Organs result from the union of different tissues that, coordinately, carry out one or several vital functions. The heart, the lungs, the kidneys and the eyes are some of the biological structures called organs.
Each one the them has a different composition (different cells and tissues) capable of carrying out its tasks. For example. the heart is made up by muscle tissue which, when contracting, produces the flow of blood, and also by a fibrous tissues that is a part of the valves that regulate heart beat frequency and rhythm.
Some organs are visible, such as the skin and the eyes, but most of them, such as the kidneys and the lungs, are found inside the body.