Gas Exchange

Question 1

How does oxygen get from the atmosphere to cells?

Answer 1

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 the lungs

Question 2

What are the structures that blood gases have to move through?

Answer 2

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

Question 3

What are the steps O2 has to take to bind to Hb?

Answer 3

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

Question 4

What are oxygenation times like?

Answer 4

→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.

Question 5

What is the rate of diffusion determined by?

Answer 5

→Partial pressure gradient between two areas
→Size of the diffusion distance
→Surface area

Question 6

What is abnormal diffusion caused by?

Answer 6

→Thickening of blood-gas barrier

Question 7

What is the equation for rate of diffusion?

Answer 7

→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.

Question 8

What are criteria for maximum diffusion?

Answer 8

↑ Surface area
↑ partial pressure gradient
↓ Distance ( barrier thickness )

Question 9

What is a result of emphysema?

Answer 9

↓Surface area

Question 10

What is a result of fibrosis?

Answer 10

↑ Basement membrane thickness

Question 11

What is a result of pulmonary oedema?

Answer 11

↑ Increased thickness of fluid layer/oedema

Question 12

What is hypoventilation and what happens as a result of it?

Answer 12

→Don’t breathe at a sufficient rate so the partial pressure gradient decreases.

Question 13

What is the relationship between ventilation and partial pressure of O2?

Answer 13

→ Hypoventilation - Decrease in PaO2

→ Hyperventilation - Increase in PaO2

Question 14

What is the relationship between ventilation and partial pressure of CO2?

Answer 14

→ Hypoventilation - Increase in PaCo2

→ Hyperventilation - Decrease in PaCo2

Question 15

What needs to be matched for efficient gas exchange?

Answer 15

→blood flow through pulmonary capillaries needs to be matched to alveolar ventilation

Question 16

Why does perfusion need to be matched to ventilation?

Answer 16

→ Each unit of blood has a finite amount of Hb and can only transport a limited amount of oxygen

Question 17

What is the V/Q ratio?

Answer 17

→ The relationship between ventilation and perfusion

Question 18

What should the V/Q ratio normally be?

Answer 18

→ 1

Question 19

What is the cause of V/Q being >1?

Answer 19

→ Hypoperfusion

Question 20

What is the cause of V/Q being <1?

Answer 20

→ Hypoventilation

Question 21

What reduces ventilation-perfusion mismatching?

Answer 21

→ Homeostatic mechanisms

Question 22

What is hypoxic vasoconstriction for?

Answer 22

→ constriction of capillaries which diverts blood flow from poor to well ventilated alveoli

Question 23

How does hypoxic vasoconstriction occur?

Answer 23

→ 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

Question 24

What is physiological dead space?

Answer 24

→ you have ventilation but not perfusion

Question 25

What happens to the V/Q to alveoli being underperfused due to an embolism and how can you compensate?

Answer 25

→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

Question 26

What are diseases that cause physiologic dead space?

Answer 26

→ Heart failure
→ Blocked vessels
→ Loss/damage to capillaries

Question 27

What is the shunt effect?

Answer 27

→ Perfusion without ventilation

→ as deoxygenated blood returns to the left side of the heart from the right, without taking part in gas exchange.

Question 28

What are diseases which cause the shunt effect?

Answer 28

→ Pneumonia
→ Acute lung injury
→ respiratory distress syndrome
→ atelectasis

Question 29

What is the respiratory exchange ratio (RER)?

Answer 29

→ The relationship between Co2 and O2 in inspired + expired air

Question 30

What is the equation for RER?

Answer 30

→ VCO2 produced/VO2 consumed

Question 31

How can the alveolar O2 pressure be calculated?

Answer 31

→ PaO2 = F1O2 x (Pb-PH2O) - (Pa Co2/RER)

Question 32

What is the difference in alveolar partial pressure and arterial partial pressure in a healthy individual?

Answer 32

→ <2kPa

Question 33

What is the A-a gradient?

Answer 33

→ Difference between arterial and alveolar partial pressure

Question 34

How do you know if the cause of hypoxaemia is hypoventilation?

Answer 34

→ PaCo2 > 6kPa

Question 35

How do you know if the oxygen reaching the alveoli is diffusing into the blood?

Answer 35

→ A-a gradient should be less than 2 kPa

Question 36

What are the percentages of nitrogen, oxygen and other gases?

Answer 36

78% nitrogen, 21% oxygen, 1% other gases

Question 37

What is partial pressure of water vapour in dry and humid air?

Answer 37

0kPa in dry air to approximately 6kPa in fully warmed, humidified air within the lung

Question 38

Equation for partial pressure of gas…

Answer 38

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%)

Question 39

Equation of concentration….

Answer 39

𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛=𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ×𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦

Question 40

What is partial pressure of a gas dissolved in a liquid?

Answer 40

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.

Question 41

What happens to partial pressures when blood passes through pulmonary capillaries?

Answer 41

→ When blood passes through the pulmonary capillaries of healthy individuals, the partial pressures of respiratory gases almost completely equilibrate with those of alveolar air.

Question 42

What must happen in order for gas exchange to take place efficiently regarding perfusion and ventilation?

Answer 42

→ 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.

Question 43

What happens in pulmonary embolism?

Answer 43

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.

Question 44

Why does oxygen therapy have limitations?

Answer 44

→ 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

Question 45

Why does oxygen therapy have limitations regarding haemoglobin?

Answer 45

→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.

Question 46

What is hypoxic vasoconstriction?

Answer 46

→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.

Question 47

What is the result of hypoxic vasoconstriction?

Answer 47

diverts blood to other capillaries that innervate better-ventilated alveoli.

Question 48

How can hypoxic vasoconstrictions be pathological?

Answer 48

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.

Question 49

Why is shunt induced hypoxaemia more resistant too oxygen therapy?

Answer 49

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)