Session 2 - The gas exchange

Question 1

What three factors determine rate of gas exchange?

Answer 1

• Area available for exchange
  
  • Resistance to diffusion

Gradient of partial pressure

Question 2

How does diffusion resistance change?

Answer 2

• Depends on the nature of the barrier
○ Cells, water
• What gas is moving through it
○ O2/CO2

Question 3

Is area a limiting factor in the lungs?

Answer 3

• No, lungs have a gas exchange area of 80m2 - huge area

Question 4

Outline the structures which provide diffusion resistance in the lungs

Answer 4

• The alveolar epithelial cell
  
  • Interstitial fluid
  • Capillary endothelial cell
  • Plasma
  • RBC membrane

Question 5

Overall, what do gases have to diffuse through from the alveoli to reach the blood cells?

Answer 5

• 5 cell membranes
  
  • 3 layers of intracellular fluid
  • 2 layers of extra cellular fluid

Question 6

How fast do gases move through other gases? How does this relate to O2 and CO2?

Answer 6

• At rate inversely proportional to molecular weight

○ CO2 moves slower than O2

Question 7

How fast do gases move through liquids? How does this relate to O2 and CO2?

Answer 7

• At rate proportional to solubility

○ CO2 much more soluble than O2, moves 21 times faster

Question 8

Is the rate of diffusion of CO2 a limiting factor?

Answer 8

• No, O2

Question 9

How fast is O2 exchange

Answer 9

• 0.5 seconds

Question 10

How long do blood cells stay in capillary?

Answer 10

How long do blood cells stay in capillary?

• 1 second

Question 11

What is the pO2 and pCO2 in the blood leaving capillaries in a normal lung?

Answer 11

• Same as in alveolar air

Question 12

Where does exchange occur in lungs?

Answer 12

• Across the alveolar membrane

Question 13

How is alveolar air different to atomspheric air?

Answer 13

• Less oxygen

* More carbon dioxide

Question 14

What is the partial pressure of O2 in the alveolar air?

Answer 14

• 13.3 kPa

Question 15

What is the partial pressure of pCO2 in the alveolar air?

Answer 15

• 5.3 kPa

Question 16

What is the pO2 in the venous blood which returns to the lungs from the body?

Answer 16

• 6.0 kPa

Question 17

What is the pCO2 in the venous blood which returns to the lungs from the body?

Answer 17

• 6.5 kPa

Question 18

Why does O2 move from alveoli to blood?

Answer 18

• Diffusion gradient

* 13.3 - 6.0

Question 19

Why does CO2 move from blood to alveoli

Answer 19

• Diffusion gradient

* 6.5 kPa –> 5.3 kPa

Question 20

What does alveolar ventilation determine?

Answer 20

• Gas composition of arterial blood

* And therefore oxygen supply to tissues

Question 21

What is ventilation of the lungs?

Answer 21

• Expansion of lungs
	• Increases volume of
		○ Respiratory bronchioles
		○ Alveolar ducts
	• So air flows down airways to them

Question 22

What parts of the lungs expand with ventilation?

Answer 22

• Respiratory bronchioles

* Alveolar ducts

Question 23

Does fresh air enter the alveoli in the lungs?

Answer 23

• No, reaches as far as the respiratory and terminal bronchioles

Question 24

Why would it be bad to have air going directly into alveoli?

Answer 24

• Make the blood impossibly alkaline on way in, very acidic on way out

Question 25

How can we measure ventilation?

Answer 25

• Using a spirometer

Question 26

What is a spirometer?

Answer 26

• A closed chamber is which gas is held at constant pressure, but the volume of which can change with ventilation

Question 27

What is anatomical/serial dead space?

Answer 27

• Volume of the airways

* Measured by nitrogen washout (typically 0.15l)

Question 28

Outline the nitrogen washout test

Answer 28

• Patient takes a maximum inspiration of 100% oxygen
  
  • Mixes with nitrogen naturally present in alveolar air
  • Air in conducting pathway will still be filled with pure O2
  • Person exhales through one way valve - nitrogen content is measured
  • A graph can be drawn plotting Nitrogen% against expired volume

Question 29

What is distributive/a;veolar dead space?

Answer 29

• Some parts of the lung are not airways and do not support gas exchange
○ Dead or damaged alveoli
○ Alveoli with poor perfusion (ventilation/perfusion ratio)

Question 30

What is physiological dead space?

Answer 30

• Serial dead space + distributive dead space (0.17l)

Question 31

What is alveolar ventilation rate?

Answer 31

• Dead space must be completely filled with air at each breath
• Dead space ventilation rate
○ Dead space volume x respiratory rate
• Subtracted from pulmonary ventilation rate to get AVR

Question 32

How is physiological dead space measured?

Answer 32

• pCO2 of expired alveolar air

* Alveolar air is diluted by dead space air

Question 33

Give the calculation for alveolar ventilation rate

Answer 33

• Pulmonary ventilation rate (tidal volume x RR) - Dead space ventilation rate (dead space volume x RR)