Living things are organized on a scale from small to large. All living things are made of cells, the smallest unit of structure and function. In most multicellular organisms, cells combine to make tissues, groups of similar cells carrying out the same function. Organs are collections of tissues grouped by a common function, an organ system is a set of functionally related organs, and an organism is an individual living thing. The heart is an organ built mostly of cardiac muscle tissue, found only in the heart. Its beats can be counted at the wrist as a pulse, a science reading the student takes at rest and after gentle exercise.