In addition to the physical external parts, the human body can also be divided by organ system and the parts that compose those systems. The main organ systems of the human body are the respiratory system, the vascular system, the skeletal system, and the digestive system.
The nine major organ systems in the human body are the integumentary system, the musculoskeletal system, the respiratory system, the circulatory system, the digestive system, the excretory system, the nervous system, the endocrine system, and the reproductive system.
Life processes of the human body are maintained at several levels of structural organization. These include the chemical, cellular, tissue, organ, organ system, and the organism level. Higher levels of organization are built from lower levels.