Organ. In biology, an organ (from the Latin "organum" meaning an instrument or tool) is a collection of tissues that structurally form a functional unit specialized to perform a particular function. Your heart, kidneys, and lungs are examples of organs.
Organs are organised into organ systems. An organ system is a group of organs that work together to do a job. The circulatory system includes the heart, veins and arteries. Its function is to transport substances in the blood, around the body.
