The digestive system

Layout of the digestive system

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Layout of the digestive system

•The digestive system is essentially a tube whose ends can be opened or closed•Its contents are outside the body although enclosed by it

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Processes in the digestive system

•Motility: moving food along the gut•Secretion: of the “tools” for digesting food (enzymes, bile etc)•Absorption: the aim of digestive function

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Parts of the digestive system

10 s

1-3 hours

7-9 hours

25-30 h

30-120 h

When food arrives at each point:

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Parts of the digestive system

Mouth

Oesophagus

Stomach

Small intestine5

Parts of the digestive system

Pancreas

Liver

Small intestine

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Parts of the digestive system

Small intestine

Large intestine

RectumAnus

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Mouth and teeth

Palatine tonsils: protect against infection entering from mouth and nose

Tongue and cheeks: hold food in place to be chewed

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Deciduous (“milk”) teeth

10 upper teeth

10 lower teeth

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Permanent teeth

16 upper teeth

16 lower teeth

Four quadrants

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Salivary glands

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Saliva contains:

•Mucin (lubrication)•Lysozyme and immunoglobulins (antibacterial)•Salivary amylase: Breaks down starch into maltose and isomaltose (both glucose + glucose)

Glucose

Starch

Maltose

Salivary amylase

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Swallowing: controlled by swallowing centre in brainstem

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Events in swallowing

Reflex triggered by contact of food with the back of the

throat

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...followed by a wave of contraction down the oesophagus (peristalsis)

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Stomach

•Acid and enzyme secretion•Agitation and mechanical breakdown•Very minor role in absorption

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Gastric secretion•Acid: hydrochloric acid•Enzyme: pepsinogen (gets converted into pepsin)•Mucus: lubrication and protection of stomach wall•Secreted from cells in the gastric pits

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Gastric motility•Gastric filling: reflex relaxation•Mixing waves (about 3 per minute): mix food with gastric secretions to produce chyme•Gastric emptying: squeezing liquid chyme through pyloric sphincter

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Small intestine anatomy

Three parts in this order (starting from the stomach end):•Duodenum: about 30 cm (12 inches) long•Jejunum: about 2.5 m long•Ileum: about 3.5 m long•Structure and function are similar all the way along

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Small intestine anatomy

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Small intestine anatomy

Absorption through:•capillaries•lacteals (lymph vessels)

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Small intestine functions

•Motility: segmental contractions mix contents, peristaltic contractions propel them

•Secretion: several enzymes secreted from intestinalmucosa and also from the pancreas

•Absorption: most absorption takes place in the duodenum and jejunum, some in the ileum

•Associated organs: liver and pancreas

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Small intestine motility

•Peristalsis: relaxation/contraction pushes food along

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Small intestine motility

•Segmental contraction: contraction at different points divides and mixes food

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Small intestine secretion

•Mucus: protects the wall of the intestine•Enzymes: bound to the microvilli- disaccheridases convert maltose/sucrose to simple sugars (glucose, fructose)- peptidases break down small amino acid chains- nucleases break down nucleic acids

•Added to the intestinal secretion are secretions from the liver and the pancreas

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Absorption in the small intestine

e.g. sugars

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What is absorbed where?

Food Water27

The liver

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The liver

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Liver functions

•Bile production:- bile helps to neutralise stomach acid- bile is a “detergent”: important role in fat digestion

•Storage of glycogen

•Interconversion of nutrients eg amino acids to glucose

•Detoxification eg alcohol dehydrogenase

•Synthesis of blood proteins

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Inside the liver

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Inside the liver

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The gallbladder

•Bile is stored in the gallbladder: enters small intestine after meals

•Gallstones can form in the gallbladder (from excess cholesterol)

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The pancreas

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The pancreas

Endocrine (blue) Exocrine (pink)(Islet of Langerhans)

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Pancreatic functions

Endocrine function (secretion into the blood):•secretion of insulin•secretion of glucagonSee Term 2

Exocrine function (secretion into the gut):•secretion of bicarbonate (HCO3

–) to neutralise stomach acid•secretion of enzymes:- trypsin, chymotrypsin, carboxypeptidase to break down proteins- amylase to break down sugars- lipases to break down fats- nucleases to break down nucleic acids

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Large intestine

•Absorption of water: contents become “faeces”•Secretion of mucus•Motility: “mass movements” 3-4 times a day (usually after meals)

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Large intestine reflexes

•Fullness of stomach and duodenum stimulate mass movements•Faeces are pushed into the rectum•Rectal fullness stimulates defecation reflex•Internal anal sphincter relaxes•When the time is right...- voluntary relaxation of external anal sphincter- inspiration, forceful contraction of abdominal muscles, faeces emerge

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Water absorption and secretion

•Food intake: 1200 ml•Secretions: 7000 ml•Absorbed: 8100 ml•Excreted: 100 ml

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Reading

Digestive system chapter inSeeley series textbooks