IntroductionMany of us would not ordinarily consider the digestive, respiratory, and urinary systems to be closely related. There are certainly differences between these systems and the functions they perform. The digestive system is primarily concerned with the breakdown and absorption of nutrients found in the food we eat, which the body uses for energy and as raw material for building macromolecules. The respiratory system is involved with the process of gas exchange, allowing oxygen to enter the body and carbon dioxide to be released to the environment. The urinary system functions primarily in ridding the body of other metabolic wastes (most importantly urea), and is also vital in the maintenance of the water/solute balance of the body. So what is it that these systems have in common? If you think about each of the functions mentioned above, a common theme emerges: each of these systems is involved with the absorption (input) or excretion (output) of needed substances or metabolic wastes, respectively. While each system deals with various substances in different ways, absorption and excretion represent the fundamental processes by which various substances are exchanged between the cells of our bodies and the external environment.