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MICROCIRCULATION AND REGULATION |FINDYOURSELF
Microcirculation
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Capillaries
are the sites for exchange of materials between blood and tissue cells.
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The
walls of the capillaries are extremely thin, constructed of single-layer of highly
permeable endothelial cells. Therefore water, cell nutrients, and cell excreta
can interchange quickly and easily between the tissues and the circulating
blood.
•
The
peripheral circulation of the whole body has about 10 billion capillaries with
a total surface area estimated to be 500 to 700 square meters.
Structure of the Capillary Wall
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The
wall is composed of a single layer of endothelial cells and is surrounded by a
very thin basement membrane on the outside of the capillary. The total
thickness of the capillary wall is only about 0.5 micrometers. The internal diameter
of the capillary is 4 to 9 micrometers.
Structure of Microcirculation and Capillary System
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Each
nutrient artery entering an organ branches six to eight times before the
arteries become small enough to be called arterioles, which generally have
internal diameter of only 10 to 15 micrometers. Then the arterioles branch two
to five times, reaching diameter of 5 to 9 micrometers at their ends where they
supply blood to the capillaries.
•
The
arterioles are highly muscular, and their diameter can change manyfold.The
metarterioles (the terminal arterioles) do not have a continuous muscular coat,
but smooth muscle fibers encircle the vessel at intermittent points.
•
At
the point where each capillary originates from a metarteriole, smooth muscle
fibers usually encircle the capillary. This is called the precapillary
sphincter. This sphincter can open and close the entrance to the capillary.
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The
venules are larger than the arterioles and have a much weaker muscular coat.
•
The pressure in the venules is much less than
that in the arterioles. The venules can contract despite the weak
muscle.
Pores in the Capillary Membrane
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An
intercellular cleft thin-slit like curving channel that lies between
adjacent endothelial cells. Each cleft is interrupted by short ridges of
protein attachments that hold the endothelial cells together, but between these
ridges fluid can percolate freely through the cleft. The cleft normally has a
uniform spacing with a width of about 6 to 7 nanometers.
•
There
are many minute vesicles called caveolae in the endothelial cells. These are
formed from protein caveolins.
•
They
are believed to play role in endocytosis.
Vasomotion
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Intermittent
contraction of the metarterioles and precapillary sphincters.
Because of
this blood does not flow continuously through the capillaries.
Regulation of Vasomotion
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The
most important factor found to affect the degree of opening and closing of the
metarterioles and precapillary sphincters is the concentration of oxygen in
the tissues.When the rate of oxygen usage by the tissue is great so that tissue
oxygen concentration decreases below normal the intermittent periods of
capillary blood flow occur more often, and the duration of each period of flow
lasts longer, thereby allowing the capillary blood to carry increased
quantities of oxygen (as well as other nutrients) to the tissues.
•
Exchange
of water, nutrients and other substances between the blood and Interstitial
fluid occurs by diffusion through the capillary membrane.
