The cardiovascular system
comprises the heart and blood vessels, arteries, veins, capillaries
and sinusoids. The arteries and veins passing
to organs and muscles are usually accompanied by the nerves, and together form
a compact neurovascular bundle.The walls of the arteries possess three coats,the
intima, composed of an endothelial lining and a small amount of connective tissue; the media, which is composed mainly of
elastic tissue in the larger arteries and almost entirely of smooth muscle in
the small arterioles
and
medium-sized arteries; and the outer fibrous adventitia. The coats of the veins
correspond to those of the arteries,but the
media contains less smooth muscle
and fewer elastic fibres.In the larger veins the adventitia is thicker than the
media.
In most veins, valves are present. These are formed of
pairedfolds of endothelium and help to determine the direction of
flow. Medium and smaller arteries
are often accompanied by two veins, the venae comitantes, rather than one. The
smallest,postcapillary veins are termed venules. The capillaries,which unite the arteries and
veins, have walls formed of a
single endothelial layer of large
angular flattened cells.The direct union between two vessels is called an anastomosis.
Arteriovenous anastomoses occur around the nail beds and
are an important mechanism in controlling digital
blood flow.
They may also exist as congenital
abnormalities of the vascular system and can be created surgically when a large
vein with an arterialized
circulation is required for regular access to the circulation.Sinusoids are
thin-walled, dilated channels uniting arteries and veins and are found in the
bone marrow, liver, spleen and suprarenal glands.
In some situations blood passes
through two capillary beds before returning to the heart: this constitutes a portal circulation.The passage of blood from the
stomach, intestine,pancreas and spleen through the liver exemplifies such a
system.Short vessels passing through foramina in the skull and joining venous
channels (sinuses) inside and veins outside are
called emissary
veins.
Reduction of the blood supply to
a region is known as ischaemia, and this is of clinical importance in the heart
and brain. One important degenerative arterial disease that can affect the
vessels is arteriosclerosis, and this is very prevalent
in developed countries. The
arterial narrowing produced by the disease may cause local intravascular
clotting to occur. A thrombus
may become
detached and flushed into the bloodstream, forming an embolus and blocking distal smaller
vessels. Local death of an area of tissue or organ owing
to reduction of its blood supply is known as
an infarction.In situations where bacteria infect the infarcted area it
undergoes putrefaction,a condition known as gangrene. In some instances it is possible surgically to bypass arterial
blockages, thus re-establishing the distal blood supply and preventing The body
responds to an injury, e.g. invading bacteria, by the process known as inflammation. The capillaries dilate and white blood
cells pass out of the circulation to phagocytose the offending organisms. The
area becomes red and hot because of the increased blood supply, and swollen
with increased tissue fluid; it is also painful. A collection of dead tissue
and dead white blood cells is called an abscess.
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