Section I. BLOOD PARTS
Adequate substitutes for human blood do not exist. There are many kinds of
blood expanders, blood thinners, and aids for ailing blood, but none of these can
substitute for the unique properties and functions of blood in the human body. Without
sufficient human blood, our bodies would stop functioning. This is why the blood banks
make frequent requests for donations from healthy individuals. As a medical
noncommissioned officer (NCO), you will be expected to perform your duties in
peacetime and on the field of battle. Your subordinates will often ask why certain
functions must be done. Using the knowledge contained in this lesson, you will have
the answers for questions about human blood.
PROPERTIES AND FUNCTIONS OF THE BLOOD
a. Amount. Blood accounts for 7 to 9 percent of the body weight. A person
weighing 150 pounds will have about 4 to 6 liters of blood. The actual amount of blood
in each person's body is affected by several factors, such as body size and age. The
older the person, the less blood in his body. A big person has more blood than a small
person and, all else being equal, a male has more blood than a female.
b. Functions. Blood is constantly in motion. Asleep or awake, the blood flows
in a circulation system at almost the same rate. This process begins before we are born
and does not stop until after death. Blood flows in virtually a closed system to all
tissues of the body. It brings oxygen and nutritive substances to the capillaries
(smallest blood vessels) and removes metabolic waste products and carbon dioxide,
which are then eliminated from the body by the excretory organs. The blood
coordinates activities of various organs by carrying chemical regulators called
hormones. See paragraph 2-14 for a discussion of hormones. Blood regulates body
temperature and protects the body against disease. Blood maintains acid-base
equilibrium of the body (about pH 7.35 in the veins and about pH 7.39 in the arteries).
See paragraph 2-6 for a discussion of acid-base balance.
c. Major Components. Blood has a liquid portion and a solid portion. The
liquid portion is called plasma, which is about 55 percent of the blood's volume. The
solid portion (red cells, white cells, and other vital factors) makes up the remaining 45