The body has cells, tissues, and organs.
Your body is made of many parts that work together as a system to help you grow and stay
healthy. The basic level of organization in your body is the cell. Next come tissues, then
individual organs, and then systems that are made up of organs. The highest level of
organization is the organism itself. You can think of the body as having five levels of
organization: cells, tissues, organs, organ systems, and the organism. Although these levels
seem separate from one another, they all work together.
Cells
The cell is the basic unit of life. Cells make up all living things. Some organisms, such as bacteria, are made of only a
single cell. In these organisms the single cell performs all of the tasks necessary for survival. That individual cell
captures and releases energy, uses materials, and grows. In more complex organisms, such as humans and many
other animals and plants, cells are specialized. Specialized cells perform specific jobs. A red blood cell, for example,
carries oxygen and other nutrients from the lungs throughout the body.
Tissues
A​
tissue ​
is a group of similar cells that work together to perform a particular function. Think of a tissue as a brick wall
and the cells within it as the individual bricks. Taken together, the bricks form something larger and more functional.
But just as the bricks need to be placed in a certain way to form the wall, cells must be organized in a tissue.
The human body contains several types of tissues. These tissues are classified into four main
groups according to their function: epithelial tissue, nerve tissue, muscle tissue, and connective
tissue.
Epithelial (ehp-uh-THEE-lee-uhl) tissue functions as a boundary. It covers all of the inner and outer surfaces of your
body. Each of your internal organs is covered with a layer of epithelial tissue.
Nerve tissue functions as a messaging system. Cells in nerve tissue carry electrical impulses between your brain and
the various parts of your body in response to changing conditions.
Muscle tissue functions in movement. Movement results when muscle cells contract, or shorten, and then relax. In
some cases, such as throwing a ball, you control the movement. In other cases, such as the beating of your heart, the
movement occurs without conscious control.
Connective tissue functions to hold parts of the body together, providing support, protection, strength, padding, and
insulation. Tendons and ligaments are connective tissues that hold bones and muscles together. Bone itself is another
connective tissue. It supports and protects the soft parts of your body.
Organs
Groups of different tissues make up organs. An ​
organ ​
is a structure that is made up of two or more types of tissue
that work together to carry out a function in the body. For example, the heart that pumps blood around your body
contain all four types of tissues. As in cells and tissues, the structure of an organ relates to its function. The stomach's
bag-shaped structure and strong muscular walls make it suited for breaking down food. The walls of the heart are also
muscular, allowing it to function as a pump.
Organ Systems
An ​
organ system ​
is a group of organs that together perform a function that helps the body meet its needs for energy
and materials. For example, your stomach, mouth, throat, large and small intestines, liver, and pancreas are all part
of the organ system called the digestive system. The body is made up of many organ systems. In this unit, you will
read about these systems. They include the skeletal, muscular, respiratory, digestive, urinary, circulatory, immune,
nervous, and reproductive systems. Together, these systems allow the human organism to grow, reproduce, and
maintain life.
The body's systems interact with one another.
The ability of your body to maintain internal conditions is called ​
homeostasis
(​
HOH​
-mee-oh-STAY-sihs). Your body is constantly regulating such things as your body
temperature, the amount of sugar in your blood, even your posture. The processes that take
place in your body occur within a particular set of conditions.
The body's many levels of organization, from cells to organ systems, work constantly to
maintain the balance needed for the survival of the organism. For example, on a hot day, you
may sweat. Sweating keeps the temperature inside your body constant, even though the
temperature of your surroundings changes.