red (venous) blood is the result of hemoglobin
combining with carbon dioxide.
Red blood cells live only about 100 to 120 days in
the body. There are several reasons for their short life
span. These delicate cells have to withstand constant
knocking around as they are pumped into the arteries
by the heart. These cells travel through blood vessels at
high speed, bumping into other cells, bouncing off the
walls of arteries and veins, and squeezing through
narrow passages. They must adjust to continual
pressure changes. The spleen is the graveyard where
old, worn out cells are removed from the blood stream.
Fragments of red blood cells are found in the spleen
and other body tissues.
WHITE BLOOD CELLS.White blood cells,
or leukocytes, are almost colorless, nucleated cells
originating in the bone marrow and in certain
lymphoid tissues of the body (fig. 1-32). There is only
one white cell to every 600 red cells. Normal WBC
count is 6,000 to 8,000 per cubic millimeter, although
the number of white cells may be 15,000 to 20,000 or
higher during infection.
Leukocytes are important for the protection of the
body against disease. Leukocytes can squeeze between
the cells that form blood cell walls. This movement,
called diapedesis, permits them to leave the blood
stream through the capillary wall and attack
pathogenic bacteria. They can travel anywhere in the
body and are often named the wandering cells. They
protect the body tissues by engulfing disease-bearing
bacteria and foreign matter, a process called
phagocytosis. When white cells are undermanned,
more are produced, causing an increase in their number
and a condition known as leukocytosis. Another way
WBC's protect the body from disease is by producing
bacteriolysins that dissolve the foreign bacteria. The
secondary function of WBCs is to aid in blood clotting.
BLOOD PLATELETS.Blood platelets, or
thrombocytes, are irregular- or oval-shaped discs in
the blood that contain no nucleus, only cytoplasm
(fig. 1-32). They are smaller than red blood cells and
average about 250,000 per cubic millimeter of blood.
Blood platelets play an important role in the process of
blood coagulation, clumping together in the presence
of jagged, torn tissue.
To protect the body from excessive blood loss,
blood has its own power to coagulate, or clot. If blood
components and linings of vessels are normal,
circulating blood will not clot. Once blood escapes
from its vessels, however, a chemical reaction begins
that causes it to become solid. Initially a blood clot is a
fluid, but soon it 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 insoluble protein, fibrin, whenever injury
Other necessary elements for blood clotting are
calcium salts; a substance called prothrombin, 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 network
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
can bleed to death as a result of minor wounds.
The heart is a hollow, muscular organ, somewhat
larger than the closed fist, located anteriorly 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
The heart is enclosed in a membranous sac, the
pericardium. The smooth surfaces of the heart and
pericardium are lubricated by a serous secretion called
pericardial fluid. The inner surface of the heart is
lined with a delicate serous membrane, the
endocardium, similar to and continuous with that of
the inner lining of blood vessels.
The interior of the heart (fig. 1-33) is divided into
two parts by a wall called the interventricular
septum. In each half is an upper chamber, the atrium,
which receives blood from the veins, and a lower
chamber, the ventricle, which receives blood from the