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
What is hypercapnia?
Abnormally high [CO₂] in the blood
What is the consequence of increased V/Q ratio?
- Heart failure (cardiac arrest)
- Blocked vessels (pulmonary embolism)
- Loss / damage to capillaries (emphysema)
What is the consequence of unperfused alveoli?
Physiological dead space as there is little or no gas exchange occurring
What is a pulmonary shunt?
Perfusion without ventilation
Blood travels from Right heart -> left heart without taking part in gas exchange
What causes a pulmonary shunt to occur?
Reduced ventilation of alveoli or limited diffusion causes decreases in V/Q ratio which may be due to:
- Pneumonia
- Acute lung injury
- Respiratory Distress syndrome
- Atelectasis
How effective is supplemental O₂ therapy to shunt induced hypoxaemia?
Shunt induced hypoxaemia responds poorly to supplemental O₂ therapy
Describe the features of healthy lung function
Pulmonary capillaries innervate alveoli before returning to systemic circulation
- Blood saturation ~60%
- Blood saturation post Oxygenation ~95%
when they mix total saturation ~90% (lower than 95%)
Describe the features of shunt lungs
- Not enough ventilation occurs; no gas exchange
- Capillaries innervating alveoli are still perfused
- Blood oxygenation enters + leaves at 60% as no
ventilation is occurring - Blood innervating working alveoli still ventilated like
normal
=> when blood mixes % saturation falls to 75%
What is the effect of ventilation during shunt lungs?
Can get O₂ into working part of lung
- increased ventilation of functional airways / alveoli
- supplies additional O₂
- removal of CO₂
BUT
shunt affected part still not ventilated
blood mixes with systemic circulation
(De)oxygenated blood mix - healthy saturation is never reached
What affect does pulmonary shunt have on CO₂?
Shunt only affects O₂ loading, CO₂ is still able to be removed
How can we determine the cause of hypoxaemia?
Calculate Alveolar O₂ pressure to determine if respiratory failure is due to hypoventilation or poor oxygenation
Outline the equation used to calculate alveolar O₂ pressure
PAO₂ = FiO₂ x (Pb - PH₂O) - (PaCO₂/RER)
PAO₂ - alveolar O₂ pressure FiO₂ - O₂ friction in inspired gas Pb - barometric pressure PH₂O - H₂O vapour pressure PaCO₂ - arterial CO₂ pressure RER - respiratory exchange ratio
What is RER?
Respiratory exchange ratio determines the relationship between CO₂ elimination and O₂ consumption
RER = VCO₂ produced / VO₂ consumed
measures difference between O₂ and CO₂ in inspired and expired air
What determines RER value?
The main determinant of RER is the particular substrate being used (e.g. fat / carbohydrate)
RER for modern diet = 0.8
How can we investigate hypoxaemia?
Alveolar gas equation and alveolar-arterial pressure gradient (PAO₂ - PaO₂) is used to investigate hypoxaemia
ABG - arterial blood gases
AGE - alveolar gas equation
A-aO₂ - pressure gradient
How do we interpret ABG readings using AGE and A-aO₂?
- See if hypoventilation is contributing to hypoxaemia?
- if PA(CO₂) > 6kPa
- Is O₂ reaching alveoli diffusing into blood?
- use AGE and ABG readings to calculate A-a gradient
- should be > 2kPa
