Functions of the Circulatory System

Organisms composed of a small number of cells do not need a circulatory system. Most cells in such organisms are in direct contact with the environment. Oxygen, nutrients, and waste products can easily diffuse back and forth across cell membranes.

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Larger organisms, however, cannot rely on diffusion. Most of their cells are not in direct contact with the environment, and substances made in one part of the organism may be needed in another part. In a way, this same problem is faced by the millions of people living in a large city. Cities have transportation systems that move people, goods, and waste material from one place to another. The transportation system of a city is its streets, highways, and rail lines. The transportation system of a living organism is its circulatory system.

Humans and other vertebrates have closed circulatory systems. This means that a circulating fluid called blood is contained within a system of vessels.   The human circulatory system consists of the heart, a series of blood vessels, and the blood that flows through them.