what is the correct order of the parts of a human body system from the most simplest to the most complex?

Cells - -> Tissues - --> Organs - -> systems

Explanation:

There are four levels of organization of life in organisms. These are the cells, tissues, organs and systems.

- Cells

This is the first and simplest level of organization of life. Cell is the smallest unit of body of living organisms. Examples of cells are rod and cone cells in the eyes, nerve cells, red blood cells etc

- Tissues

A tissue is the second level of organization of life. It is a group of similar cells forming a layer in an organism which performs a particular function. Examples of tissues include blood and muscles

- Organs

An organ is the third level of organization of life. It is a group of similar tissues forming a layer which performs a specific function. Examples of organs in the body include skin and eyes

- System

An system is the fourth level of organization of life. It is a group of similar organs which work together to perform specific functions. Examples of systems in the body of animals include nervous and digestive system. The systems then work together to make up the body of an organism.

Thus, the correct order from the most simplest to the most complex is:

Cells - -> Tissues - --> Organs - -> systems