Lecture 5 - Partial pressure and diffusion

Question 1

What would methotrexate induced pulmonary fibrosis do to FRC?

Answer 1

Decrease FRC

As elastic recoil increases and compliance decreases

Question 2

Equation for pressure

Answer 2

P= (n x R x T)/ V

n = no. of moles present
R= gas constant 
T = Temperature 
V = Volume

Question 3

Dalton’s law

Answer 3

In a mixture of gases, each individual gas exerts a partial pressure in proportion to its volume percentage in the mixture

Question 4

Atmospheric pressure

Answer 4

101.1 kPa

Question 5

What happens to atmospheric pressure at higher altitudes?

Answer 5

Decreases as less weight of air pressing down

Question 6

Equation for partial pressure of gas

Answer 6

PP = Total pressure x Proportion of gas in mixture

Question 7

Equilibrium of gas in contact with liquid

Answer 7

Rate of gas dissolved = rate of gas coming out of the solution

pp of dissolved gas = pp of free gas

Question 8

Equation for the amount of dissolved gas

Answer 8

Amount of gas dissolved = partial pressure of gas x solubility coefficient

Question 9

Solubility coefficient

Answer 9

Amount of gas that dissolves in 1 L of plasma at body temperature (37degrees) when exposed to a given partial pressure

Question 10

pO2 in atmosphere, conducting airways, alveoli, arteries and vein

Answer 10

Atmosphere - 21.3 kPa
Conducting airways - 20 kPa
Alveoli - 13.3 kPa
Arteries - 13.3 kPa
Veins - 6kPa

Question 11

Why does pO2 decrease in alveoli and conducting airways

Answer 11

In the conducting airways, air is warmed and moistened therefore there is a water vapour pressure exerted.

In the alveoli, new air (10%) is mixed with old air. The old air loses oxygen and gains CO2

Question 12

What percentage of the tidal volume is used by the anatomical dead space

Answer 12

30%

150ml

Question 13

pCO2 in atmosphere, conducting airways, alveoli, arteries and vein

Answer 13

Atmosphere - 0.04 kPa
Conducting airways - 0.04 kPa
Alveoli - 5.3 kPa
Arteries - 5.3 kPa
Vein - 6.1 kPa

Question 14

Solubility coefficient of O2

Answer 14

0.01 mmol/L/kPa

Question 15

Content of gas

Answer 15

Amount of gas chemically bound + amount of free gas in solution

Question 16

Why is O2 bound to Hb

Answer 16

More O2 can be carried in blood so more O2 can be dissolved.

Keeps partial pressure in blood low to maintain a strong diffusion gradient

Question 17

Percentage of O2 bound to Hb

Answer 17

98-99%

Question 18

Factors affecting diffusion

Answer 18

Surface area
Diffusion distance
Partial pressure gradient
Diffusion coefficient - depending on solubility and molecular weight of gas

Question 19

Diffusion coefficient

Answer 19

Solubility / √ molecular weight

Molecular weight is square rooted therefore solubility matters more

Question 20

Why does CO2 have a higher diffusion coefficient compared to O2?

Answer 20

More soluble - diffuses faster
Has a higher molecular weight but solubility matters more

CO2 diffuses 20 x faster than O2

Question 21

What compensates for the low diffusion coefficient of O2

Answer 21

Higher partial pressure gradient

Question 22

In a diseased state is CO2 or O2 affected more? why?

Answer 22

O2 as diffuses slower

CO2 affected in late-stage disease

Question 23

Diffusion barriers

Answer 23

Fluid film lining the alveoli
Epithelial cells of the alveoli 
Interstitial fluid 
Endothelium of the capillary 
Plasm
RBC membrane

Question 24

How has the lungs adapted for diffusion

Answer 24

Large SA
Short diffusion pathways as thin
100% oxygen exchange occurs through 1/3rd of the capillary length - plenty of reserve

Question 25

What causes diffusion impairment?

Answer 25

Fibrotic lung - thickened alveolar membrane
Pulmonary oedema - Fluid in interstitial space
Emphysema - decreases alveolus SA