The major functions of the digestive system are digestion, absorption and defaecation. Digestion involves the mechanical breakdown of foods such as chewing by teeth and churning of food in the stomach.Chemical digestion occurs in a series of steps whereby food is broken down by enzymes into small simple molecules that can be absorbed through the lining of the gastrointestinal tract into the body. Thefollowing nutrients are absorbed: carbohydrates e.g. glucose; proteins in the form of simple amino acids; lipids or fats e.g. fatty acids; electrolytes or ions e.g. Na+ and Ca+ as well as vitamins, nucleic acidsand water. Food that cannot be digested is eliminated from the body via the anus.