9 Alveolar gases and diffusion EDIT

Question 1

Why don’t gas tensions change too much in the alveolar space when you breathe in and out?
What is the significance of this?

Answer 1

we take little air into this space, we don’t empty and fill it as we’re in the conducting zone
the movement is only via diffusion

the alveolar gas tensions are essentially stable, allowing the blood to equilibrate ot these levels

Question 2

What are the venous and alveolar partial pressures of oxygen and carbon dioxide?

Answer 2

alveolar O2-13, CO2-5

venous O2-5, CO2-6

Question 3

How can we control the alveolar PCO2?

What is the metabolic hyperbola?

Answer 3

concentrate it by increasing metabolism, and dilute it by increasing ventilation
the alveolar PCO2 at any VA and VCO2

Question 4

What is the relationship between alveolar CO2 and CO2 production, and alveolar ventilation?

Answer 4

alveolar CO2 is proportional to CO2 productino

alveolar CO2 is inversely proportional to alveolar ventilation

Question 5

What can we conclude from the metabolic hyperbola?

Answer 5

PaCO2 is a clinical measure of the adequacy of VA

Question 6

How might we calculate the amount of oxygen in the alveoli?

Answer 6

alveolar gas equation

PAO2 = PiO2 - (PACO2 / R)

Question 7

What is the clinical relevance of the alveolar gas equation?

Answer 7

PaCO2 whic can easily be obtained from ABG’s can replace PACO2 in hte alveolar gas equation

PiO2 is easily determined and taken as its value when dry at 37 degrees

R (essentially RQ) will be 0.8 unless the diet is unusual

Question 8

What is Graham’s Law?

Answer 8

diffusion in gaseuous phase is dependent on change in P and is proportional to 1/√MW

bigger molecules diffuse slower than smaller ones
MW CO2 = 44
MW O2 = 32

therefore CO2 diffuses in gas 0.85x as fast as O2

Question 9

In liquid, what does diffusion of gas depend on?

Answer 9

its’ concentration gradient (proportional to change in pressure in a gas)

Question 10

What is Henry’s law?

Answer 10

[gas] = PP x solubility coefficient

Question 11

What is the solubility coefficient of CO2?

Answer 11

5.30 ml L-1 kPa-1

Question 12

What is the solubility coefficient of O2?

Answer 12

0.23 ml L-1 kPa-1

Question 13

What is the change in pressure and hence uptake of gases into blood dependent on?

Answer 13

solubility

chemical combination

Question 14

What limits the exchange of N20, O2, and CO?

Answer 14

N20 - perfusion (low solubility / no chemical combination)
O2 - perfusion limited
CO - diffusion limited (high solubility / high chemical combination)

Question 15

how much time does the blood spend in transit?

Answer 15

0.75

Question 16

What is significant about the transit time for CO?

Answer 16

every molecule of CO that binds to haem limits the partial pressure of CO in fluid in blood, so more CO moves, but the patial pressure barely changes

it is hence diffusion limited, and CO takes way way longer than 0.75 seconds to equilibrate

Question 17

For what proportion of the transit time has O2 equilibrated?

Answer 17

2/3 of the 0.75 seconds

we could hence decrease transit time

Question 18

What happens to transit time in exercise?

What about abnormal people?

Answer 18

it decreases, in abnormal partients people may become hypoxic

the severely abnormal person is hypoxic at rest

Question 19

What, broadly, is the pulmonary diffusing capacity?

Answer 19

the ability to diffuse gases across lungs

Question 20

What 3 variables dictate the diffusion of gases across the lungs?

Answer 20

gas diffusability
area
thickness
pressure gradient

flow = d x (A/T) x (pressure gradient)

Question 21

What happens to the diffusing capacity of an animal with an increase in body mass?

Answer 21

it increases

Question 22

What is the diffusability constant?

Answer 22

flow = d x (A/T) x (pressure gradient)

BUT in the lung, A, and T are difficult to measure so are combined to form D(L), the diffusability constant

D(L) = flow / pressure gradient ml min(-1) kPa(-1)

Question 23

What is the problem with the diffusion constant equation?

What is the solution to this problem?

Answer 23

it’s difficult to measure directly for O2 as PO2 changes along pulmonary capillary

we use CO at low concentrations at a single breath (it binds well to haem)

this allows us to measure CO for P1 and P2

Question 24

What is the new diffusion constant equation accounting for the use of CO?

Answer 24

D(L)CO = V(dot)CO / PACO = 175 ml min-1 kPa-1

we use a conversion factor of 1.25 to calculate, normal D(L)O2 = 220 ml min-1 kPa-1

the D(L)CO is converted to D(L)O2 to be clear

Question 25

What 2 things is pulmonary diffusing capacity reduced by?

Answer 25

reducing SA
increasing diffusion path length

NOTE - D(L) tests can’t distinguish between theses

Question 26

How might effective surface area be decreased?

Answer 26

loss of lung tissue
airway obstruction
capillary obstruction
ventilation / perfusion mismatch

Question 27

How might diffusion distance be increased?

Answer 27

thickened alveolar - capillary membrane (fibrosis)
fluid build up
increased intracapillary distance (low haemoglobin)