Organ (biology)

Structure and characteristics

Organs combine tissues with complementary roles: parenchyma or functional cells, connective scaffolding, blood vessels, nerves and, in some organs, specialized ducts. Typical features include a defined shape or boundary, a blood supply in animals, and coordinated regulation so the organ responds to physiological signals. Size and complexity range widely—from microscopic glands to large organs like the liver.

Development and evolution

During embryonic development organs form by organogenesis: groups of progenitor cells differentiate and arrange into the organ's pattern under genetic and molecular control. Over evolutionary time organs have arisen by modification of simpler structures; analogous organs can evolve independently in different lineages while homologous organs reflect common ancestry.

Functions and examples

• Heart: pumps blood and maintains circulation.
• Lungs/gills: exchange respiratory gases with the environment.
• Kidneys: filter blood, conserve water and regulate electrolytes.
• Leaves (plants): capture light and perform photosynthesis.

Organs are central to physiology and medicine. Clinical problems such as organ failure, infection, congenital malformation and cancer target organs directly, and treatments include surgery, transplantation and regenerative approaches.

Notable distinctions: an organ differs from a tissue (a tissue is a population of similar cells) and from an organ system (a coordinated set of organs). Some structures called "organs" by tradition may be specialized glands or composite organs with multiple functional zones.