Fundamentals of Anatomy and Physiology

The Stomach.

Leukocytes are far less numerous than RBCs,
-averaging from 5000 to 9000 per mm^3 of blood. They can
phagocytize only a certain number of substances before
these materials interfere with the leukocyte’s ­normal
metabolic activity. Therefore, their life span is quite short.
In a healthy body, some WBCs will live only a few days.
During infections, they may live for only a few hours.

Thrombocytes or platelets are disk-shaped cellular
fragments with a nucleus. They range in size from 2 to 4
micrometers in diameter. They prevent fluid loss when blood
vessels are damaged by initiating a chain of reac-tions that
result in blood clotting. They have a life span of about a week.
They are produced in red bone marrow from large
megakaryocytes (meg-ah-KAIR-ee-oh-sightz). Refer to
Figure 13-4 for the life span and functions of blood cells.

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The Clotting Mechanism

When we injure ourselves through a fall or scrape, blood

vessels are damaged and blood flows into tissues and can

be lost from the body. Fortunately, the body has a

mechanism to stop the loss of blood and repair the dam-

aged blood vessels and tissues. The clotting mechanism is a

process that the body uses to stop the loss of blood.

When small blood vessels are damaged, smooth mus-

cles in the vessel’s walls contract. This can stop blood loss.

When larger vessels are damaged, the constriction of the

smooth muscles in the vessel walls only slows down blood

loss and the clotting mechanism takes over. A cut in a

blood vessel causes the smooth walls of the vessel to

-become rough and irregular. Clotting or coagulation is a

complex process that proceeds in three stages (Figure 13-

5).

In the first stage, the roughened surface of the cut

vessel causes the platelets or thrombocytes to aggregate, or

clump together, at the site of injury. The damaged tissues

release thromboplastin (throm-boh-PLAST-in). The

thromboplastin causes a series of reactions that re-sult in

the production of prothrombin activator. These activities

require the presence of calcium ions and -certain proteins

and phospholipids.

Health Alert

MAINTAINING CIRCULATION

Diet is important to maintaining healthy red

blood cells. Since hemoglobin contains iron

atoms, iron in our diet helps the RBCs carry

more oxygen. Moderate amounts of red meat

and liver are good sources of iron in our diet.

At the same time, we need to main-tain heart-

healthy diets by eating the right kinds of

meats. To maintain good blood cir-culation, we

need to have low blood choles-terol levels.

Low-fat meats, such as fish and poultry, and

increased fiber in our diet from fresh fruits and

vegetables help lower blood cholesterol levels.

High cholesterol levels can lead to arterial

plaque formation, which blocks blood flow by

causing clots to form in unwanted places.

There are activities that can lead to

preventing unwanted clotting. Regular ex-

ercise increases blood plasma volume, thus

keeping the blood “thin.” This results in less

fibrinogen and RBCs per volume of blood,

reducing the risk of blood clotting. People who

exercise regularly are at low risk of heart

attacks. Sedentary lifestyles produce the

opposite effects. Based on this fact, pa-tients

are encouraged to walk very soon af-ter

operations to prevent the possibility of blood

clot formation. If you smoke, try to quit.

Smoking increases fibrinogen levels in the

blood, thus increasing the possibility of

unwanted clotting. Moderate alcohol intake is

also associated with reduced heart disease

possibilities. One or two drinks per day or one

glass of red wine can help maintain a healthy

heart and good blood circulation in many

individuals. Remember, a combination of a

healthy diet and exercise helps main-tain good

blood circulation.