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Dave Story writes:
>> [....]  In the
>> pulmonary circulation, everything works almost the opposite of the
>> systemic side with hypoxia causing quite marked vasoconstriction (to the
>> point of right heart failure in some of my patients).
>
>This is very interesting....could you comment more?  It seems to me
>that the lungs, acting as a microfilter for bubbles, would be
>particularly susceptible to general hypoxia as a result of bubble
>blockages.  If hypoxia affects even non-blocked capillaries in the
>lungs, it would seem that microbubbles in the lungs could more
>seriously affect general oxygenation than I previously thought.

The effect seems to be a combination of response to O2 pressure in the
alveolar space but also shows some response to O2 pressure in the blood
flowing into the capillaries.  A capillary blocked by a bubble would not
lower the distal PO2 and, in fact, the blood in the capillary beyond the
bubble is quite high but the blood doesn't go anywhere.  Blocking
capillaries reduces the overall contact surface for inert gas and CO2
removal although the latter is fairly easily compensated for by
increased flow anywhere else since CO2 is quite soluble in plasma and 
diffuses across membranes quite readily (as opposed to O2 or N2).  How
great the effect of hypoxia, when it occurs in pulmonary capillaries, on
nearby capillaries is not easily measured as far as I know (although
there is a lot of work ongoing in this area).  Oxygenation in the lungs
is affected by 5 distinct processes.  These are overall ventilation
(exchange of gas between alveoli and the outside), the matching of local
ventilation with perfusion of the capillaries in contact with the local
alveoli, shunt (blood passing directly from the venous side to the
arterial side without contact with alveoli, diffusion (the rate of
passage of O2 across 2 cells, some plasma and into a red cell), and the
overall pressure of oxygen (related to pressure and O2 fraction).

Tom

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