BODY

The body is the physical structure of a living organism. In biology, it is organized in hierarchical levels, from cells to tissues to organs and organ systems, which interact to sustain life. In humans and many animals, the body possesses specialized tissues such as muscle, nerve, and epithelial tissue that perform distinct functions.

Major body systems include the skeletal system, which provides support and protection; the muscular system, which enables movement; and the integumentary system, comprising skin and its derivatives, which guards against injury and regulates temperature. Internal systems include the cardiovascular system, the respiratory system, the digestive system, the excretory system, the immune and lymphatic system, the endocrine system, and the reproductive system. The nervous system coordinates sensation and response, while the circulatory and respiratory systems supply oxygen and nutrients and remove waste.

Physiology describes how the body maintains homeostasis, balancing variables such as temperature, pH, fluid levels, and energy supply through feedback mechanisms. Metabolism converts food into energy and building blocks necessary for growth and repair.

Body development begins before birth and continues through life, shaped by genetics and environment. Variation arises from genetic differences, aging, health, and lifestyle. The study of the body encompasses anatomy, physiology, and medicine, and its insights inform fields ranging from healthcare to sports and ergonomics.