Gas Exchange: Properties Of Gases And Diffusion In The Lung

Question 1

What would happen to a patients function residual capacity if they experienced methotrexate induced pulmonary fibrosis and why?

Answer 1

FRC would decrease because pulmonary fibrosis causes increased lung elastic recoil and decreased compliance

Question 2

What is Dalton’s law on gases?

Answer 2

The total pressure of the gases = the sum of the partial pressures of the individual gases

Question 3

At sea level what is 1 atmosphere equivalent to in mmHg and Kpa?

Answer 3

101Kpa

760mmHg

Question 4

Why is atmospheric pressure lower at high altitudes?

Answer 4

Air molecules are dispersed = thin air = less air molecules available when you breathe

Question 5

What are the rough percentages of gases in ambient air?

Answer 5

Nitrogen 79%
Oxygen 21%
Carbon dioxide 0.04%

Question 6

How could you work out the partial pressure of O2 if you knew the atmospheric pressure?

What do you need to consider when using this for inspired air?

Answer 6

Fraction of oxygen in the air (21% in ambient air) X atmospheric pressure (101Kpa at sea level)

Inspired gas is warmed and humidified in the URT (add water vapour) so need to subtract the water vapour pressure from the atmospheric pressure (about 6.3kpa) so 101-6.3 = 94.7Kpa

0.21 X 94.7

Question 7

What’s the PO2 in conducting airways of our respiratory tract? Why is it lower than atmospheric?

Answer 7

20Kpa

It’s humidified so water vapour is added to the mix when it’s humidified

Question 8

What is alveolar/ arterial partial pressure of O2? Why is this different to the pressure in the conducting part of the respiratory tract?

Answer 8

13.3Kpa

Tidal volume (air inspired/ expired at rest) = 450ml, 30% of this fills the anatomical dead space so 150ml

Anatomical dead space - air conducting space from nostrils-> bronchi where gas exchange can’t occur

So 300ml new air reaches respiratory portion (lungs air volume about 3000ml) so 10% of all air in lungs is fresh

Question 9

What’s the partial pressures for CO2 in the conducting airways, the alveoli, arteries and veins?

Answer 9

Conducting: 0.04KPa (same at atmospheric)

Alveoli= 5.3Kpa
Arterial= 5.3Kpa 

Veins = 6.1Kpa

Question 10

What’s the mixed venous partial pressure of O2?

Answer 10

6Kpa

Mixed - from SVC, IVC and coronary veins (average) most distal part of pulmonary artery

Question 11

Dissolved gases exert pressure in the liquid. When is equilibrium reached? What’s another term for partial pressure of a gas in the liquid? When does this occur physiologically?

Answer 11

When the rate of gas entering the liquid = rate of gas leaving the liquid

Partial pressures of gas dissolved = partial pressure of gas in air above

Tension

Arterial blood oxygen and Caron dioxide equilibrium to alveolar levels bc equilibrium is reached

Question 12

How can you work out the amount of a gas dissolved mmol/L?

Answer 12

Partial pressure(Kpa) X solubility coefficient of gas (tendency to dissolve)

Question 13

What is the solubility coefficient for O2 In Plasma?

Answer 13

0.01mmol/L/Kpa (at 37dc)

Question 14

How does O2 binding to Hb affect O2 equilibrium between the alveolar and blood?

Answer 14

It doesn’t

Hb must be fully Saturated (with 4O2 molecules) then O2 will continue to dissolve till equilibrium is reached

Maintains a strong gradient for diffusion

Question 15

What proportion of O2 in blood is bound to Hb and what proportion is dissolved? How does this O2 enter the tissues?

Answer 15

98-99% bound Hb
1-2% dissolved

Dissolved O2 diffuses into tissues and then is replaced by O2 from Hb

Question 16

What’s is the rate of diffusion directly proportion to? (Equation)
What’s the diffusion coefficient directly proportional to?

Answer 16

Surface area available for diffusion X diffusion coefficient of individual gas X partial pressure difference across membrane

All divided by the distance molecules must diffuse

A x D x (P1-P2)
———————
T

D directly proportional to solubility divided by the square root of Mr

Question 17

Out of O2 and CO2 which has the smaller diffusion coefficient and why? What doe this mean for diffusion speed?

Answer 17

CO2 much more soluble then O2 but has a higher molecular weight

However the effect of solubility is greater so CO2 diffuses 20X faster

However, larger difference in partial pressure for O2 compensates for slower diffusion

Question 18

Why in a diseases lung with lower oxygen levels is O2 gas exchange more impaired than CO2?

Answer 18

Bc O2 has a slower diffusion rate

Question 19

What happens to the levels of CO2 and O2 in the blood in hypoventilation?

Answer 19

CO2 increases (hypercapnia) bc air needs to be delivered for gas exchange

And O2 decreases

Question 20

What diffusion barriers does alveolar air need to cross to reach RBCs in capillary?

Answer 20

Fluid film lining alveoli 
Epithelium alveoli
Interstitial space
Endothelium capillary 
Plasma 
RBC membrane

Question 21

How much of the time blood spends in the lungs is O2 exchange completed in? Why is this a benefit?

Answer 21

About 1/3

Leaves a reserve for exercise

Question 22

Factors affecting rate of gas diffusion in disease

Answer 22

Thickness:

• increased oedema as fluid in interstitial space and alveoli
• increased in lung fibrosis of alveolar and capillary membranes and interstitium

Surface area:

• decreased removal of lung
• decreased emphysema

Diffusion coefficient of the gas:
- CO2 always diffuses faster so diffusion of O2 is affected and PO2 is low in diseased lungs but CO2 not affected until late stages of disease

Question 23

What is ‘the bends’ how can it be prevented?

Answer 23

Decompression sickness in divers

Nitrogen moves from high pressure in lungs into the blood (normally nitrogen too big to diffuse but under high pressure) swimming up too quickly means N2 can form painful bubbles and embolise

✅ a slow return to the surface lets the nitrogen return to the lungs where it’s breathed out