In biology, an organ (Latin organum: instrument, tool) is a group of tissues, which 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

• circulatory system
• digestive system
• endocrine system
• immune system
• integumentary system
• lymphatic system
• muscular system
• nervous system
• reproductive system
• respiratory system
• skeletal system
• urinary system