Levels of Complexity

There are different levels of organization and complexity in the human body. These are:

Molecular levelThe molecular level is the lowest level of organization. There are a vast number of different molecules in the human body.The simplest level of organization is a single organelle, which is composed of macromolecules.
Cellular levelThe cell is the smallest independent unit of life. There are millions of kinds of cells in the human body. Cells cannot be seen with the naked eye. However, under a microscope different types of cells can be distinguished by size and shape and also by the dyes they absorb.
Cells of the body are specialized and perform specific functions, contributing to the functioning of the whole body. Cells having similar structures and functions form tissues.
Tissue levelTissues are made up of cells that have similar structure and function and which work together to perform a specific activity. Examples of tissues include blood, nervous, bone, etc.
There are four basic tissues in the human body: connective, epithelial, muscle, and nerve
Organ levelOrgans are made of different types of tissues. Organs have specific functions. Examples include heart, brain, skin, etc.
Organ system levelA group of organs and tissues constitute a system.The human body has several systems which work interdependently and perform specific functions. Proper functioning of all systems is necessary for the body to be healthy.
The human body has 11 organ systems: integumentary, muscular, skeletal, nervous, circulatory, lymphatic, respiratory, endocrine, urinary/excretory, reproductive and digestive.