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6.5 Capillary in greater omentum (dog). The capil-
lary wall comprises a tube of squamous epithelial cells
(endothelium). The nuclei of the endothelial cells
protrude slightly into the lumen. Endothelial cells are
often accompanied by pericytes, cells of mesodermal
origin that lie on the outer surface of the endothe-
lium. Goldner’s Masson trichrome stain (x250).

Based on its multiple functions, the endothelium lining arterioles (metarterioles). These vessels provide temporary
capillary walls is classified as a transport epithelium (see resistance to blood flow by way of precapillary sphincters.
Chapter 2, ‘Epithelial tissue’). They are innervated by adrenergic nerve fibres. Changes
Three types of capillary can be distinguished; their in the diameter of these vessels alters vascular resistance
structure is related to the tissues and organs in which they and thus has a direct influence on blood pressure and blood
are located (Figure 6.2). flow through the capillary bed (Figure 6.12).
Capillary blood flow is also regulated by arteriovenous
Continuous capillaries anastomoses. These specialised structures shunt blood
Capillaries lacking physical interruptions (non-fenestrated directly from arteries to venules, bypassing the capillary
capillaries, endotheliocytus nonfenestratus) are by far the bed (Figure 6.12, see also below).
most common type of capillary found in the body. Their The role of the capillary endothelium in the transfer
wall is relatively thick, limiting rapid trans-epithelial trans- of substances between the blood and the tissues has been
port. They contain abundant pinocytotic vesicles. The described above. In addition, the vascular endothelium has
barrier function of continuous capillaries is further rein- several other important functions:
forced by a layered basal lamina and numerous pericytes
(e.g. in brain and muscle capillaries). · synthesis of vasoconstrictors (endothelin) and vaso-
dilators (nitrous oxide and prostacyclin),
Fenestrated capillaries · regulation of inflammation and immune responses
This type of capillary is found mainly in tissues in which large (synthesis of IL-1, IL-6, IL-8, expression of adhesion
volumes of fluid are exchanged at a rapid rate (e.g. capil- molecules and histocompatibility antigens),
laries involved in filtration in the glomerulus, absorption in · synthesis of anticoagulants (thrombomodulin, hepa-
the renal tubules, capillary loops in the intestinal mucosa, rin-like molecules) and
exocrine and endocrine organs). The endothelium (endothe- · regulation of cell growth by synthesis of growth
liocytus fenestratus) is markedly flattened and is interrupted factors and inhibitors.
in a sieve-like fashion by numerous openings (fenestrae). The
fenestrae are usually covered by a diaphragm. When the dia- Blood vessels
phragm is lacking, the openings are referred to as pores.
Structure
Sinusoidal capillaries (vas capillare sinusoideum) For functional reasons, the structure of larger vessels
Sinusoidal (discontinuous) capillaries, or sinusoids, are (arteries, arterioles, veins and venules) varies between
characterised by intercellular spaces, a typically discon- the high- and low-pressure systems, and within individual
tinuous or absent basal lamina, numerous pores and organs. Nevertheless, the basic structure of the vascular
an irregular shape. This type of capillary enables rapid wall is consistent (Figures 6.6 to 6.12 and Table 6.1), con-
exchange of metabolic substances (e.g. in the liver, spleen, stituting three concentrically positioned tubes:
adenohypophysis, adrenal cortex and bone marrow).
Phagocytic cells of the mononuclear phagocytic system · tunica intima (interna) comprising the
are often associated with the wall of the sinusoid. − endothelium,
Blood flow through the capillary bed is regulated by the − stratum subendotheliale (subendothelial layer) and
contraction of smooth muscle in the wall of precapillary

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