Section II. THE BLOOD--THE VEHICLE OF THE CARDIOVASCULAR SYSTEM
Blood is the vehicle of the cardiovascular system. Thus, the component actually
Plasma makes up about 55 percent of the total blood volume.
a. Water. The major constituent of plasma is water. The physical
characteristics of water make it a very good vehicle.
(1) Since water is fluid, it can flow through the conduits.
(2) Since most substances can be dissolved in water, it is often known as
the "universal solvent."
(3) At ordinary pressures, water is essentially non-compressible.
(4) In addition, water has important temperature characteristics.
(a) Water has an ample heat-carrying capacity. It can carry heat
readily throughout the body.
(b) Some of this heat is transferred to the water of the sweat glands.
Since water can dissipate great quantities of heat through evaporation, excess heat can
be efficiently disposed of at the surface of the skin.
b. Dissolved and Suspended Substances. To some extent, all transported
substances are dissolved or suspended in the water of the plasma. These substances
include various gases, end products of digestion, various control substances, and waste
products. Also, there are three major plasma proteins--albumin, globulins, and
fibrinogen. Together with dissolved salts (electrolytes), these plasma proteins help to
maintain the tonicity of the plasma. In addition, fibrinogen is important to blood clotting.
10-11. FORMED ELEMENTS
The remainder of the blood volume consists of the formed elements--the red
blood cells, the white blood cells, and the platelets. In adults, these formed elements
normally make up 40 percent to 45 percent of the total blood volume. (This measure is
called the hematocrit.)