To protect the body from excessive blood loss,
blood has its own power to coagulate, or clot. If
blood constituents and linings of vessels are nor-
mal, circulating blood will not clot. Once blood
escapes from its vessels, however, a chemical reac-
tion begins that causes it to become solid. The clot
formed is at first fluid but soon becomes thick
and then sets into a soft jelly that quickly becomes
firm enough to act as a plug. This plug is the result
of a swift, sure mechanism that changes one of
the soluble blood proteins, fibrinogen, into an in-
soluble protein, fibrin, whenever injury occurs.
Other necessary elements for blood clotting
are calcium salts, a substance called prothrom-
bin, which is formed in the liver, blood platelets,
and various factors necessary for the completion
of the successive steps in the coagulation process.
Once the fibrin plug is formed, it quickly
enmeshes red and white blood cells and draws
them tightly together. Blood serum, a yellowish
clear liquid, is squeezed out of the clot as the mass
shrinks. Formation of the clot closes the wound,
preventing blood loss. A clot also serves as a net-
work for the growth of new tissues in the process
of healing. Normal clotting time is 3 to 5 minutes,
but if any of the substances necessary for clotting
are absent, severe bleeding will occur.
HEMOPHILIA is an inherited disease
characterized by delayed clotting of the blood and
consequent difficulty in controlling hemorrhage.
Hemophiliacs may bleed to death as a result of
even a trivial wound.
The heart is a hollow, muscular organ, some-
what larger than the closed fist, located anteri-
orly in the chest and to the left of the midline.
It is shaped like a cone, its base directed upward
and to the right, the apex down and to the left.
Lying obliquely in the chest, much of the base of
the heart is immediately posterior to the sternum.
The heart is enclosed in a membranous sac,
the PERICARDIUM. The smooth surfaces of the
heart and pericardium are lubricated by a serous
secretion, the pericardial fluid. The inner surface
of the heart is lined with a delicate serous mem-
brane, the ENDOCARDIUM, similar to and con-
tinuous with that of the inner lining of blood
The interior of the heart (fig. 3-30) is divided
into two parts by a wall called the INTER-
VENTRICULAR SEPTUM. In each half is an
Figure 3-30.Diagram of the heart.