Build from smaller parts
A cell is the smallest independently functioning unit of a living organism. Groups of similar cells that cooperate in a particular job form tissues.
Two or more tissue types combine to form an organ. Multiple organs then work together as an organ system, and all of the body’s systems contribute to the functioning organism.
Use the level to describe the job
The levels are not just size labels. Each level describes a different kind of teamwork. A tissue coordinates cells, an organ combines multiple tissues, and an organ system coordinates multiple organs around larger needs.
For example, the heart is an organ because it contains multiple tissue types working together. The heart and blood vessels belong to the cardiovascular system. This organizational model explains normal body structure; it is not a tool for diagnosing a health concern.
Remember these ideas
Key ideas
- Cells are the smallest independently functioning units of living organisms.
- A tissue is a group of similar cells working together.
- An organ contains two or more tissue types.
- An organ system is a group of organs working together to meet major body needs.
Make the reasoning visible
Build a cardiovascular example
- 1
Start with one cardiac muscle cell.
- 2
Many cardiac muscle cells contribute to cardiac muscle tissue.
- 3
Cardiac muscle and other tissue types combine in the heart, making the heart an organ.
- 4
The heart works with blood vessels in the cardiovascular system, which contributes to the functioning of the whole organism.
Try the next step
Put the levels in order
Arrange these from simpler to more complex: digestive system, intestinal epithelial cell, person, epithelial tissue lining the intestine, small intestine.
Why is the small intestine an organ instead of a tissue or an organ system?