1. Human Body

2. What is the body composed of?
The body has 5 levels of organization: cells, tissues, organs, organ systems, and the organism itself
The cell is the basic unit of life and makes up all living things
Depending on the type of organism, the cells will have different functions
For example, in a single celled organism, the individual cell is responsible for capturing and releasing energy
In more complex organisms, cells are specialized and perform specific jobs
Red blood cells carry oxygen from the lungs throughout the body; white blood cells help fight off infection

3. There are between 4,000-10,000 white blood cells per microliter of blood in a normal adult body

4. Tissues
A tissue is a group of similar cells that work together to perform a particular function
The human body contains several types of tissues and they can be classified into 4 main groups according to their functions
Epithelial tissue functions as a boundary covering all of the inner and outer surfaces of your body

5. Nerve tissue functions as a messaging system by carrying electrical impulses between your brain and the various parts of your body in response to changing conditions
Muscle tissue functions in movement resulting when muscle cells contract and relax
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

7. Organs
An organ is a structure that is made up 2 or more types of tissue that work together to carry out a function in the body
As in cells and tissues, the structure of an organ relates to its function

8. 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
The body is made up of many organ systems

9. Organisms
The 5th level of organization is the organism itself.