ANATOMY AND PHYSIOLOGY
Section I. THE CIRCULATORY SYSTEM AND THE LYMPHATIC SYSTEM
NEED FOR A CIRCULATORY SYSTEM
a. Unicellular Animals. In primitive animals which are composed of one, or
only a very few cells, there is no need for a circulatory system. All of the cells are
exposed to the environment (energy source and waste removal) and are equally
capable of carrying on the processes, which are necessary to sustain life. No cell is
dependent on any other cell to any great extent, and no circulatory system is necessary
b. Complex Organisms. Unlike the simple onecelled organisms, the human
being has many specialized cells. Most of these cells are not exposed to the
environment. Therefore, they are unable to obtain the materials needed for life or to
remove their waste products on their own. The circulatory system serves to meet the
needs of the cells of the body. Food, water, oxygen, and all the other substances
required by the cells are delivered by the circulatory system. Likewise, the waste
products of the cells are removed by the circulatory system. The general composition of
the bloodstream remains very uniform, despite the fact that chemical substances and
cellular elements are continually leaving and entering the bloodstream.
BASIC FUNCTIONS OF THE CIRCULATORY SYSTEM AND BLOOD
Because blood flows through every organ in the body, blood participates in every
major function of the body.
a. Respiration. Blood carries oxygen from the lungs to all parts of the body.
The oxygen capacity of the blood depends directly on the hemoglobin concentration.
b. Nutrition. Through the bloodstream, food materials are absorbed from the
digestive tract and carried to the tissue cells all over the body. Such absorbed food
products may be used immediately by active cells or stored. If nutritional materials are
stored, they may be used at any time by being transported by the blood when the need
c. Excretion. In respiration, the blood carries carbon dioxide from the tissues to
the lungs. The blood also carries waste products for elimination by the excretory
d. AcidBase Balance. The blood has certain buffer systems so that neither a
too acid nor a too alkaline condition occurs. The buffers in the blood include the protein,
bicarbonate, and phosphate buffers. If CO2 were not removed by the lungs and