a. General Construction. The blood vessels in general are tubular and have a
(1) Intima. A layer of smooth epithelium known as the intima lines the
lumen (hollow central cavity).
Media. A middle layer of smooth muscle tissue is called the media.
(3) Adventitia. The adventitia is the outer layer of fibrous connective tissue
that holds everything together.
b. Types of Blood Vessels. See Figure 2-1 for a diagram of the human
circulatory system. We recognize three types of blood vessels:
The arteries carry blood away from the chambers of the heart.
The veins carry blood to the chambers of the heart.
(3) Capillaries are extremely thin-walled vessels having only the intimal
layer through which exchanges can take place between the blood and the tissue cells.
c. Relationships. Arteries and veins are largest where they are closest to the
heart. Away from the heart, they branch into smaller and smaller and more numerous
vessels. The branching continues until the smallest arteries (arterioles) empty into the
capillaries. The capillaries in turn are drained by the venules of the venous system.
d. Valves. Within the heart and the veins are structures known as valves.
Valves function to insure that the blood flows in only one direction.
2-7. BLOOD CLOTTING
Blood clotting is a process that is dependent on several different factors. This
process is also known as hemostasis. There are three general mechanisms involved in
blood clotting: vascular spasm, the platelet plug, and the clotting mechanism.
a. Vascular Spasm. When a blood vessel is cut, the vascular spasm causes
rapid constriction of the cut blood vessel. This decreases the amount of blood lost. The
mechanism by which this mechanism occurs is not fully known, but it appears to be a
reflex response initiated by pain. It is interesting to note that when a vessel is cut by
crushing, the vascular spasm response seems to occur more rapidly and more intensely
than if the vessel is quickly cut (as with a knife). After the vascular spasm has occurred,
the second mechanism involved with the clotting process--the platelet plug--occurs.