9 Alveolar gases and diffusion EDIT Flashcards
Why don’t gas tensions change too much in the alveolar space when you breathe in and out?
What is the significance of this?
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
What are the venous and alveolar partial pressures of oxygen and carbon dioxide?
alveolar O2-13, CO2-5
venous O2-5, CO2-6
How can we control the alveolar PCO2?
What is the metabolic hyperbola?
concentrate it by increasing metabolism, and dilute it by increasing ventilation
the alveolar PCO2 at any VA and VCO2
What is the relationship between alveolar CO2 and CO2 production, and alveolar ventilation?
alveolar CO2 is proportional to CO2 productino
alveolar CO2 is inversely proportional to alveolar ventilation
What can we conclude from the metabolic hyperbola?
PaCO2 is a clinical measure of the adequacy of VA
How might we calculate the amount of oxygen in the alveoli?
alveolar gas equation
PAO2 = PiO2 - (PACO2 / R)
What is the clinical relevance of the alveolar gas equation?
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
What is Graham’s Law?
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
In liquid, what does diffusion of gas depend on?
its’ concentration gradient (proportional to change in pressure in a gas)
What is Henry’s law?
[gas] = PP x solubility coefficient
What is the solubility coefficient of CO2?
5.30 ml L-1 kPa-1
What is the solubility coefficient of O2?
0.23 ml L-1 kPa-1
What is the change in pressure and hence uptake of gases into blood dependent on?
solubility
chemical combination
What limits the exchange of N20, O2, and CO?
N20 - perfusion (low solubility / no chemical combination)
O2 - perfusion limited
CO - diffusion limited (high solubility / high chemical combination)
how much time does the blood spend in transit?
0.75
What is significant about the transit time for CO?
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
For what proportion of the transit time has O2 equilibrated?
2/3 of the 0.75 seconds
we could hence decrease transit time
What happens to transit time in exercise?
What about abnormal people?
it decreases, in abnormal partients people may become hypoxic
the severely abnormal person is hypoxic at rest
What, broadly, is the pulmonary diffusing capacity?
the ability to diffuse gases across lungs
What 3 variables dictate the diffusion of gases across the lungs?
gas diffusability
area
thickness
pressure gradient
flow = d x (A/T) x (pressure gradient)
What happens to the diffusing capacity of an animal with an increase in body mass?
it increases
What is the diffusability constant?
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)
What is the problem with the diffusion constant equation?
What is the solution to this problem?
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
What is the new diffusion constant equation accounting for the use of CO?
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
