CARDIOVASCULAR AND OTHER CIRCULATORY SYSTEMS OF THE
Section I. INTRODUCTION
NEED FOR A CIRCULATORY SYSTEM
In simple organisms such as unicellular and one-or two-layer organisms,
materials can be transferred among cells by simple processes of diffusion. However, in
large organisms, a system is needed for the distribution and collection of materials.
This is because diffusion does not occur fast enough to carry the large volumes of
materials necessary through the greater distances required.
DISTRIBUTION OF SUBSTANCES
a. Products of the Digestive System. Some of the substances distributed to
the body cells are products of the digestive system. These materials meet individual
cell requirements for energy, growth, repair, synthesis of new materials, and storage for
b. Oxygen. In the lungs, oxygen is obtained by the blood through the process
of external respiration. Oxygen is then transported to the individual body cells, where it
is used in metabolic oxidation. This provides energy for production of ATP (adenosine
triphosphate), which is necessary for carrying on the life processes of the body.
COLLECTION OF SUBSTANCES
Some substances are collected from the body cells for elimination. These
include carbon dioxide, nitrogenous wastes, and other potentially harmful substances
that are carried to organs like the lungs, liver, or kidneys for elimination from the body.
HORMONES AND OTHER CONTROL SUBSTANCES
Hormones are the products of endocrine glands (see lesson 11). Hormones and
other control substances are distributed throughout the body by circulatory systems.
The tissues or organs affected by these substances are usually called target organs. In
turn, substances released by the target organs often affect the original endocrine gland.
This results in a feedback system.