Human beings are arguably the most complex organisms on this
planet. Imagine billions of microscopic parts, each with its
own identity, working together in an organized manner for the
benefit of the total being. The human body is a single structure
but it is made up of billions of smaller structures of four
major kinds:
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Cells
Cells have long been recognized as the simplest units
of living matter that can maintain life and reproduce
themselves. The human body, which is made up of numerous
cells, begins as a single, newly fertilized cell.
Tissues
Tissues are somewhat more complex units than cells.
By definition, a tissue is an organization of a great
many similar cells with varying amounts and kinds
of nonliving, intercellular substance between them.
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Organs
Organs are more complex units than tissues. An organ is an
organization of several different kinds of tissues so arranged
that together they can perform a special function. For example,
the stomach is an organization of muscle, connective, epithelial,
and nervous tissues. Muscle and connective tissues form its
wall, epithelial and connective tissues form its lining, and
nervous tissue extends throughout both its wall and its lining.
Systems
Systems are the most complex of the component units of the
human body. A system is an organization of varying numbers
and kinds of organs so arranged that together they can perform
complex functions for the body. Ten major systems compose
the human body:
Detailed discussions on the body systems will be provided
in the following learning units.
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