Food constitutes of a mix of nutrients, which cannot be used unchanged from the organism to cover its special needs. Digestive system is designed to convert complex molecules (like carbohydrates and proteins) to simple ones (like glucose and amino acids), which can be used by cells in order to develop and be auto-repaired. Digestive system begins from the mouth, where the food is received, and ends up in the colon which is responsible for the elimination of waste material of the body. Digestion starts in the oral cavity, where salivary glands reside. Salivary glands are cells producing saliva, which contains digestion enzymes. These enzymes are the first after chewing to meet food and start degrading them. Food molecules are then delivered through the throat to the esofagus, and they end up in the stomach afterwards through a series of contractions. The main digestion processing is happening there, because the pH of the stomach is acidic. That means that it contains acids (like hydrochloric acid) which have the ability to degrade food molecules, without destroying them. From there, food in a more liquid state is delivered to the small intestine, which is responsible both for the further cleavage of the food and for the absorption of nutrients in the blood circulation. As aforementioned, food molecules that are not transfered in the blood circulation are discharged by colon.