Organs
Two or more tissues working together.
E.g. heart, lungs and brain.
Tissues
A group of similar cells working together.
E.g. muscle and nervous tissue.
Systems
A number of organs working together.
E.g. the sensory and the digestive systems.
Organisms
An organism is a living thing. It is made up of many systems working together.
Organism are arranged into: Cells
Why is blood considered to be a tissue
It contains cells