The human boy is the entire physical structure of a human organism. The human body consists of a head, neck, torso, two arms and two legs. The average height of an adult human is about 1.7 m (5 to 6 feet) tall. This size is largely determined by genes. Body type and body composition are influenced by postnatal factors such as diet and exercise. By the time the human reaches adult-hood, the body consists of close to 100 trillion cells. Each is part of an organ system designed to perform essential life functions. The body's organ systems include: the circulatory system, immune system, respiratory system, digestive system, excretory system, urinary system, musculoskeletal system, nervous system, endocrine system and the male and / or female reproductive system.