Plasma Skimming

[E.S.Prakash]

I read about plasma skimming - that the haematocrit of capillary is
lower than in arteries because of this phenomenon. But what about the
function of capillaries as exchange vessels especially at the lung
areas? Wouldn't it be less efficient due to plasma skimming as the
number of rbc has reduced? Can you enlighten me further on this
concept of plasma skimming?

Thanks

Cindy Teh
Year 2 Student

[Nicholas Cheong]

Dear Cindy Teh,
Thank you for sharing your doubt. I never thought along this line
before but I'm very intrigued by
your question.

I'll try to answer your question using the:Fahraeus-Lindqvist Effect.
This phenomenon is as follows:
Below a critical vessel caliber, blood viscosity becomes dependent
upon vessel radius.
The critical radius is approximately 1 mm. Viscosity falls sharply
down to a vessel caliber of approximately 12-15 um. (Note that typical
capilary diameter ranges from 5 - 10 um)

As you mentioned, hematocrit is much less in the capillaries as
compared to the larger vessels. The reason for this reduction in
hematocrit is due to plasma skimming. There is a correlation between
dynamic hematocrit and blood viscosity. As dynamic viscosity decreases
due to plasma skimming, viscosity of blood drops sharply.

The fact that blood viscosity declines in smaller vessels is
beneficial since less resistance is incurred in the vascular circuit.
Therefore lower perfusion pressures, lower transmural (blood)
pressures, and a smaller pump (heart) are required to adequately
perfuse the tissues.

Since resistance is less, we can say that blood flow increases. Now to
answer your question. In my opinion,
plasma skimming allows pulmonary blood flow to increase and more
erythrocytes can be brought to the alveoli to take part in gaseous
exchange per unit time.

If a large number of erythrocytes are allowed into the capillaries,
sludging may occur. Gas exchange may not take place efficiently. In
spite of the large number of erythrocytes, their presence in the
pulmonary capillary may be futile since there may be cramping of
erythrocytes and total surface area of erythrocytes involved in gas
exchanged may be significantly reduced. Pulmonary capillary viscosity
may also increase
and this may not be good

This is just what I feel. I believe Dr Prakash will enlighten us on
this matter.

All best,
Nicholas Cheong
MBBS Batch 14

[Nicholas Cheong]

Correction:

As dynamic viscosity decreases
due to plasma skimming, viscosity of blood drops sharply.

I mistyped the above line. I meant to say "As dynamic HEMATOCRIT
decreases( due to plasma skimming), viscosity of blood drops sharply"

aLL BEST,
nicholas MBBS Batch 14
On Sep 19, 11:24 pm, Nicholas Cheong <nicholascheon...@gmail.com>
wrote: