HUMAN BIOLOGY

150 Chapter 8

Blood flow

Fibrinogen

Prothrombin

Thrombin

Fibrin threads (clot)

Ca^2 +

Prothrombin Ca^2 +

activator

1 Injury to

blood vessel

Blood flow

decreases

2 Wall of vessel

contracts

4 More permanent

clot forms and no more

bleeding occurs

Damaged cells and platelets

release substances that

activate clotting factors

3 Platelets stick

to collagen fibers

of damaged vessel

wall

Platelet

plug

B

A

1 2 3 4

hemostasis The name for
mechanisms that slow or
stop the loss of blood from
a ruptured blood vessel.

Hemostasis and Blood Clotting

Factors in blood or damaged tissue may

cause a clot to form

Two different mechanisms can produce a blood clot. The

first is an “intrinsic” clotting mechanism that involves

substances in the blood itself (Figure 8.7B). It begins when a

“factor X” protein in blood plasma is activated. This triggers

reactions that produce thrombin. This is an enzyme that

acts on a protein called fibrinogen. Rods of fibrinogen stick

together, forming long threads of fibrin. The fibrin threads

also stick to one another. The result is a net that entangles

blood cells and platelets, as you can see in the micrograph

in Figure 8.7. The entire mass is a blood clot. As the clot

compacts it draws the torn walls of the vessel back together.

Blood also can coagulate through an “extrinsic” (out-

side) clotting mechanism triggered by the release of sub-

stances outside the blood. Damaged blood vessels or nearby

tissue produce the substances, which in turn trigger the

formation of thrombin. The remaining steps are like those

of the intrinsic pathway.

Aspirin reduces the aggregation of platelets, so small

doses may be prescribed to help prevent blood clots. You

may recall from Section 7.8 that a clot lodged in an unbro-

ken blood vessel is called a thrombus. The condition is

called a thrombosis.

n Small blood vessels can easily be damaged by a cut

or blow. To maintain homeostasis, small tears must be

quickly repaired.

Hemostasis prevents blood loss

Hemostasis means “stopping bleeding.” It involves

responses that halt bleeding when a small vessel is torn or

punctured and so helps limit blood loss. Although hemosta-

sis can only seal tears or punctures in relatively small blood

vessels, most cuts and punctures fall into this category.

When a blood vessel is ruptured (Figure 8.7A), smooth

muscle in the damaged vessel wall contracts in an auto-

matic response called a spasm. The muscle contraction con-

stricts the blood vessel, so blood flow

through it slows or stops. A spasm can

last for up to 30 minutes and is vital

in stemming the immediate loss of

blood. While the flow of blood slows,

platelets arrive and clump together,

creating a temporary plug in the damaged wall. They also

release the hormone serotonin and other chemicals that help

prolong the spasm and attract more platelets. Lastly, blood

coagulates—that is, it converts to a gel—and forms a clot.

8.7

Figure 8.7 Animated! A blood clot forms in four steps. The micrograph

shows red blood cells trapped in a fibrin net. (© Cengage Learning) Eye of Science/Science Source

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