538
UNIT FOUR
tonin, which contracts smooth muscles in the blood vessel
walls. This vasoconstriction further helps to reduce blood loss.
Platelet Plug Formation
Platelets adhere to exposed ends of injured blood vessels.
They adhere to any rough surface, particularly to the colla-
gen in connective tissue underlying the endothelial lining of
blood vessels.
When platelets contact collagen, their shapes change
drastically, and many spiny processes begin to protrude
from their membranes. At the same time, platelets adhere to
each other, forming a platelet plug in the vascular break. A
plug may control blood loss from a small break, but a larger
break may require a blood clot to halt bleeding.
Figure 14.17
shows the steps in platelet plug formation.
PRACTICE
37
What is hemostasis?
38
How does a blood vessel spasm help control bleeding?
39
Describe the formation of a platelet plug.
Blood Coagulation
Coagulation
(ko-ag
u-la
shun), the most effective hemostatic
mechanism, forms a
blood clot
in a series of reactions, each
from the metabolism of creatine. As discussed in chapter 9
(p. 294), creatine is present as
creatine phosphate
in muscle
and brain tissues as well as in the blood, where it stores energy
in phosphate bonds, much like those of ATP molecules.
Normally, the concentration of nonprotein nitrogenous
substances in plasma remains relatively stable because pro-
tein intake and use are balanced with excretion of nitrogenous
wastes. Because about half of the NPN substances is urea,
which the kidneys ordinarily excrete, a rise in the blood urea
nitrogen (BUN) may suggest a kidney disorder. Excess pro-
tein catabolism or infection may also elevate BUN.
Plasma Electrolytes
Recall that electrolytes release ions when dissolved in water.
Plasma contains a variety of these ions, also called
electrolytes.
Plasma electrolytes are absorbed from the intestine or released
as by-products of cellular metabolism. They include sodium,
potassium, calcium, magnesium, chloride, bicarbonate, phos-
phate, and sulfate ions. Of these, sodium and chloride ions
are the most abundant. Bicarbonate ions are important in
maintaining the osmotic pressure and the pH of plasma, and
like other plasma constituents, they are regulated so that their
blood concentrations remain relatively stable. These electro-
lytes are discussed in chapter 21 (pp. 815–818) in connection
with water and electrolyte balance.
PRACTICE
34
What is a nonprotein nitrogenous substance?
35
Why does kidney disease increase the blood concentration of
these substances?
36
What are the sources of plasma electrolytes?
14.4
HEMOSTASIS
Hemostasis
(he
mo-sta
sis) refers to the stoppage of bleed-
ing, which is vitally important when blood vessels are dam-
aged. Following an injury to the blood vessels, several actions
may help to limit or prevent blood loss, including blood ves-
sel spasm, platelet plug formation, and blood coagulation.
These mechanisms are most effective in minimizing blood
losses from small vessels. Injury to a larger vessel may result
in a severe hemorrhage that requires special treatment.
Blood Vessel Spasm
Cutting or breaking a smaller blood vessel stimulates the
smooth muscles in its wall to contract, called
vasospasm.
Blood loss lessens almost immediately, and the ends of the
severed vessel may close completely. This effect results from
direct stimulation of the vessel wall as well as from refl exes
elicited by pain receptors in the injured tissues.
Although the refl ex response may last only a few minutes,
the effect of the direct stimulation usually continues for about
thirty minutes. By then, a blockage called a
platelet plug
has
formed, and blood is coagulating. Also, platelets release sero-
Endothelial lining
Collagen fiber
Break in
vessel wall
Platelet
Red blood cell
Blood escaping
through break
Platelets adhere
to each other,
to end of broken
vessel, and to
exposed collagen
Platelet plug
helps control
blood loss
1
2
3
4
FIGURE 14.17
Steps in platelet plug formation.
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