Gas Exchange in the Lungs Flashcards
How is O₂ transported 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
haemoglobin - O₂ diffuses into cells/tissues for use in aerobic
respiration - CO₂ diffuses from respiring tissues to blood -
exchanged at lungs
How does gas exchange occur?
Gas exchange involves diffusion of blood gases through multiple structures and mediums
What structures does O₂ diffuse through to reach red blood cells?
Airspace ↓ Alveolar Lining fluid ↓ Alveolar epithelial layer ↓ Basement membrane and interstitial fluid ↓ Capillary endothelial layer ↓ Dissolves in blood plasma ↓ RBCs - O₂ binds to Hb molecules
When does blood oxygenation occur?
Oxygen of blood must occur in the brief time taken for RBCs to flow through pulmonary capillaries
Outline the equation used to determine rate of diffusion
Rate of diffusion ∝ SA / d² x (Pa - Pc)
SA - alveolar surface area
d² - endothelial + epithelial cell, basement membrane
thickness and fluid layer depth
(PA - Pc) - Partial pressure gradient between alveolar air
and capillary blood
How can we alter factors to ensure maximum diffusion?
- Increased SA
- Increased Partial pressure gradient
- Decreased distance (barrier thickness)
What defects can occur at gas exchange surfaces?
Hypoventilation
- Type II respiratory failure
Emphysema
- decreased SA
Fibrosis
- increased basement membrane thickness
Pulmonary Oedema
- (e.g. pneumonia)
- increased fluid layer thickness
What is the role of ventilation in gas exchange?
Adequate ventilation maintains the pressure gradient between alveoli and blood
Describe the relationship between pP(O₂) and ventilation?
The partial pressure of O₂ within alveoli increases with increased ventilation
Describe the relationship between CO₂ and ventilation
The partial pressure of CO₂ in alveoli decreases as ventilation increases
What is the significance of good perfusion in gas exchange?
Maintaining pressure gradients for diffusion also requires adequate perfusion
Describe the partial pressure of O₂ and CO₂ during hyperventilation.
O₂ levels remain the same - once PA(O₂) is 100%, no more oxygen can be obtained (can’t exceed 100%) so levels remain constant
PA(CO₂) decreases, as hyperventilating (breathing in and out v. quickly) so blood CO₂ decreases rapidly
Describe the partial pressure of CO₂ and O₂ during hypoventilation
Insufficient ventilation so decreased PA(O₂) and not enough CO₂ removed so PA(CO₂) increases
How do we ensure 100% oxygen saturation isn’t constantly reached to cause hyperventilation?
V/Q ratio:
Blood flow through pulmonary capillaries (Q - perfusion), needs to be matched to alveolar ventilation (Va) to enable efficient gas exchange as there is a maximum amount of O₂ each unit of blood can carry
What is the ideal V/Q ratio?
Ideally V/Q should = 1
At rest, ventilation and perfusion both = 5 L/min
so V/Q = 1 : 0.8
How much O₂ can 1 L of blood carry?
1 L of blood can carry 200 ml of O₂
How much O₂ does dry air carry?
1 L of dry air can carry 200 ml of O₂
How is ventilation-perfusion ratio maintained?
Ventilation-Perfusion coupling is maintained by hypoxic vasoconstriction
How is ventilation-perfusion mismatching prevented?
V/Q mismatching is prevented by homeostatic mechanisms
How does hypoxic vasoconstriction maintain V/Q ratio?
Hypoxic vasoconstriction of capillaries diverts blood from poor to well ventilated alveoli
What is the consequence of V/Q mismatching?
In situations where ventilation and perfusion to individual alveolar units aren’t matched, gas exchange will be reduced
How does V/Q inequality affect partial pressure of CO₂ and O₂?
V/Q mismatch affects both O₂ and CO₂
Increased PA(CO₂) induces reflex hyperventilation to wash out excess CO₂, but doesn’t increase PA(O₂)
What is the effect of a pulmonary embolism on the V/Q ratio?
Embolism occludes pulmonary artery supplying a region of the lung
Where blood flow is blocked:
- Unperfused alveoli = ↑V/Q
Where excess perfusion
- Perfusion to these vessels / alveoli increases as CO
is diverted = ↓V/Q
- unless ventilation of these alveoli increases to match
perfusion, hypoxaemia and hypercapnia will occur
What is hypoxaemia?
Abnormally low [O₂] in the blood
