All living cells require metabolic substrates
(e.g., oxygen, amino acids, glucose) and a
mechanism by which they can remove byproducts
of metabolism (e.g., carbon dioxide,
lactic acid). Single-cell organisms exchange
these substances directly with their environment
through diffusion and cellular transport
systems. In contrast, most cells of large organisms
have limited or no exchange capacity
with their environment because their cells are
not in contact with the outside environment.
Nevertheless, exchange with the outside environment
must occur for the cells to function.
To accomplish this necessary exchange, large
organisms have a sophisticated system of
blood vessels that transports metabolic substances
between cells and blood, and between
blood and environment. The smallest of these
blood vessels, capillaries, are in close proximity
to all cells in the body, thereby permitting
exchange to occur. For example, each cell in
skeletal muscle is surrounded by two or more
capillaries. This arrangement of capillaries
around cells ensures that exchange can occur
between blood and surrounding cells