The human body is the most complicated machine in the world. We see with it, hear with it, breathe with it, walk and run with it, and sense pleasure with it. Its bones, muscles, arteries, veins and internal organs are organized with marvellous design, and when we examine this design in detail we find even more amazing facts. Every part of the body, though each may seem to be so different from another, is made up of the same material: cells.
The human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs. By the time the human reaches adulthood, the body consists of close to 10 trillion cells, the basic unit of life. Groups of cells combine and work in tandem to form tissue, which combines to form organs, which work together to form organ systems.
The organ systems of the body include the musculoskeletal system, cardiovascular system, digestive system, endocrine system, integumentary system, urinary system, lymphatic system, immune system, respiratory system, nervous system and reproductive system.