Plasma components. Proteins
Proteins are large molecules formed of chains of amino
acids, organic acids that contain both an acidic and
a nitrogenous basic (amino) group. Chains (polypeptides)
are formed by linkageof the acid group of one amino
acid to the amino group of the next (peptide bond).
The characteristics of a protein are determined by the
number and types of amino acids and the sequence in
which they are arranged.
When dietary protein is digested in the gastrointestinal
tract, individual amino acids are released from the
chains and are absorbed. The amino acids are transported
through the plasma to all parts of the body, where they
are taken up by cells and are assembled in specific
ways to form proteins of many types. These plasma proteins
are released into the blood from the cells in which
they were synthesized. Much of the protein of plasma
is produced in the liver. The major plasma protein is
albumin, a relatively small molecule, the principal
function of which is to retain water in the bloodstream
by its osmotic effect. The amount of albumin in the
blood is a determinant of the total volume of plasma.
Depletion of albumin permits fluid to leave the circulation
and to accumulate and cause swelling of soft tissues
(edema). Albumin binds certain other substances that
are transported in plasma and thus serves as a nonspecific
carrier protein. Bilirubin, for example, is bound to
albumin duringits passage through the blood.
Albumin has physical properties that permit its separation
from other plasma proteins, which as a group are called
globulins. In fact, the globulins are a heterogeneous
array of proteins of widely varying structure and function,
only a few of which will be mentioned here. The immunoglobulins,
or antibodies, are produced in response to a specific
antigen. For example, administration of poliomyelitis
vaccine is followed by the appearance in the plasma
of antibodies that react with poliovirus and effectively
prevent that infection. Antibodies may be induced by
many foreign substances in addition to microorganisms;
immunoglobulins are involved in some hypersensitivity
and allergic reactions. Other plasma proteins are concerned
with the coagulation of the blood (see bleeding and
blood clotting).
Many proteins are involved in highly specific ways
with the transport function of the blood. Blood lipids
are incorporated into protein molecules as lipoproteins,
substances important inlipid transport. Iron and copper
are transported in plasma by unique metal-binding proteins
(transferrin and ceruloplasmin, respectively). Vitamin
B1 2, an essential nutrient, is bound to a specific
carrier protein. Although hemoglobin is not normally
released into the plasma, a hemoglobin-binding protein
(haptoglobin) is available to transport hemoglobin should
hemolysis (breakdown) of red cells occur.
