THE HUMAN CARDIOVASCULAR AND LYMPHATIC SYSTEMS
Section I. INTRODUCTION
2-1. NEED FOR CIRCULATORY SYSTEMS
The need for circulatory systems is based on two criteria:
Number of cells. Multicellular animals are animals with great numbers of
(2) Size. In larger animals, most cells are too far away from sources of food
and oxygen for simple diffusion to provide sufficient amounts. Also, distances are too
great for simple removal of wastes.
b. Because of these criteria, we need a system (or systems) to carry materials to
all cells. To get food and oxygen to the cells and to remove waste products, we need a
transport system, or circulatory system. Human circulatory systems are so effective that
few cells are more than the width of two cells away from a capillary.
2-2. BASIC COMPONENTS OF ANY CIRCULATORY SYSTEM
The four basic components of any circulatory system are a vehicle, conduits, a
motive force, and exchange areas.
a. Vehicle. The vehicle is the substance that actually carries the materials
Conduits. A conduit is a channel, pipe, or tube through which a vehicle
c. Motive Force. If we say that a force is motive, we mean that it produces
movement. Systems providing a motive force are often known as pumps.
d. Exchange Areas. Since the materials being transported must eventually be
exchanged with a part of the body, special areas are developed for this purpose. They
are called exchange areas.
2-3. CIRCULATORY SYSTEMS IN THE HUMAN BODY
a. The cardiovascular system is the circulatory system involving the heart and