Blood Vessels
Blood vessels form a network of tubes carrying blood between the heart and body cells
and tissues. These tubes are an essential part of the cardiovascular system because they
provide the environment in which the blood cells can perform their function.
Vessel
Artery
Arteriole
Structure
Thick, strong wall with 3 layers –
endothelial lining, middle layer of
smooth muscle and elastic tissue, and
outer layer of connective tissue
Thinner wall than artery, but with 3
layers; smaller arterioles have
endothelial lining, some smooth muscle
tissue and small amount of connective
tissue
Function
Carries high-pressure blood from
heart to arterioles.
Connects artery to capillary;
helps to control blood flow into
capillary by undergoing
vasoconstriction or vasodilation.
Capillary
Single layer of squamous epithelium.
Provides semipermeable
membrane through which
nutrients, gases, and wastes are
exchanged between blood and
tissue cells; connects arteriole to
venule.
Venule
Thinner wall, less smooth muscle and
elastic tissue than arteriole.
Connects capillary to vein.
Vein
Thinner wall than artery, but with
similar layers; middle layer more poorly
developed; some with flaplike valves
Carries low-pressure blood from
venule to heart; valves prevent
back flow of blood; serves as
blood reservoir.
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All blood vessels except the capillaries have three distinct layers of
cells.
Each layer is called a tunic, which surrounds the tubular opening of
the blood vessel called the lumen
The inner lining consists of a layer of endothelium (simple squamous
epithelium lying on a connective tissue basement membrane)
The middle layer consists primarily of smooth muscle cells and fibers
of elastic connective tissue.
The outer layer is mainly made up of collagen fibers. The fibers
protect the blood vessels and provide anchorage to the surrounding
structures.
The inner core/cavity is called the lumen.
Arteries and Arterioles: have smooth muscle in their lining which
allows them to contract and control blood flow. The narrowing of the
artery lumen = vasoconstriction (induced by nerve impulses), and an
increase in lumen size = vasodilation (occurs in the absence of
impulses)
Arterioles are smaller than arteries – consists mainly of endothelium
with a small amount of smooth tissue
Capillaries can be found near almost every body cell; exchange of
nutrients and gases occur between the blood and the cells in the
capillaries; only have a single layer of endothelium --- allows small
molecules to pass through them easily
o Most have sphincters at entrance – circular muscles that open
and close to regulate the flow of blood through the capillary
Venules form when several capillaries combine with one another –
collect blood and deliver to the veins, similar in structure as arterioles
Veins are similar to arteries in structure, except that their external
layer is thicker and their middle layer is a little thinner
Blood flowing through a vein lacks pressure, so the blood flows
smoothly as compared to blood in the arteries, which pulses
Veins have valves which prevent the backflow of blood (when the
valves weaken, blood leaks backwards and the pressure of blood
dilates the wall of the vein --- called varicose veins