An organism is a complex structure made out of smaller integrating parts.
So, the greatest level of integration is the organism.
The lower level is the level of organs.
Even smaller is the level of tissue and in the end
we have the cells, the smallest functioning units of the body.
The smallest units in an organism are the cells. There many different types of cells. We can take a muscle cell as an example.
Each type of cell integrates with other cells of this type and makes a tissue. Therefore, muscle cells integrate into a muscle tissue.
Afterward, tissue forms different organs. Muscle tissue can form the skeletal muscle, the heart, etc.
Finally, all of the organs form a functioning organism.