Gas exchange in the lungs Flashcards
How does oxygen get from the atmosphere to cells?
O2 inhaled from atmosphere into alveoli within lungs
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O2 diffuses from alveoli into blood within pulmonary capillaries
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O2 transported in blood predominantly bound to haemoglobin
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O2 diffuses into cells/tissues for use in aerobic respiration
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CO2 diffuses from respiring tissues to blood - exchanged at the lungs
What are the structures that blood gases have to move through?
1) Airspace
2) Alveolar lining fluid
3) Alveolar epithelial layer
4) Basement membrane + interstitial fluid
5) Capillary endothelial layer
6) Blood plasma
7) Lung capillary
What are the steps O2 has to take to bind to Hb?
O2 enters the alveolar airspace from the atmosphere ↓ O2 dissolves in ALF ↓ O2 diffuses through alveolar epithelium, basement membrane + capillary endothelial cells ↓ O2 dissolves in blood plasma ↓ O2 binds to Hb molecule
What are oxygenation times like?
→Oxygenation of blood must be rapid
→In typical conditions it takes 0.75 seconds for a RBC to pass through a pulmonary capillary
→During intensive exercise it takes 0.25 seconds.
What is the rate of diffusion determined by?
→Partial pressure gradient between two areas
→Size of the diffusion distance
→Surface area
What is abnormal diffusion caused by?
→Thickening of blood-gas barrier
What is the equation for rate of diffusion?
→Rate of diffusion ∝
(Surface area/Distance^2) x (Pa - Pc)
→SURFACE AREA : Alveolar surface area
→DISTANCE = Epithelial + endothelial cell thickness + basement membrane thickness + fluid layer depth
→(PA-PC) = Partial pressure gradient between alveolar air and capillary blood.
What are criteria for maximum diffusion?
↑ Surface area
↑ partial pressure gradient
↓ Distance ( barrier thickness )
What is a result of emphysema?
↓Surface area
What is a result of fibrosis?
↑ Basement membrane thickness
What is a result of pulmonary oedema?
↑ Increased thickness of fluid layer/oedema
What is hypoventilation and what happens as a result of it?
→Don’t breathe at a sufficient rate so the partial pressure gradient decreases.
What is the relationship between ventilation and partial pressure of O2?
→ Hypoventilation - Decrease in PaO2
→ Hyperventilation - Increase in PaO2
What is the relationship between ventilation and partial pressure of CO2?
→ Hypoventilation - Increase in PaCo2
→ Hyperventilation - Decrease in PaCo2
What needs to be matched for efficient gas exchange?
→blood flow through pulmonary capillaries needs to be matched to alveolar ventilation
Why does perfusion need to be matched to ventilation?
→ Each unit of blood has a finite amount of Hb and can only transport a limited amount of oxygen
What is the V/Q ratio?
→ The relationship between ventilation and perfusion
What should the V/Q ratio normally be?
→ 1
What is the cause of V/Q being >1?
→ Hypoperfusion
What is the cause of V/Q being <1?
→ Hypoventilation
What reduces ventilation-perfusion mismatching?
→ Homeostatic mechanisms
What is hypoxic vasoconstriction for?
→ constriction of capillaries which diverts blood flow from poor to well ventilated alveoli
How does hypoxic vasoconstriction occur?
→ If ventilation of a specific alveolus decreases
→ Pa Co2 will rise
→ Pa O2 will fall
→ Less oxygenation of blood flowing through innervating capillaries
→ decreased Pa O2 induces vasoconstriction = decreased blood flow
→ Blood is diverted to alveoli with more ventilation
What is physiological dead space?
→ you have ventilation but not perfusion
What happens to the V/Q to alveoli being underperfused due to an embolism and how can you compensate?
→V/Q ratio increases in alveoli that is not being perfused
→ Increased perfusion to other alveoli and their V/Q ratio decreases
→you can compensate by ventilating more
What are diseases that cause physiologic dead space?
→ Heart failure
→ Blocked vessels
→ Loss/damage to capillaries
What is the shunt effect?
→ Perfusion without ventilation
What are diseases which cause the shunt effect?
→ Pneumonia
→ Acute lung injury
→ respiratory distress syndrome
→ atelectasis
What is the respiratory exchange ratio (RER)?
→ The relationship between Co2 and O2 in inspired + expired air
What is the equation for RER?
→ VCO2 produced/VO2 consumed
How can the alveolar O2 pressure be calculated?
→ PaO2 = F1O2 x (Pb-PH2O) - (Pa Co2/RER)
What is the difference in alveolar partial pressure and arterial partial pressure in a healthy individual?
→ <2kPa
What is the A-a gradient?
→ Difference between arterial and alveolar partial pressure
How do you know if the cause of hypoxaemia is hypoventilation?
→ PaCo2 > 6kPa
How do you know if the oxygen reaching the alveoli is diffusing into the blood?
→ A-a gradient should be less than 2 kPa
