 The stomach is an amazing organ. It varies in size from person to person but can contain up to two litres of food and liquid. The stomach is one of the organs in the digestive system. It is essentially a bag of muscle where food is kept for up to two hours whilst various things happen to it. As discussed in the earlier video, what is digestion? There are two types of digestion that occur in the human body. Both types of digestion, mechanical and chemical, occur in the stomach. The innermost tissue in the stomach is called the mucosa. This layer contains cells that secrete something called gastric juice into the stomach. This isn't the kind of juice that I would have with my breakfast in the morning. It contains mucous, digestive enzymes and hydrochloric acid. The enzymes are used to chemically break down the food. The main enzyme released is called pepsin and it breaks down protein in the food into amino acids. A lot of people think that the stomach acid is also used to break down food but in fact it's actually there to protect us from infection. There are lots of nasty bacteria and fungi that can grow in food and you don't want these bacteria to be absorbed into the blood in the small intestine along with the food. This can make you very sick and is called food poisoning which will involve the nasty food being ejected by either one or both ends of the body. Therefore the acid which is pH 1 and very strong kills a lot of these microorganisms and keep us healthy. The stomach has a layer of mucous lining which stops the cells being damaged. However if the acid comes up, the esophagus, it can feel like a burning sensation. This is called acid reflux. The stomach also mechanically breaks down food due to it being made of muscle. This muscle is folded in order to increase the contact it has with the food as it mixes and churns the food around like a cement mixer. This muscular contraction is called peristalsis. This makes the food smaller but also increases the surface area for the stomach acid and enzymes to work more effectively. Food is now called chyme and is released into the small intestine for the next stage of digestion. When the food moves on to the next stage of digestion in the intestine, alkali from the pancreas is added to neutralise the acid and stop damage to cells. So there you have it, the stomach, a bag of muscle filled with acidic juice. Lovely!