Maintaining normal pH in body tissues. Many blood pro-
teins and other bloodborne solutes act as buﬀers to prevent
excessive or abrupt changes in blood pH that could jeopar-
dize normal cell activities. Additionally, blood acts as the res-
ervoir for the body’s “alkaline reserve” of bicarbonate ions.
Maintaining adequate ﬂuid volume in the circulatory system.
Blood proteins prevent excessive ﬂuid loss from the blood-
stream into the tissue spaces. As a result, the ﬂuid volume in
the blood vessels remains ample to support eﬃcient blood
circulation to all parts of the body.
of blood include
Preventing blood loss. When a blood vessel is damaged,
platelets and plasma proteins initiate clot formation, halting
Preventing infection. DriFing along in blood are antibodies,
complement proteins, and white blood cells, all of which help
defend the body against foreign invaders such as bacteria
Discuss the composition and functions of plasma.
is a straw-colored, sticky ﬂuid (±igure 17.1). Al-
though it is mostly water (about 90%), plasma contains over
100 diﬀerent dissolved solutes, including nutrients, gases, hor-
mones, wastes and products of cell activity, proteins, and in-
organic ions (electrolytes). Electrolytes (Na
, etc.) vastly
outnumber the other solutes.
summarizes the major
Although outnumbered by the lighter electrolytes, the heav-
ier plasma proteins are the most abundant plasma solutes by
weight, accounting for about 8% of plasma weight. Except for
hormones and gamma globulins, most plasma proteins are pro-
duced by the liver. Plasma proteins serve a variety of functions,
but they are
taken up by cells to be used as fuels or metabolic
nutrients as are most other organic solutes, such as glucose, fatty
acids, and amino acids.
min) accounts for some 60% of plasma pro-
tein. It acts as a carrier to shuttle certain molecules through the
circulation, is an important blood buﬀer, and is the major blood
protein contributing to the plasma osmotic pressure (the pres-
sure that helps to keep water in the bloodstream).
Te composition of plasma varies continuously as cells re-
move or add substances to the blood. However, assuming a
healthy diet, plasma composition is kept relatively constant by
various homeostatic mechanisms. ±or example, when blood
protein levels drop undesirably, the liver makes more proteins.
When the blood starts to become too acidic (acidosis), both the
lungs and the kidneys are called into action to restore plasma’s
normal, slightly alkaline pH. Body organs make dozens of ad-
justments, day in and day out, to maintain the many plasma
solutes at life-sustaining levels.
Check Your Understanding
What is the hematocrit? What is its normal value?
List two protective functions of blood.
Are plasma proteins used as fuel for body cells? Explain your
For answers, see Appendix H.
Composition of Plasma
DESCRIPTION AND IMPORTANCE
90% of plasma volume; dissolving and
suspending medium for solutes of
blood; absorbs heat
Most abundant solutes by number;
cations include sodium, potassium,
calcium, magnesium; anions include
chloride, phosphate, sulfate, and
bicarbonate; help to maintain plasma
osmotic pressure and normal blood pH
8% (by weight) of plasma; all
contribute to osmotic pressure and
maintain water balance in blood
and tissues; all have other functions
(transport, enzymatic, etc.) as well
60% of plasma proteins; produced
by liver; main contributor to osmotic
36% of plasma proteins
Produced by liver; most are transport
proteins that bind to lipids, metal ions,
and fat-soluble vitamins
Antibodies released by plasma cells
during immune response
4% of plasma proteins; produced by
liver; forms ﬁbrin threads of blood clot
By-products of cellular metabolism,
such as urea, uric acid, creatinine, and
Materials absorbed from digestive tract
and transported for use throughout
body; include glucose and other simple
carbohydrates, amino acids (protein
digestion products), fatty acids,
glycerol and triglycerides (fat digestion
products), cholesterol, and vitamins
Oxygen and carbon dioxide; oxygen
mostly bound to hemoglobin inside
RBCs; carbon dioxide transported
dissolved as bicarbonate ion or CO
bound to hemoglobin in RBCs
Steroid and thyroid hormones carried
by plasma proteins