Capillaries
The sum of the diameters of all capillaries is
significantly larger than that of the aorta (by about three orders of
magnitude), which results in decreases in blood pressure and flow rate. Also,
capillaries are very small vessels. Their diameter ranges from 4-15 µm. The
wall of a segment of capillary may be formed by a single endothelial cell. This
results in a very large surface to volume ratio. The low rate of blood flow and
large surface area facilitate the functions of capillaries in
- providing
nutrients and oxygen to the surrounding tissue, in
- the
absorption of nutrients, waste products and carbon dioxide, and in
- the
excretion of waste products from the body.
These functions are also facilitated by a very
simple organisation of the wall of capillaries. Only the
tunica intima is present, which typically only consists of the endothelium,
its basal lamina and an incomplete layer of cells surrounding the capillary,
the pericytes. Pericytes have contractile
properties and can regulate blood flow in capillaries. In the course of
vascular remodelling and repair, they can also differentiate into endothelial
and smooth muscle cells.
Three types of capillaries can be
distinguished based on features of ethe endothelium.
Continuous
capillaries
are formed by
"continuous" endothelial cells and basal lamina. The endothelial cell
and the basal lamina do not form openings, which would allow substances to pass
the capillary wall without passing through both the endothelial cell and the
basal lamina. Both endothelial cells and the basal lamina can act as selective
filters in continuous capillaries.
Fenestrated
capillaries
The endothelial cell body forms
small openings called fenestrations, which allow
components of the blood and interstitial fluid to bypass the endothelial cells
on their way to or from the tissue surrounding the capillary. The fenestrations
may represent or arise from pinocytotic vesicles which open onto both the
luminal and basal surfaces of the cell. The extent of the fenestration may
depend on the physiological state of the surrounding tissue, i.e. fenestration may
increase or decrease as a function of the need to absorb or secrete. The
endothelial cells are surrounded by a continuous basal lamina, which can act as
a selective filter.
Discontinuous
capillaries
are formed by fenestrated
endothelial cells, which may not even form a complete layer of cells. The basal
lamina is also incomplete. Discontinuous capillaries form large irregularly
shaped vessels, sinusoids or sinusoid capillaries. They are found where a very free
exchange of substances or even cells between bloodstream and organ is
advantageous (e.g. in the liver, spleen, and red bone
marrow).
Suitable Slides
Sections of any well preserved
tissue - H&E, Whipf's
polychrome
cardiac and skeletal muscle, glands or the papillary
layer of the skin contain dense capillary beds.
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Cardiac Muscle, sheep -
Whipf's polychrome
Large numbers of capillaries are present in almost all
tissues. At least a few dozen cross sections are present in every sqr. mm of
section of poorly vascularised tissues. There may be thousands in highly
vascularised tissues. However, a "good" capillary is not that easy
to find because of their small size and because the capillary walls are very
thin, which often leads to the collapse of the capillary during tissue
preparation.
Cardiac muscle is highly vascularised. Each muscle cell is surrounded by one
or more capillaries. The capillaries roughly follow the course of the muscle
cells. To find capillaries in transverse and longitudinal sections it is
easiest to first find areas in which the muscle cells have been cut in these
planes. Only one or two red blood cells fit side by side in the capillary. A
single endothelial cell forms the wall around the entire circumference of a
segment of the capillary. Endothelial cell nuclei are therefore not always
visible, and some red blood cells are only surrounded by a fine line
representing the capillary wall.
Identify and draw a few capillaries. Include some of the
surrounding tissue features - maybe a cardiac muscle cell, a venule or
arteriole - as a scale.
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![thumbnail]( "Capillaries - Digital Signal")
Title:
Capillaries - Digital Signal
by:
om
at
2013-02-15T03:16:00+07:00
Rating: 4.8
of 5 Reviews
Capillaries - Digital Signal