Chapter
7
The
Microcirculation
7.1 Structure
of
the Microcirculation
7.1.1
Functional Organization
of
the Microvasculature
The function
of
the cardiovascular system is to provide
a
homeostatic
environment for the cells of the organism. The exchange of the essential
nutrients and gaseous materials occurs in the microcirculation at the level
of
the capillaries. These microvessels are of extreme importance for the
maintenance
of
a balanced constant cellular environment. Capillaries and
venules are known as exchange vessels where the interchange between
the contents in these walls and the interstitial space occur across their
walls.
The microcirculation can be described in terms
of
a
network such as
that shown in Fig.7.1.1. It consists of an arteriole and its major branches,
the metarterioles. The metarterioles lead to the true capillaries via a
precapillary sphincter. The capillaries gather to form small venules,
which in turn become the collecting venules. There can be vessels going
directly from the metarterioles to the venules without supplying capillary
beds. These vessels form arteriovenous
(A-V)
shunts and are called
arteriovenous capillaries. The thickness
of
the wall and endothelium of
these structures and the proportionate amounts of the various vascular
wall components have been shown in Chapter
2.
The capillary and
venule have very thin walls. The capillary as mentioned before, lacks
smooth muscle and only has a layer of endothelium. The smooth muscle
and elastic tissue are present in greater amounts in vessels having
vasoactive capabilities, such as arterioles. This is also the site of greatest
drop in mean blood pressure. For this reason, arterioles are the principal
