Body

Body (biology) is the entire physical structure of a living organism, comprising all interconnected physical parts: cells, tissues, organs, and systems. The term is used particularly of animals and humans, but also applies to other organisms with visible form.

The study of bodies divides into two fields: anatomy, which examines the structure and arrangement of parts, and physiology, which examines how those parts function. Both fields explain how the body maintains internal stability (homeostasis).

Levels of structure

The body has hierarchical levels. A cell is the smallest living unit. Cells of the same type that come together form tissue, such as muscle or connective tissue. Different tissues that come together form an organ, such as the heart or brain. Multiple organs that share a function form a system, such as the respiratory system or digestive system.

Major systems

Human and most animal bodies are characterized by systems such as: the skeletal and muscular system (movement and support), the circulatory system, the respiratory system, the digestive system, the urinary system, the nervous system, the endocrine system, the immune system, and the reproductive system. Each system depends on the others; blood, for example, carries oxygen, nutrients, and hormones.

Variation across organisms

Body structure varies by organism type. Some animals have bilateral symmetry, with left and right sides that mirror each other, while others such as starfish have radial symmetry. Plants and fungi are generally not described using the term body, but they do have distinct parts such as roots, stems, and leaves.