Gas Exchange 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
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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
↓
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
→ as deoxygenated blood returns to the left side of the heart from the right, without taking part in gas exchange.
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
What are the percentages of nitrogen, oxygen and other gases?
78% nitrogen, 21% oxygen, 1% other gases
What is partial pressure of water vapour in dry and humid air?
0kPa in dry air to approximately 6kPa in fully warmed, humidified air within the lung
Equation for partial pressure of gas…
𝑃_𝑔𝑎𝑠=〖(𝑃〗𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐−𝑃(𝐻_2 𝑂))×𝑛_𝑔𝑎𝑠
→ Partial pressure of individual constituent gas
→ Atmospheric pressure
→ Water vapour pressure
(0 kPa in dry air, 6 kPa in fully humidified air)
→ Mole fraction: the % of total moles represented by the individual gas (e.g. in air at sea level, O2 = 21%)
Equation of concentration….
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛=𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ×𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦
What is partial pressure of a gas dissolved in a liquid?
reflects the amount of gas that would dissolve (at equilibrium) if the liquid was placed in contact with a gas phase of equivalent partial pressure.
What happens to partial pressures when blood passes through pulmonary capillaries?
→ When blood passes through the pulmonary capillaries of healthy individuals, the partial pressures of respiratory gases almost completely equilibrate with those of alveolar air.
What must happen in order for gas exchange to take place efficiently regarding perfusion and ventilation?
→ there must be sufficient blood (specifically haemoglobin binding sites) to absorb the quantity of oxygen arriving at the alveoli. →Therefore the level of ventilation (supply of oxygen) and perfusion (supply of blood) need to be closely matched.
What happens in pulmonary embolism?
the overall perfusion of the lungs as a whole may not decrease if blood is simply diverted through other pulmonary arteries/capillaries
→ In theory, increased ventilation of these areas of the lungs may compensate for the reduction in gas exchange in others.
Why does oxygen therapy have limitations?
→ regardless of degree of oxygenation occurring in blood perfusing well-ventilated alveoli, it will eventually mix with deoxygenated blood returning from areas affected by shunt,
→ reducing the overall PaO2
Why does oxygen therapy have limitations regarding haemoglobin?
→The vast majority (>98%) of oxygen carried in blood is transported bound to haemoglobin.
→As saturation of haemoglobin is typically >95% at physiological oxygen pressures, administering supplemental oxygen cannot increase oxygen saturation in well-ventilated regions of the lung sufficiently to compensate for the deoxygenated blood with which it will eventually mix.
What is hypoxic vasoconstriction?
→occurs when ventilation to an alveoli is reduced;
→the decreased ventilation results in rising CO2 and falling O2 levels,
→cause contraction of the vascular smooth muscle within nearby capillaries.
What is the result of hypoxic vasoconstriction?
diverts blood to other capillaries that innervate better-ventilated alveoli.
How can hypoxic vasoconstrictions be pathological?
the chronic hypoventilation that occurs within large sections of the lungs,
→leads to prolonged and widespread pulmonary vasoconstriction.
→ increases resistance within the pulmonary vasculature,
→in pulmonary hypertension.
→lead to right heart hypertrophy and (eventually) right heart failure.
Why is shunt induced hypoxaemia more resistant too oxygen therapy?
there will always be a volume of relatively deoxygenated blood returning to the systemic circulation, for which additional ventilation of more functional parts of the lung cannot compensate
→(as there isn’t sufficient ‘spare’ unbound haemoglobin within the blood perfusing the better ventilated parts of the lung – O2 saturation is >95% at baseline)
