535
CHAPTER FOURTEEN
Blood
14.3
BLOOD PLASMA
Plasma is the clear, straw-colored, liquid part of the blood in
which the cells and platelets are suspended. It is approximately
92% water and contains a complex mixture of organic and inor-
ganic biochemicals. Functions of plasma constituents include
transporting nutrients, gases, and vitamins; helping to regulate
fl uid and electrolyte balance; and maintaining a favorable pH.
Plasma Proteins
By weight,
plasma proteins
are the most abundant dissolved
substances (solutes) in plasma. These proteins remain in the
blood and interstitial fl uids and ordinarily are not used as
energy sources. The three main types of plasma proteins are
albumins, globulins, and fibrinogen. The groups differ in
composition and function.
Albumins
(al-bu
minz) are the smallest of the plasma
proteins, yet account for 60% of these proteins by weight.
They are synthesized in the liver, and because they are so
plentiful, albumins are an important determinant of the
osmotic pressure
of the plasma.
Recall from chapter 3 (pp. 93–94) that the presence of
an impermeant solute on one side of a selectively perme-
able membrane creates an osmotic pressure and that water
always diffuses toward a greater osmotic pressure. Plasma
proteins are too large to pass through the capillary walls,
they are impermeant, and they create an osmotic pressure
that holds water in the capillaries despite blood pressure forc-
ing water out of capillaries by ± ltration (see chapter 3, p. 94).
The term
colloid osmotic pressure
is used to describe this
osmotic effect due to the plasma proteins.
By maintaining the colloid osmotic pressure of plasma,
albumins and other plasma proteins help regulate water
movement between the blood and the tissues. In doing so,
they help control blood volume, which, in turn, directly
affects blood pressure (see chapter 15, p. 582). For this rea-
son, it is important that the concentration of plasma proteins
remains relatively stable. Albumins also bind and transport
certain molecules, such as bilirubin, free fatty acids, many
hormones, and certain drugs.
If the concentration of plasma proteins falls, tissues swell, a condition
called
edema.
This may result from starvation or a protein-deF
cient
diet, either of which requires the body to use protein for energy, or
from an impaired liver that cannot synthesize plasma proteins. As the
concentration of plasma proteins drops, so does the colloid osmotic
pressure, sending ±
uids into the interstitial spaces.
Globulins
(glob
u-linz), which make up about 36% of
the plasma proteins, can be further subdivided into
alpha,
beta,
and
gamma globulins.
The liver synthesizes alpha and
beta globulins, which have a variety of functions, including
transport of lipids and fat-soluble vitamins. Lymphatic tis-
sues produce the gamma globulins, which are a type of anti-
body (see chapter 16, p. 633).
Fibrinogen
(± -brin
o-jen), which constitutes about 4% of
the plasma proteins, plays a primary role in blood coagulation.
Synthesized in the liver, it is the largest of the plasma proteins.
The function of fibrinogen is discussed later in this chapter
under the section “Blood Coagulation” on page 538.
Table 14.6
summarizes the characteristics of the plasma proteins.
TABLE
14.5
|
Cellular Components of Blood
Component
Description
Number Present
Function
Red blood cell (erythrocyte)
Biconcave disc without a nucleus, about one-third
hemoglobin
4,200,000 to 6,200,000
per microliter
Transports oxygen and carbon dioxide
White blood cell (leukocyte)
4,500 to 10,000
per microliter
Destroys pathogenic microorganisms
and parasites and removes worn cells
Granulocytes
About twice the size of red blood cells; cytoplasmic
granules are present
Neutrophil
Nucleus with two to F
ve lobes; cytoplasmic granules stain
light purple in combined acid and base stains
54%–62% of white
blood cells present
Phagocytizes small particles
Eosinophil
Nucleus bilobed; cytoplasmic granules stain red in acid
stain
1%–3% of white blood
cells present
Kills parasites and moderates allergic
reactions
Basophil
Nucleus lobed; cytoplasmic granules stain blue in basic
stain
Less than 1% of white
blood cells present
Releases heparin and histamine
Agranulocytes
Cytoplasmic granules are absent
Monocyte
Two to three times larger than a red blood cell; nuclear
shape varies from spherical to lobed
3%–9% of white blood
cells present
Phagocytizes large particles
Lymphocyte
Only slightly larger than a red blood cell; its nucleus nearly
F
lls cell
25%–33% of white
blood cells present
Provides immunity
Platelet (thrombocyte)
Cytoplasmic fragment
130,000 to 360,000
per microliter
Helps control blood loss from broken
vessels
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