The human body is made up of a number of different systems. Each system has a separate function, but some work together. Each system is made up of organs. One system is the skeleton, which serves to support the body and protect the internal organs. The heart is an organ in the circulatory system. The respiratory system enables us to breathe and take oxygen into the blood,, which moves around the body by means of the circulatory system. The lungs, for example, are part of the respiratory system. The digestive system enables us to take in food needed for growth. The liver functions as part of the digestive system and other systems. The endocrine system consists of various glands, such as the thyroid, sex and adrenal glands. Waste matter is ejected from the body by means of the urinary system. The nervous system controls the other systems and enables human beings to think.