There are different levels of organization and complexity in the human body. These are:
|Molecular level||The 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 level||The 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 level||Tissues 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 level||Organs are made of different types of tissues. Organs have specific functions. Examples include heart, brain, skin, etc.|
|Organ system level||A 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.