Primarily, oxygen is absorbed from the atmosphere into the body and carbon dioxide is expelled from the body. The main role of the digestive system is to breakdown and absorb nutrients that are necessary for growth and maintenance. The main role of the excretory system is to filter out cellular wastes, toxins and excess water or nutrients from the circulatory system. The main role of the endocrine system is to relay chemical messages through the body. In conjunction with the nervous system, these chemical messages help control physiological processes such as nutrient absorption, growth, etc.
Many glands exist in the body that secrete endocrine hormones.
Among these are the hypothalamus, pituitary, thyroid, pancreas and adrenal glands. The main role of the reproductive system is to manufacture cells that allow reproduction. In the male, sperm are created to inseminate egg cells produced in the female. The Informative Graphics Corp. has put together a wonderful Human Anatomy On-line program. The University of Washington's Digital Anatomist Interactive Atlas has some interesting computer generated illustrations of the brain, the heart and a knee cross-section.