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Digestion is the mechanical and chemical breakdown offoodinto smaller components that aremore easily absorbed into a blood stream, for instance. Digestion is a form ofcatabolism: abreakdown of large food molecules to smaller ones.When food enters the mouth, its digestion starts by the action ofmastication, a form ofmechanical digestion, and the contact ofsaliva. Saliva, which is secreted by thesalivary glands,contains salivary amylase, an enzyme which starts the digestion of starch in the food. Afterundergoing mastication and starch digestion, the food will be in the form of a small, round slurrymass called a bolus. It will then travel down theesophagusand into the stomach by the actionofperistalsis. Gastric juicein the stomach starts protein digestion. Gastric juice mainlycontains hydrochloric acid andpepsin. As these two chemicals may damage the stomachwall, mucusis secreted by the stomach, providing a slimy layer that acts as a shield against thedamaging effects of the chemicals. At the same time protein digestion is occurring, mechanicalmixing occurs by peristalsis, which are waves of muscular contractions that move along thestomach wall. This allows the mass of food to further mix with the digestive enzymes. After sometime (typically an hour or two in humans, 46 hours in dogs, somewhat shorter duration in housecats), the resulting thick liquid is called chyme. When thepyloric sphincter valve opens, chymeenters the duodenumwhere it mixes with digestive enzymes from the pancreas, and then passesthrough thesmall intestine, in which digestion continues. When the chyme is fully digested, it isabsorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water andminerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly

acidic about 5.6 ~ 6.9. Some vitamins, such as biotin andvitamin K(K2MK7) produced by bacteriain the colon are also absorbed into the blood in the colon.[1] Waste material is eliminated fromthe rectumduring defecation.[2]

Contents

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1 Digestive systems

o 1.1 Secretion systems

1.1.1 Channel transport system

1.1.2 Molecular syringe

1.1.3 Conjugation machinery

1.1.4 Release of outer membrane vesicles

o 1.2 Gastrovascular cavity

o 1.3 Phagosomeo 1.4 Specialised organs and behaviours

1.4.1 Beaks

1.4.2 Tongue

1.4.3 Teeth

1.4.4 Crop

1.4.5 Abomasum

1.4.6 Specialised behaviours

o 1.5 In earthworms

2 Overview of vertebrate digestion

3 Human digestion process

o 3.1 Phases of gastric secretion

o 3.2 Oral cavity

o 3.3 Pharynxo 3.4 Esophagus

o 3.5 Stomach

o 3.6 Small intestine

o 3.7 Large intestine

4 Breakdown into nutrients

o 4.1 Protein digestion

o 4.2 Fat digestion

o 4.3 Carbohydrate digestion

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o 4.4 DNA and RNA digestion

5 Digestive hormones

6 Significance of pH in digestion

7 Uses of animal's internal organs by humans

8 See also

9 References

10 External links

Digestive systems

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