> Introduction to Digestive System 1 - Oesophagus, Stomach & Duodenum

When we see or smell food, or anticipate a good meal, the brain not only tells certain cells in the stomach lining to start secreting digestive chemicals but also tells others to secrete the hormone gastrin, which sets full-scale production of digestive juices in train. The most important chemicals produced by the stomach are hydrochloric acid and the enzymes pepsin and lipase. The acid produced by the stomach is of sufficient strength to liquefy meat and to kill the bacteria in food, efficiently protecting the rest of the digestive system from infection caused by putrefaction; it is also necessary for the absorption of calcium, iron, and other nutrients. Pepsin breaks down proteins into smaller units and lipase has a similar effect on fats; both enzymes require an acid environment. However, if the stomach did not also have millions of mucus-producing cells in its walls, it would start to digest itself; ulcers are, in effect, areas where the mucous lining has been destroyed.

Although the stomach mostly secretes substances which break food down, it also absorbs small molecules such as water, salts, Vitamin B₁₂, and alcohol, and feeds them into the bloodstream. Once in the duodenum, semi-digested food is made alkaline and further broken down by secretions from the pancreas, and any untreated fat in it is emulsified or broken down into very small droplets by bile from the gall bladder. Both the pancreas and gall bladder have ducts which open into the duodenum, and the flow of digestive juices through them is controlled by a series of hormones secreted as soon as food mixed with acid enters the duodenum.

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