170207 digestive system - epu.ucc.ie
Transcript of 170207 digestive system - epu.ucc.ie
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The digestive system
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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
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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
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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