The human body is organized into hierarchical systems that work together to maintain life. The largest organ is the skin, part of the integumentary system, covering approximately 1.5–2 m² in adults and serving as a barrier against infection, regulator of temperature, and sensory interface. The adult skeleton contains 206 bones, divided into the axial skeleton (80 bones: skull, spine, ribs) and the appendicular skeleton (126 bones: limbs, shoulders, pelvis). Three types of muscle tissue perform different mechanical roles: skeletal muscle enables voluntary movement, cardiac muscle drives involuntary heart contractions, and smooth muscle produces involuntary contractions in organs and blood vessels.
The body's internal environment relies on homeostasis, the active maintenance of stable internal conditions such as temperature, pH, blood sugar, and fluid balance despite external fluctuations. This regulation depends on feedback loops involving sensors that detect changes, control centers (often the hypothalamus) that integrate information, and effectors that carry out corrective responses. Body temperature is one well-regulated variable: the hypothalamus acts as a thermostat, triggering sweating and peripheral vasodilation when core temperature rises, and shivering with vasoconstriction when it falls. Normal body temperature is approximately 36.5–37.5°C (97.7–99.5°F), with fever generally defined at 38°C (100.4°F) or higher.
The major organ systems each have specialized roles. The digestive system includes the mouth, esophagus, stomach, small intestine, large intestine (colon), rectum, and anus, with accessory organs (liver, gallbladder, pancreas) that aid digestion. The respiratory and circulatory systems cooperate closely: the lungs exchange gases in tiny air sacs called alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out, a process called gas exchange. The heart, with its four chambers (right atrium, right ventricle, left atrium, left ventricle) and four valves (tricuspid, pulmonary, mitral, aortic), pumps blood through pulmonary circulation to the lungs and systemic circulation to the body in approximately 60 seconds. The brain, divided into cerebrum, cerebellum, and brainstem, controls all bodily functions, while the spinal cord relays signals between brain and body and controls reflexes. The lymphatic system, including lymph nodes, spleen, and thymus, drains excess tissue fluid and houses immune cells. The kidneys filter blood to produce urine, regulate electrolytes, maintain acid-base balance, and produce hormones such as erythropoietin and renin.