Today people strive to great lengths to keep their body in a good, healthy condition. Every system in the body, let it be digestion, circulatory (of blood), immune, respiratory, cellular respiratory or kidneys and excretion are all vital. Each have their separate roles, but not one works separately. The human body works as a whole in order to survive, even though not all goes well all the time.
            
             The digestive system is in charge of breaking down complex food particles into small molecules that can be transported in the blood plasma. It involves ingestion (taking in food), digestion (breaking down food), and absorption (passing nutrients from the digestive tract to blood stream). Chemicals such as mucin (which line the walls of the stomach to protect it from the acid) and rennin (which aids in the coagulation and digestion of milk proteins) are involved in this complex process. The end result is urine and solid waste.
             Circulation of blood allows the transportation of some waste products. The liver takes excessive amino acids from the excess protein out of the blood and removes the amino groups in order to convert the rest of the molecule into a useful form for energy production by body cells. Pancreatic hormones such as insulin, increase the removal of glucose from the blood stream; glucagon that stimulates the release of glucose into the blood stream and somatostatin that inhibits the action of insulin and other hormones.
             Without a proper digestive system, the immune system would not be able to fight off diseases that try and sometimes do infect the body. Vitamins, such as the fat-soluble vitamins (A, D, E, and K) are important for the performance of the immune system. Other vitamins such as Vitamin C and B diffuse across the intestinal lining and move directly into the capillaries. Iron is a very important mineral that is required for the production of hemoglobin which is part of the red blood cell (important for immunity). The l...