=Angiology= is the description of the organs of circulation of the blood
and lymph—the heart and vessels. The =heart= is the central hollow
muscular organ which functions as a suction and force pump; the
differences in pressure caused by its contraction and relaxation
determine the circulation of the blood and lymph. It is situated in the
middle mediastinal space of the thorax, between the two lungs, and is
inclosed in a fibro-serous sac—the pericardium. The vessels are tubular
and run through almost all parts of the body. They are designated
according to their contents as =blood-= and =lymph vessels=.
The =blood-vascular system= consists of: (1) the =arteries=, which
convey blood from the heart to the tissues; (2) the =capillaries=,
microscopic tubes in the tissues which permit of the necessary
interchange between the blood and the tissues; (3) the =veins=, which
convey the blood back to the heart.
The blood-vessels are divided into the =pulmonary= and the =systemic=.
The =pulmonary artery= conveys the blood from the right ventricle of the
heart to the lungs, where it is arterialized, and is returned by the
=pulmonary veins= to the left atrium of the heart, and passes into the
left ventricle. The =systemic arteries= convey the blood from the left
ventricle all over the body, whence it is returned by the =venæ cavæ= to
the right atrium, and passes into the right ventricle.
It should be noted, however, that the lungs receive arterial blood
through the systemic bronchial arteries. This blood is returned in
part by the bronchial veins to the right atrium (indirectly), in part
by the pulmonary veins to the left atrium.
The term =portal system= is often applied to the portal vein and its
tributaries which come from the stomach, intestine, pancreas, and
spleen. The vein enters the liver, where it branches like an artery, so
that the blood in this subsidiary system passes through a second set of
capillaries before being conveyed to the heart by the hepatic veins and
the posterior vena cava.
The =arteries= (Arteriæ), as a rule, divide at an acute angle, giving
off finer and finer branches. In some cases branches come off at a right
angle, and others are recurrent, _i. e._, run in a direction opposite to
that of the parent stem. The intercommunication of branches of adjacent
arteries is termed =anastomosis=. Most commonly the connections are made
by a network of numerous fine branches (Plexus vasculosus). Relatively
large communicating branches (Rami communicantes) occur in certain
places; they may be transverse or in the form of arches. Wide-meshed
networks of vessels are termed =Retia vasculosa=. =Terminal= or =end
arteries= are such as form isolated networks, _i. e._, do not anastomose
with adjacent arteries. A =rete mirabile= is a network intercalated in
the course of an artery. A =collateral vessel= (Vas collaterale) is one
which pursues a course near and similar to that of a larger vessel.
The =veins= (Venæ) are in general arranged like the arteries, but are
usually of greater caliber. When a vein accompanies an artery, it is
termed a =vena comitans= or =satellite vein=; in many places two veins
accompany a single artery. The primitive venous trunks do not run with
the arteries, and most of the superficial veins (Venæ cutaneæ) pursue
independent courses. They anastomose even more freely than the arteries,
and large communicating branches are very common. The veins form very
rich plexuses (Plexus venosi) in many places. Some veins which are
without independent walls and are inclosed by dense membranes and run
usually in bony grooves are termed (venous) =sinuses=; examples of this
are the sinuses of the dura mater of the brain.
=Structure of Arteries.=—The wall consists of three coats. The =external
coat= (Tunica externa) consists chiefly of fibrous connective tissue. In
the deeper part are some elastic fibers, and in some arteries there are
also longitudinal unstriped muscle-fibers. The =middle coat= (Tunica
media) is composed of unstriped muscle and elastic tissue in
medium-sized arteries. In small vessels there is only the muscular
tissue, and in the largest trunks only elastic tissue. The =internal
coat= or =intima= (Tunica intima) consists of a layer of endothelial
cells, resting on an elastic membrane. The =sheath= (Vagina vasis) is a
condensation of the surrounding connective tissue, and is attached more
or less closely to the external coat.
=Structure of Veins.=—The walls of veins are similar in structure to
those of the arteries, but are very much thinner, so that veins collapse
more or less completely when empty, while arteries do not. The =middle
coat= is very thin and consists to a large extent of ordinary connective
tissue. The =intima= is also less elastic than in the arteries. In many
veins this coat forms semilunar =valves=, the free edges of which are
directed toward the heart. They are most numerous in the veins of the
skin and the deep veins of the extremities (except the foot), while in
most veins of the body cavities and viscera they are absent or occur
only where the veins open into larger ones or where two veins join.
The walls of the vessels are supplied with blood by numerous small
arteries, called =vasa vasorum=. These arise from branches of the artery
which they supply or from adjacent arteries, ramify in the external
coat, and enter the middle coat also.
The =nerves= of the vessels consist of both medullated and
non-medullated fibers. They form plexuses around the vessels, from which
fibers pass mainly to the muscular tissue of the middle coat.