In biology, an organ (Latin: organum, "instrument, tool", from Greek όργανον - organon, "organ, instrument, tool") is a group of tissues that perform a specific function or group of functions. Usually there is a main tissue and sporadic tissues. The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are the nerves, blood, connective etc.

Animal organs

Animal organs include the heart, lungs, brain, eye, stomach, spleen, bones, pancreas, kidneys, liver, intestines, skin (the largest human organ), urinary bladder, and the sex organs (uterus in female animals, testicles in male). The internal organs collectively are often called viscera, the plural of the rarely-used term viscus.

Plant organs

Plant organs can be divided into vegetative and reproductive. Vegetative plant organs are root, stem and leaf, while reproductive are flower, seed and fruit.

The vegetative organs are essential for maintaining the life of a plant (they perform the vital functions, such as photosynthesis), while the reproductive organs are essential in reproduction. But, if there is asexual vegetative reproduction, the vegetative organs are those which create the new generation of plants (see clonal colony).

Organ systems

main: Biological system

A group of related organs is an organ system. Organs within a system may be related in any number of ways, but relationships of function are most commonly used. For example the urinary system comprises organs that work together to produce, store, and carry urine.

The functions of organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system, which involves the relationship between the muscular and the skeletal system.

List of major human organ systems

There are typically considered to be eleven major organ systems of the human body.

• Digestive system - Absorption of nutrients and excretion of waste.

• Skeletal system - Support and movement, lymphocyte production

• Muscular system - Support and movement, production of heat

• Nervous system - Integration and coordination through electrochemical signals

• Endocrine system - Integration and coordination through hormones

• Respiratory system - Elimination of CO₂ and absorption of O₂