JOBR: “2121_c001” — 2006/2/17 — 14:06 — page 3 — #3
An Outline of Cardiovascular Structure and Function
1-3
The primary function of erythrocytes is to aid in the transport of blood gases — about 30 to 34%
(by weight) of each cell consisting of the oxygen- and carbon dioxide-carrying protein hemoglobin
(64,000
≤
MW
≤
68,000) and a small portion of the cell containing the enzyme carbonic anhydrase,
which catalyzes the reversible formation of carbonic acid from carbon dioxide and water. The primary
function of leukocytes is to endow the human body with the ability to identify and dispose of foreign
substances such as infectious organisms) that do not belong there — agranulocytes (lymphocytes and
monocytes) essentially doing the “identifying” and granulocytes (neutrophils, basophils, and eosinophils)
essentially doing the “disposing.” The primary function of platelets is to participate in the blood clotting
process.
Removal of all hematocytes from blood centrifugation or other separating techniques leaves behind
the aqueous (91% water by weight, 94.8% water by volume), saline (0.15 N) suspending medium called
plasma — which has an average mass density of 1.035
±
0.005 g/cm
3
and a viscosity 1
1
2
to 2 times that of
water. Some 6.5 to 8% by weight of plasma consists of the plasma proteins, of which there are three major
types — albumin, the globulins, and fibrinogen — and several of lesser prominence (Table 1.2).
The primary functions of albumin are to help maintain the osmotic (oncotic) transmural pressure
differential that ensures proper mass exchange between blood and interstitial fluid at the capillary level
and to serve as a transport carrier molecule for several hormones and other small biochemical constituents
(such as some metal ions). The primary function of the globulin class of proteins is to act as transport
carrier molecules (mostly of the
α
and
β
class) for large biochemical substances, such as fats (lipoproteins)
and certain carbohydrates (muco- and glycoproteins) and heavy metals (mineraloproteins), and to work
together with leukocytes in the body’s immune system. The latter function is primarily the responsibility
of the
γ
class of immunoglobulins, which have antibody activity. The primary function of fibrinogen is
to work with thrombocytes in the formation of a blood clot — a process also aided by one of the most
abundant of the lesser proteins, prothrombin (MW
62, 000).
Of the remaining 2% or so (by weight) of plasma, just under half (0.95%, or 983 mg/dl plasma) consists
of minerals (inorganic ash), trace elements, and electrolytes, mostly the cations sodium, potassium,
calcium, and magnesium and the anions chlorine, bicarbonate, phosphate, and sulfate — the latter three
helping as buffers to maintain the fluid at a slightly alkaline pH between 7.35 and 7.45 (average 7.4). What
is left, about 1087 mg of material per deciliter of plasma, includes (1) mainly (0.8% by weight) three major
types of fat, that is, cholesterol (in a free and esterified form), phospholipid (a major ingredient of cell
membranes), and triglyceride, with lesser amounts of the fat-soluble vitamins (A, D, E, and K), free fatty
acids, and other lipids, and (2) “extractives” (0.25% by weight), of which about two-thirds includes glucose
and other forms of carbohydrate, the remainder consisting of the water-soluble vitamins (B-complex and
C), certain enzymes, nonnitrogenous and nitrogenous waste products of metabolism (including urea,
creatine, and creatinine), and many smaller amounts of other biochemical constituents — the list seeming
virtually endless.
Removal from blood of all hematocytes and the protein fibrinogen (by allowing the fluid to completely
clot before centrifuging) leaves behind a clear fluid called
serum, which has a density of about 1.018
±
0.003 g/cm
3
and a viscosity up to 1
1
2
times that of water. A glimpse of Table 1.1 and Table 1.2, together
with the very brief summary presented above, nevertheless gives the reader an immediate appreciation for
why blood is often referred to as the “river of life.” This river is made to flow through the vascular piping
network by two central pumping stations arranged in series: the left and right sides of the human heart.
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