The
Microcirculation
197
processes to take place. These processes are slow in comparison to blood
perfusion.
For this reason, a capillary is normally in the neighborhood
and within reach to any single cell at a distance of about three to four
cells apart.
A
given capillary can have a length of about
1
mm and a
diameter of 4-10 pm.
Two principal types of capillaries are found.
The true capillaries
have no vascular smooth muscle and form network with other capillaries.
The arterio-venous or
A-V
capillaries have some amount of smooth
muscle and are directly connected to muscular arterioles and small
venules. Phasic flow patterns are determined by the vasomotion,
attributed to the small arterioles, the
A-V
capillaries, the pre-capillary
sphincters. These latter give rise to the constant dilation and constriction
that modulate the flowing blood. Local control dominates, as the pre-
capillary sphincters do not have nerve connections. The true capillary
walls are without connective tissue and smooth muscle and consist of a
single layer of endothelial cells surrounded by a basement membrane of
collagen and mucopolysaccharides.
Differences in capillary endothelium structure give rise to different
capillary function in different tissues.
This is well illustrated in Fig.
7.1.4, *here a continuous capillary, a fenestrated capillary and a sinusoid
capillary are shown. In a continuous capillary (Fig. 7.1.4
(A)),
4 nm
clefts, a complete basement membrane and numerous vesicles are seen.
(Fig. 7.1.4
(B))
shows the fenestrated capillary with pores through a thin
portion of the wall, few vesicles and a complete basement membrane.
(Fig. 7.1.4
(C))
shows a sinusoidal capillary with large para-cellular gaps
extending through the discontinuous basement membrane. Because of
the structural differences, the continuous capillary has been found to be
the least permeable, while the sinusoidal capillary, the most permeable.
Continuous capillaries are found, for instances, in muscle, nervous tissue
and the lungs. The fenestrated capillaries are found, for example, in
intestines. The sinusoidal capillaries are located, for instances, in the
liver, adrenal cortex and bone marrow.
Nutrients and formed substances can move across the wall of
continuous capillaries either through or between the endothelial cells.
Lipid-soluble substances diffuse through the cell membrane. Water and
ions diffuse through the water-filled clefts between cells. There are
