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Blood Vessels

Blood Vessels
Blood Vessels
• The vascular network through which blood flows to all
parts of the body comprises of arteries, arterioles,
capillaries, veins and venules.
• Arteries and arterioles
• Arteries are high pressure vessels which carry blood
from the heart to the tissues.
• The largest artery in the body is the aorta which is the
main artery leaving the heart.
• The aorta constantly subdivides and gets smaller.
• The constant subdivision decreases the diameter of the
vessel arteries, which now become arterioles.
Structure of Arteries
• Arteries are composed of three
layers of tissue:
• 1 an outer fibrous layer — the
tunica adventitia or tunica
• 2 a thick middle layer — the
tunica media
• 3 a thin lining of cells to the
inside — the endothelium or
tunica intima.
• The tunica media is comprised
of smooth muscle and elastic
tissue, which enables the
arteries and arterioles to alter
their diameter.
Structure of Arteries
• Arteries tend to have more
elastic tissue, while arterioles
have greater amounts of
smooth muscle; this allows the
vessels to increase the
diameter through vasodilation
or decrease the diameter
through vasoconstriction.
• It is through vasoconstriction
and vasodilation that the
vessels can regulate blood
pressure and ensure the
tissues are receiving sufficient
blood — particularly during
Arteries and arterioles have three
basic functions:
• to act as conduits carrying and controlling
blood flow to the tissues
• to cushion and smooth out the pulsate flow
of blood from the heart
• to help control blood pressure.
Veins and venules
• Veins are low pressure vessels which return
blood to the heart. The structure is similar to
arteries, although they possess less smooth
muscle and elastic tissue.
• Venules are the smallest veins and transport
blood away from the capillary bed intothe veins.
• Veins gradually increase in thickness the nearer
to the heart they get, until they reach the largest
vein in the body, the venae cavae, which enters
the right atrium of the heart.
Veins and Venules
• The thinner walls of the veins often distend
and allow blood to pooi in them. This is
also allowed to happen as the veins
contain pocket valves which close
intermittently to prevent back flow of blood.
• This explains why up to 70% of total blood
volume is found in the venous system at
any one time.
• Capillaries are the functional units or the
vascular system.
• Composed of a single layer of endothelial cells,
they are just thin enough to allow red blood cells
to squeeze through their wall.
• The capillary network is very well developed as
they are so small; large quantities are able to
cover the muscle, which ensures efficient
exchange of gases.
• If the cross-sectional area of all the
capillaries in a muscle cell were to be
added together, the total area would be
much greater than that of the aorta.
• Distribution of blood through the capillary
network is regulated by special structures
known as pre-capillary sphincters, the
structure of which will be dealt with later in
this chapter.