Session 2 - The gas exchange Flashcards
What three factors determine rate of gas exchange?
- Area available for exchange
- Resistance to diffusion
Gradient of partial pressure
How does diffusion resistance change?
• Depends on the nature of the barrier
○ Cells, water
• What gas is moving through it
○ O2/CO2
Is area a limiting factor in the lungs?
• No, lungs have a gas exchange area of 80m2 - huge area
Outline the structures which provide diffusion resistance in the lungs
- The alveolar epithelial cell
- Interstitial fluid
- Capillary endothelial cell
- Plasma
- RBC membrane
Overall, what do gases have to diffuse through from the alveoli to reach the blood cells?
- 5 cell membranes
- 3 layers of intracellular fluid
- 2 layers of extra cellular fluid
How fast do gases move through other gases? How does this relate to O2 and CO2?
• At rate inversely proportional to molecular weight
○ CO2 moves slower than O2
How fast do gases move through liquids? How does this relate to O2 and CO2?
• At rate proportional to solubility
○ CO2 much more soluble than O2, moves 21 times faster
Is the rate of diffusion of CO2 a limiting factor?
• No, O2
How fast is O2 exchange
• 0.5 seconds
How long do blood cells stay in capillary?
How long do blood cells stay in capillary?
• 1 second
What is the pO2 and pCO2 in the blood leaving capillaries in a normal lung?
• Same as in alveolar air
Where does exchange occur in lungs?
• Across the alveolar membrane
How is alveolar air different to atomspheric air?
- Less oxygen
* More carbon dioxide
What is the partial pressure of O2 in the alveolar air?
• 13.3 kPa
What is the partial pressure of pCO2 in the alveolar air?
• 5.3 kPa
What is the pO2 in the venous blood which returns to the lungs from the body?
• 6.0 kPa
What is the pCO2 in the venous blood which returns to the lungs from the body?
• 6.5 kPa
Why does O2 move from alveoli to blood?
- Diffusion gradient
* 13.3 - 6.0
Why does CO2 move from blood to alveoli
- Diffusion gradient
* 6.5 kPa –> 5.3 kPa
What does alveolar ventilation determine?
- Gas composition of arterial blood
* And therefore oxygen supply to tissues
What is ventilation of the lungs?
• Expansion of lungs • Increases volume of ○ Respiratory bronchioles ○ Alveolar ducts • So air flows down airways to them
What parts of the lungs expand with ventilation?
- Respiratory bronchioles
* Alveolar ducts
Does fresh air enter the alveoli in the lungs?
• No, reaches as far as the respiratory and terminal bronchioles
Why would it be bad to have air going directly into alveoli?
• Make the blood impossibly alkaline on way in, very acidic on way out
How can we measure ventilation?
• Using a spirometer
What is a spirometer?
• A closed chamber is which gas is held at constant pressure, but the volume of which can change with ventilation
What is anatomical/serial dead space?
- Volume of the airways
* Measured by nitrogen washout (typically 0.15l)
Outline the nitrogen washout test
- 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
What is distributive/a;veolar dead space?
• 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)
What is physiological dead space?
• Serial dead space + distributive dead space (0.17l)
What is alveolar ventilation rate?
• 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
How is physiological dead space measured?
- pCO2 of expired alveolar air
* Alveolar air is diluted by dead space air
Give the calculation for alveolar ventilation rate
• Pulmonary ventilation rate (tidal volume x RR) - Dead space ventilation rate (dead space volume x RR)
