Organ
An organ is a body structure made of two or more tissues that work together to perform a specific function, such as the heart, lung, or leaf.
Definition
An organ is a body part made of different tissues working together to do one main job; your heart, lungs, stomach, and brain are organs. It is a structure built from at least two tissue types organized to carry out a specific function, and animal organs include the heart, lungs, kidneys, liver, skin, and brain, while plant organs are roots, stems, leaves, and flowers. Organs cooperate in organ systems, and the skin, the largest human organ, protects the body and helps regulate temperature. Anatomically, an organ is a distinct structure with a defined functional role in which multiple tissues are integrated in a specific architecture, supplied by blood vessels and nerves, and often bounded by a connective tissue capsule. Function emerges from arrangement: the nephron of the kidney filters and reabsorbs because epithelial tubules are wrapped by capillaries, and the alveolus exchanges gas because a thin epithelium sits against a dense capillary bed. Organogenesis is directed by conserved signaling pathways, and organ failure is treated by support, transplantation, or, increasingly, engineered tissue.
Example
The heart is built from muscle tissue that squeezes, nerve tissue that sets the rhythm, and connective tissue that holds it together, and its job is to pump blood. The stomach contains epithelial tissue that secretes acid and enzymes, smooth muscle that churns food, connective tissue that supports it, and nervous tissue that controls secretion, and an adult stomach can hold about $1$ to $1.5\ \text{L}$ after a meal. Each human kidney contains about $1$ million nephrons, and together the two kidneys filter roughly $180\ \text{L}$ of plasma per day while returning about $99$ percent of it to the blood; the liver, with about $500$ known functions, regenerates to its original size after up to two thirds of it is removed.
Key Insight
Plants have organs too: a leaf is an organ for making food, a root for taking in water, and a flower for reproduction. The same tissue types appear in many organs, and what makes an organ unique is how they are arranged, so muscle in the heart pumps, muscle in the stomach churns, and muscle in the eye focuses. Organs are efficient because they concentrate a function in one place, but that also creates single points of failure: a blocked coronary artery can stop a heart even though every other organ is healthy.