HUMAN BIOLOGY

BLooD 151

What are hemostasis and blood clotting?

• Mechanisms of hemostasis prevent major blood loss when
  small vessels are torn or punctured.


• The mechanisms include spasms that constrict blood vessel
  walls, the formation of platelet plugs, and blood clotting.


• Blood clotting can be triggered by substances in the blood
  itself or by reactions involving substances in damaged tissue.

taKe-Home message

An embolus is a clot that circulates in the bloodstream.
Someone who suffers an embolism in the heart, lungs,
brain, or some other organ may suddenly die when the
roving clot shuts down the organ’s blood supply. Many
cases of stroke involve an embolus that blocks the flow
of blood to some part of the brain and the affected tissue
dies. Some strokes are mild, but in serious cases the person
may be paralyzed on one side of the body and have trouble
speaking. Physical therapy and speech therapy may help
minimize the long-term effects.
Formation of a blood clot is a first step in healing
wounds. When the skin is punctured or torn, blood clot-
ting gets under way immediately to help seal the breach
(Figure 8.8). With minor cuts, it usually takes less than
30 minutes for a clot to seal off injured vessels. In a few
more hours, phagocytes are at work cleaning up debris
and a scab has begun to form. This quick action is vital to
minimize blood loss and the chances of infection.

Regenerating

Scab epithelium

Collagen

fibers

Healed

damage

Clot in damaged

epidermis

Figure 8.8 Animated! Blood clotting helps heal a wound in
the skin. (© Cengage Learning)

Cat Gwynn/Cusp/Corbis

save Lives—

be a blood donor

every 2 seconds, someone in the United States needs a

blood transfusion. Worldwide, hospital patients require

almost 80 million units of donated blood. many recipients

are people who have lost blood due to a serious injury—

blood that must come from donors. according to the

american red Cross, a typical 1-pint donation helps save

three lives (Figure 8.9).

If you are considering becoming a blood donor, what

can you expect? the procedure starts with an ID check and

a short, confidential interview about your health history

and any recent travel (to determine possible exposure to

infectious agents). next, a technician will take your pulse,

check your blood pressure and body temperature, and

do a finger-prick test to confirm that your blood contains

adequate hemoglobin. (If you are anemic—that is, your

blood hemoglobin is below a set point—you’ll be asked to

defer your donation.) next, you’ll be seated in a comfortable

chair or recliner and probably will be offered water to

ensure you’re well hydrated.

Blood donation itself usually takes no more than 10 minutes.

the process is painless except for an initial “pinch” when a

needle attached to a collection bag is inserted into a large

vein in your arm. you’ll be given a small ball to squeeze, which

helps keep the blood flowing smoothly. after your blood is

drawn and the needle site covered with a small bandage, you’ll

receive a snack that includes juice or water. Following about

15 minutes of recuperation, you’ll be free to go about the rest

of your day knowing that you’ve helped others in need.

most donors give whole blood, which later is typed and

separated into transfusable elements including plasma, red

blood cells, and platelets. Because it takes several weeks for

the body to replenish donated blood, a given individual is

restricted to giving blood no more often than every 56 days.

anyone in good general health can give blood. as a

precaution, however, organizations that process blood

donations can’t rely on a donor’s self-reported health

history to determine if the donated blood is safe. By law, all

donations must be rigorously tested for infectious agents

including hepatitis C and hIV, the virus that causes aIDS.

Figure 8.9 Blood donations save lives.

8.8

FOCUS ON HUmAN impACT

Antonia Reeve/Science Source

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