answer 24

• Surfactant layer: A layer of fluid lining the alveolus and containing surfactant that reduces the surface tension of the alveolar fluid
• Alveolar epithelium: The alveolar epithelium composed of thin epithelial cells
• Epithelial basement membrane
• Interstitial space: between the alveolar epithelium and the capillary membrane
• Capillary basement membrane: fuses with the alveolar epithelial basement layer
• Capillary endothelium

Question 25

What factors affect rate of gas diffusion through respiratory membrane?

Answer 25

Thickness of the membrane (increase in edema)
Surface area of the membrane (decrease if removal of lung or in emphysema, where many of the alveoli coalesce, with dissolution of alveolar walls)
Diffusion coefficient of the gas in the substance of the membrane (CO2 is faster than O2 which is faster than N)
Partial pressure difference of the gas between the two sides of the membrane

Question 26

Oxygen partial pressure in alveoli is?

Answer 26

> than the pressure of gas in blood –> net diffusion from alveoli into blood

Question 27

Carbon dioxide pressure of gas in blood is?

Answer 27

> than the partial pressure in alveoli –> net diffusion from blood to alveoli