The
Microcirculation
195
pulsatile or intermittent in nature (e.g. Zweifach,
1974).
It has also been
shown that the rhythmic vasomotor activity
of
the precapillary sphincters
is responsible for the observed intermittency. The sphincters may also
exhibit
constriction and dilation in response to changes in local
metabolites, chemicals, or sympathetic stimuli.
Together with the
arterioles, the precapillary sphincters serve to adjust the amount
of
blood
flow to meet the demands in tissues.
Fig.
7.1.3:
Photomicrographic reconstruction
of
the microcirculation from a terminal
arteriole through capillaries to a collecting venule. The flow into the capillaries remains
pulsatile or intermittent. Direct pulsatile pressure recordings are shown. From Zweifach
(1
974).
If we compare the size of red cells from various mammals, we find
the perhaps surprising fact that their diameters seem to be rather uniform
and independent of mammalian body size (Li,
1996).
This is summarized
