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In biology, an organ (Latin: organum, "instrument, tool") is a group of tissues that perform a specific function or group of functions. Common animal organs include the heart, lungs, brain, eye, stomach, spleen, pancreas, kidneys, liver, intestines, skin, uterus, bladder, bone, etc. A group of related organs is an organ system.

Organelles are analogous sub-cellular structures.

Organ systems

Organ system – a system composed of organs working together to carry out a function.

• Circulatory system
• Digestive system
• Endocrine system
• Integumentary system
• Lymphatic system
• Muscular system
• Nervous system
• Reproductive system
• Respiratory system
• Skeletal system
• Urinary system