Lecture 5 - Gas Exchange

Question 1

What is Dalton’s Law?

How do you calculate the partial pressure of an individual gas within a mixture?

Answer 1

Dalton’s Law = The sum of the partial pressures of all the gases = Total pressure (P1 + P2 + P3 = Pt)

Percentage of the gas (in decimal form) x total pressure - e.g.:

• oxygen makes up 21% of total gas in 100kPa total pressure
• 0.21 x 100 = 21kPa

Question 2

Is atmospheric pressure always the same?

Answer 2

No, at sea level atmosphere is 101kPa, at the top of mount everest it is 31kPa. At high altitudes, PP is lower.

Question 3

How are partial pressure denoted in the alveoli, in arterial blood & in venous blood?

Answer 3

Alveoli = PAO2 (big A)
Arterial Blood = PaO2 (little a)
Venous = PVO2 (big V)

Question 4

Why is the total gas pressure in the upper respiratory tract 94.72 kPa rather than 101 kPa?
Therefore, what is the PP of oxygen, nitrogen & CO2 in the URT?

Answer 4

• Air entering URT is humidified
• Saturated Vapour Pressure (SVP) = 6.28 kPa at body temperature
• Water vapour pressure displaces 6.28 kPa of the atmospheric gas pressure, so 101 - 6.28 = 94.72 kPa.
• 0.21 x 94.72 = 19.8kPa (O2)
• 0.78 x 94.72 = 73.8 kPa (N2)
• 0.03 x 94.72 = 0.03 kPa (CO2)

Question 5

PO2 in the URT is 19.8 kPa but alveolar PO2 is 13.3 kPa, what accounts for this difference?

Answer 5

• Tidal volume is 500mL, 30% of this fills anatomical dead space (150mL) so only 350mL of air reach the respiratory portion (alveolar ventilation)
• Total lung volume at rest is 3L. Fresh air breathed in is diluted with old air instantaneously. Older air has O2 continually extracted and CO2 constantly added
• Hence PAO2 is lower than in the URT and PACO2 is higher.

Question 6

How are PAO2 and PACO2 maintained in a steady state at normal alveolar ventilation (5L/min)?

What are the normal values?

What happens if alveolar ventilation becomes extremely low (below 2.5L/min)?

Answer 6

• Rate of extraction and replenishment (via alveolar ventilation) are kept constant for O2 and CO2
• PAO2 = 13.3 kPa (lower than inhaled air)
• PACO2 = 5.3 kPa (higher than inhaled air)
• PCO2 will rise steeply, as O2 will be extracted, CO2 will be added, but O2 will not be replenished.

Question 7

How does gas move in the from the pulmonary artery into the alveoli & in which direction does O2 and CO2 move?

Answer 7

• Mixed venous blood delivered by pulmonary artery
• PVCO2 is higher than PACO2 (6 vs 5.3 kPa), so CO2 diffuses from blood into alveolus to be exhaled out
• PVO2 is lower than PAO2 (6 vs 13.3 kPa), so O2 diffuses from alveoli into blood

Question 8

How do you calculate the amount of gas dissolved in a liquid

Answer 8

Amount dissolved = partial pressure x solubility coefficient (Henry’s Law)

E.g.: solubility coefficient for O2 in plasma = 0.01

13.3 x 0.01 = 0.13 mmol of O2

Question 9

Partial pressure difference (gradient) affects rate of gas diffusion:

• Give the equation for rate of diffusion & explain what other factors affect this rate?

Answer 9

P1-P2 = gradient, higher gradient means higher diffusion
A = surface area, increased SA means increased diffusion
T = thickness, decreases thickness/distance to diffuse means increased diffusion
D = diffusion coefficient, solubility/SR of MW, solubility of the gas in liquid most important, if solubility is higher, faster the rate of diffusion.

Question 10

Therefore, why does CO2 diffuse 20 x faster than CO2?

Answer 10

• CO2 is much more soluble than O2, despite it having a higher molecular weight and therefore has a higher diffusion coefficient.

Question 11

How is the alveolar capillary adapted for efficient gas exchange? (3)

Answer 11

• High surface area (A) - roughly 100M^2
• Low diffusion distance (T) - less than 0.4 uM thick
• Oxygen exchange complete in 1/3 time blood spends in capillary, leaving plenty of reserve for situations such as exercise or disease states.

Question 12

Name 3 conditions where there is impairment of alveolar diffusion due to problems with the alveolar capillary membrane

Answer 12

1) Fibrotic/Interstitial Lung Disease - Increased thickness of alveolar membrane (T)
2) Pulmonary oedema - fluid in interstitial space increases T
3) Emphysema - destruction of alveoli reduces SA for gas exchange (A)

Question 13

Why do healthy lungs have a ventilation:perfusion (V:Q) ratio of 0.9 rather than 1?
What does V:Q matching sure?

Answer 13

• So we ave a reserve, allowing V:Q match to approach 100% during exercise
• Ensures optimal gas exchange, in alveoli with high ventilation, it should have high perfusion

Question 14

DO SLIDES ON V:Q mismatch - fucking confused out my mind.

Answer 14

FUCK SAKE

Question 15

Why does decompression sickness (the bends) occur in deep sea divers if they surface too quickly?

Answer 15

• Pressure at 10M underwater = 2atm/202kPa
• Therefore, PN2 = 202 x 0.78 = 157.5kPa
• Nitrogen moves from high pressure in lungs to low pressure in blood
• A slow return lets nitrogen return to lungs where it is breathed out, however swimming up too quickly doesn’t give nitrogen sufficient time to leave blood, forming painful bubbles which can lead to death