INSIDE THE HUMAN BODY: The Stomach
The major role of the stomach is functioning as a reservoir for the food we eat; this allows large meals to be consumed in one sitting before it gradually gets emptied into the small intestine. A combination of acid, protein-digesting enzymes and vigorous churning action breaks the content within the stomach into an easier-to-process liquid form, preparing food for absorption in the bowels.
In the resting state of the stomach, it is constructed and the internal surface of the organ folds into characteristic ridges. When we start to eat, the stomach begins to distend; the ridges flatten in turn allowing the stomach to expand while the outer muscles relax. The stomach is able to accommodate approximately a liter (1.8 pint) of food without discomfort.
The expansion of the stomach activates stretch receptors, which trigger nerve signals resulting in increased acid production and powerful muscle contractions to mix and churn the contents. Gastric acid causes proteins within our food to unravel, allowing enzyme pepsin access to break down the proteins revealed. The presence of partially digested proteins stimulate enteroendocrine cells (G-cells) to produce a hormone called gastrin thus encouraging even more acid production within the stomach.
The stomach then empties its content into the small intestine through what is known as the pyloric sphincter; it serves as a kind of gateway between the stomach and the small intestine. Liquids are able to pass through the sphincter with ease; however, solids are smaller than two millimetres (0.04-0.08 inches) before they are able to pass. Anything that is larger is ‘refluxed’ back into the main chamber for further enzymatic breakdown. On average, it only takes about two hours for half a meal to pass into the small intestine and the process is generally complete within four to five hours.
Why Does It Not Digest Itself?
The Stomach is full of corrosive acid and enzymes capable of breaking down protein – if left unprotected the stomach lining will quickly be destroyed. To prevent destruction on the stomach lining, cells on the stomach wall produce carbohydrate-rich mucus, forming a slippery, gel-like barrier. This gel-like barrier contains bicarbonate, which serves as an alkaline and buffers the ph on the surface of the stomach lining, preventing damage that could be caused by the acid. For added protection, the protein-digesting enzyme pepsin is produced from a zymogen (the enzyme in its inactive form) – pepsinogen and only become active when in contact with the acid, at a safe distance away from the cells that produce it.
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