Organ system is present in higher animals. Organs (an independent part of the body) are the functional units of an animal's body that are made up of more than one type of tissue. Example includes the heart, lungs, liver, spleen and kidney.

The next higher level of structural organization in animals is the organ system. An organ system is an association of organs that together perform an over all function. The organ systems in higher vertebrate animals are the integumentary, skeletal, muscular, nervous, endocrine, circulatory, lymphatic, respirator, digestive, urinary and reproductive systems.

The highest level or organization in an animal body is the orgnasimic level. All parts of the animal body function with one another to contribute to the total organism a living entity or individual.

Animals need energy in order to survive. Enzymes regulate many of the chemical reactions that produce energy. Together energy and enzymes are the driving and controlling forces in animals. All animals harvest energy from nutrients to fuel their metabolism with energy from a ATP.