Gas exchange in the lung Flashcards
How does oxygen get from the atmosphere to cells?
- O2 inhaled from atmosphere into alveoli within lungs
- O2 diffuses from alveoli into blood within pulmonary capillaries
- O2 transported in blood, predominantly bound to Hb.
- O2 diffuses into cells/tissues for use in aerobic respiration
- CO2 diffuses from respiring tissues to blood-exchanged at lungs
What does gas exchange involve?
Gas exchange involves diffusion of blood gases through multiple structures and mediums
Movement of O2 through different surfaces
- O2 enters the alveolar airspace from the atmosphere
- O2 dissolves in alveolar lining fluid(ALF)
- O2 diffuses through alveolar epithelium, basement membrane and capillary endothelial cells
- O2 dissolves in blood plasma
- O2 binds Hb molecules
When must the oxygenation of blood occur?
Oxygenation of blood must occur during brief time taken for RBC’s to flow through pulmonary capillaries
What factors determine rate of diffusion?
- Surface area(Alveolar surface area)
- Distance^2(Epithelial and endothelial cell thickness+basement membrane thickness+fluid layer depth)
- Partial pressure gradient between alveolar air and capillary blood
What will an increase in distance do to the rate of diffusion?
Decrease diffusion
How do gases behave?
Gases behave in terms of partial pressure.
How would you change certain factors for maximum diffusion?
- Increase partial pressure gradient
- Increase in surface area
- Decrease in distance
What does fibrosis do and how does it affect rate of diffusion?
Fibrosis increases basement membrane thickness and causes rate of diffusion to decrease
What does a pulmonary oedema do and how does it affect rate of diffusion?
Increases thickness of fluid layer and causes a decrease in rate of diffusion
What is maintained between alveoli and blood and how?
Pressure gradients between alveoli and blood are maintained by adequate ventilation
What happens as ventilation increases?
Partial pressure of O2 within alveoli increases as ventilation increases
What does an increase in alveolar partial pressure of oxygen result in?
Results in very diminishing returns for arterial partial pressure of oxygen
Affects of a decrease in ventilation to alveolar and arterial partial pressure of oxygen
A decrease in ventilation decreases both alveolar and arterial partial pressure of oxygen
What happens as ventilation decreases?
Decrease in ventilation will increase both partial pressure of CO2 in alveoli and arterieals
What happens to arterial partial pressure of CO2 in hypoventialtion and hyperventilation?
Hypoventilation-Increase in partial pressure of CO2
Hyperventialtion-Decrease in partial pressure of CO2
What happens to arterial pressure of O2 in hypoventilation and hyperventilation?
Hypoventilation-Approx same partial pressure of O2 to normal value
Hyperventilation-Decrease in partial pressure of O2
What other factor is required for maintaining pressure gradients for diffusion?
-Adequate perfusion
What 2 factors need to be matched to enable efficient gas exchange?
Perfusion(Q) needs to be matched to alveolar ventilation(VA)
Normal V/Q ratio
1
Likely cause of a V/Q ratio >1
Hypoperfusion(“dead space effect”)
Likely cause of a V/Q ratio <1
Hypoventilation (Shunt)
What is ventilation-perfusion coupling maintained by?
Maintained by hypoxic vasoconstriction
What is the homeostatic mechanism to reduce ventilation-perfusion mismatching?
1)Under normal conditions, blood flow and ventilation are matched
2)If ventilation of a specific alveoli decreases, PACO2 will rise and PAO2 will fall therefore causing a decreased oxygenation of blood flowing through innervating capillaries
3)A decrease in PAO2 induces vasoconstriction, reducing blood flow
Blood flow diverted to another alveoli with higher ventilation
What happens in most cases of V-Q inequality?
Increase in PaCO2 induces a reflex hyperventilation that clears excess CO2, but doesn’t increase PaO2
What does an embolism cause?
- Embolism occludes pulmonary artery supplying a region of the lung
- Results in the increase of the V/Q ratio of the unperfused alveoli and a decrease in V/Q ratio of the other alveoli
- Perfusion of the other alveoli increases due to cardiac output being diverted to this aveoli because of the embolism
- Unless ventilation of the other alveoli increases, to match perfusion, hypoxaemia and hypercapnia will occur
What can causes an increase in V/Q ratio?
- Heart faliure
- Blocked vessels
- Loss/damage to capillaries
What is physiologic dead space?
Ventilation without perfusion
What is shunt?
Perfusion without ventilation
What can cause a decrease in V/Q ratio ?
- Pneumonia
- Acute lung injury
How is the respone of a pulmonary shunt to O2 therapy?
Poor
What is the respiratory exchange ratio?
Relationship between CO2 elimination and O2 consumption
What is the main determinant in of RER and approx value?
Main determinant is the particular metabolic substrate being used and value is approx 0.8 for the modern diet
What is A-a O2 gradient normally?
<2kPa
When do you know that hypoventilation is contributing to hypoxaemia?
when PaCO2>6kPa
How do you know that the oxygen reaching the alveoli is diffusing into the blood?
Calculate A-a gradient and should be approx 2kPa or less
