Phys - Resp - Diffusino

Front 1

is carbon monoxide uptake diffusion or perfusion limited? why?

Back 1

diffusion

since the partial pressure of CO in the blood doesn't really change with uptake (Hb binds CO tightly enought hat partial pressure is not affected), the blood can continue rapidly accepting CO through the length of the capillary and therefore relies on the ability of the blood gas barrier to diffuse gas, not the amount of blood available

(nb need to take into account rate of reaction of CO with Hb which changes things slightly)

Front 2

is nitrous oxide uptake diffusion or perfusion limited? why?

Back 2

perfusion limited

the partial pressure of NO is virtually the same as the alveolar gas very early in the capillary so no more will be picked up. therefore to remove more NO from the alveolar gas, increased blood flow (perfusion) is required

Front 3

explain why CO2 diffuses more rapidly than O2 in terms of Fick's law

Back 3

Fick's law states that the diffusion constant is proportional to the solubility of the gas and inversely proportional to (the square root of) its molecular weight

CO2 has a much higher solubility than O2, even though the molecular weight is much the same

Front 4

What is Fick's law?

Back 4

Amount of gas transferred is
proportional to:
area
diffusion constant
difference in partial pressure between sides
gas solubility (diffusion constant)

and inversely proportional to
thickness
square root of molecular weight

Front 5

is oxygen uptake diffusion or perfusion limited & why?

Back 5

both, but mainly perfusion limited in a healthy lung

there is already 4/10 partial pressure in RBC as it enters the capillary due to O2 in mixed venous blood.

O2 still combines with Hb but has much less affinity for it than CO. Capillary pO2 matches alveolvar pO2 about 1/3 of the way along the capillary

however if there is some pathology which causes thickening of the blood gas barrier, O2 uptake will also be diffusion limited

Front 6

how does exercise change diffusion of oxygen across the blood gas barrier?

Back 6

it doesn't if the lung is healthy

exercise reduces the time spent in the capillary to 0.25 second ie reduces time to oxygenation

but this is enough time in a healthy lung to reach alveolar Po2

if the lung is thickened (ie in disease), oxygen diffusion is impeded and alveolar pO2 is not reached until about 0.5 sec in capillary

Front 7

how does altitude change diffusion of oxygen across blood gas barrier?

Back 7

alveolar pO2 is lowered so driving force (pressure difference) is less and diffusion is slower

also when the pO2 is low, the slope of the dissociation curve is steep ie hb saturation drops off quickly

in a healthy subject, increased blood flow (eg exercise) means that the full amount of O2 may not diffuse in time

in an unhealthy subject, exercising at altitude may mean completely inadequate perfusion

Front 8

?? do we need to be able to discuss measurement of diffusing capacity?

Back 8

.

Front 9

what increases the diffusing capacity of carbon monoxide?

Back 9

exercise ie increased blood flow (recruitment & distention of pulmonary capillaries)

Front 10

what is the other kind of resistance (apart from diffusion/perfusion) against O2 entering capillaries?

Back 10

reaction rate w Hb

Front 11

what are the two components of O2 uptake into the capillary?

Back 11

diffusion across blood gas barrier

reaction of O2 w Hb

Front 12

what is θ with respect to rate of reaction of a gas w Hb?

Back 12

it is the rate of reaction w Hb

ie

rate in mL/min O2/CO which combines w 1mL blood per mm Hg partial pressure O2/CO

Front 13

what is the formula for diffusing capacity for rate of reaction of O2/CO w Hb?

Back 13

θ x Vc

where Vc is volume of capillary blood

Front 14

what reduces θ for CO and why? what implications does this have for measurement?

Back 14

if the subject breathes a high O2 mixture because O2 competes w CO for Hb

can therefore measure Vc by changing alveolar Po2

Front 15

what proportion has each component of O2 transfer across the blood gas barrier

Back 15

diffusion and rate of reaction w Hb each contribute about 50%

ie both lung thickening and decreased Hb or blood flow can have a significant contribution

Front 16

what determines if a gas is diffusion limited?

Back 16

the slope on the dissociation curve

ie how solubility in blood gas barrier compares to the solubility in blood