The large intestine is 1.5–1.8m in length and is divided into the caecum, colon and rectum. By the time food remains have reached this point, about 5–12 hours have elapsed.
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Undigested remains of food are passed through a one-way muscular valve into the first part of the large intestine known as the caecum – a small pouch that acts as a temporary storage site. Its main function is to absorb the end products of digestion and release hormones that regulate feelings of fullness. Absorption: Ileumīy the time chyme has reached the ileum, most of the digestion processes involving carbohydrate, protein and fats have occurred. These assist with the absorption of the end products of digestion into the bloodstream. It has a huge surface area created by finger-like structures called villi. Peristaltic waves of muscular contraction mix and move the chyme down the duodenum and into the jejunum. These fluids contain bicarbonate, enzymes and bile salts essential to the digestion process. The chyme is mixed with secretions from the pancreas and gall bladder. Small amounts of chyme are ejected approximately every 20 seconds from the stomach into the duodenum. The small intestine’s structure of folds, villi and microvilli increases the absorptive surface area and allows maximum exposure to enzymes and complete absorption of the end products of digestion. It takes 3–5 hours from entry to the duodenum to exit from the ileum. The pyloric sphincter at the lower end of the stomach slowly releases chyme into the duodenum. Strong muscular contractions in the stomach wall reduce the food to chyme – a thick milky material. Time to empty: Stomachįood is mixed with gastric juice. For a medium-sized bolus, it takes about 5–8 seconds to reach the stomach. In the oesophagus, the bolus is moved along by rhythmic contractions of the muscles present in its walls. This swallowing reflex takes about 1–3 seconds. Involuntary muscle contractions in the pharynx then push the bolus down towards the oesophagus. The food is formed into a small ball called a bolus, which is pushed to the back of the mouth by the tongue.
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Each mouthful takes approximately 30–60 seconds. The mixing process is lubricated by mucin, a slippery protein in saliva.
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Amylase in saliva chemically digests starch in the food. Chewing mechanically mixes food with saliva from the salivary glands.