The |pancreas| and |spleen| lie just behind the lower part of the |stomach|. The |pancreas| is responsible for secreting digestive ~enzymes~ into the |duodenum|. The |spleen|'s function is closely related to the cardiovascular and lymphatic systems, but because of its physiological proximity to the |pancreas| and |stomach|, it is also worthwhile to discuss it in conjunction with these organs. The |spleen| is the only lymphatic organ responsible for filtering ~blood~. It removes worn out |blood cells| from the bloodstream, converts their ~hemoglobin~ to bilirubin, and then releases their iron back into the bloodstream to be used in the manufacture of other |blood cells|.
The |pancreas| is the second largest gland in the body and is both an endocrine and an exocine gland. Its exocrine function is to produce digestive juices (pancreatic juices) and release them through a tube, the pancreatic duct, to the intestine. The endocrine function of the |pancreas| is controlling the amount of sugar in the ~blood~. The cells that control ~blood~ sugar levels are called islands of Langerhans. These islands are microscopic clumps of cells scattered throughout the pancreatic tissue among the other pancreatic cells but are concentrated somewhat in the tail of the |pancreas|.
There are two kinds of cells in the islands: alpha and beta. The alpha cells secrete a ~hormone~ called glucagon and the beta cells secrete insulin. Insulin and glucogon work as a check and balance system regulating the body's ~blood~ sugar level. Glucogon accelerates the process of |liver| glycogenesis (a chemical process by which the glucose stored in the |liver| cells in the form of glycogen is converted to glucose. This glucose then leaves the |liver| cells and enters the ~blood~). This process tends to increase the concentration of glucose in the ~blood~. Insulin is an antigen to glucagon. It decreases the amount of ~blood~ glucose concentration. Insulin decreases ~blood~ glucose by accelerating its movement out of the ~blood~, through cell membranes, and into cells. As glucose enters the cells at a faster rate, the cells increase their metabolism of glucose. All sugary and starchy foods, such as bread, potatos, and cakes, are broken down into glucose. In this form they can be absorbed by every cell in the body, including the cells in the |liver|, one of whose major roles is to store sugar. Cells absorb glucose and burn it in structures called |mitochondria|, using the energy it contains and producing carbon dioxide and water as by-products. This burning up process is the body's principle source of energy. It cannot take place without insulin. |Diabetes| occurs when the |pancreas| fails to secrete enough insulin and so fails to regulate the glucose concentration in the ~blood~. The normal glucose level for an average adult is about 80 to 120 milligrams of glucose in every 100 milliliters of ~blood~. If the islands of langerhans secrete too little insulin an excess of glucose develops, a characteristic of |diabetes| mellitus the most common disorder of the endocrine system.