nutrition 2

• Peristalsis

  • Alternating waves of contraction and relaxation in smooth muscle lining alimentary canal
• Sphincters

  • Muscular layer forms these ring-like valves, which regulate the passage of material between compartments
• How does food pass along the digestive tract

  1. Food is chewed in mouth and swallowed

  2. 5-10 seconds in oesophagus

  3. 2-6 hours in stomach

  4. 5-6 hours small intestine

  5. 12-24 hours large intestine

  6. Faeces expelled through anus
• Mouth / Oral Cavity

  • Teeth - mechanical digestion, food easier to swallow and increasing surface area
  • Salivary glands - secrete saliva (sometimes before food is even present - smell, time of day etc.)
  • Amylase - hydrolyzes starch (from plants) and glycogen (from animals) into polysaccharides and disaccharide maltose
  • Mucin - slippery glycoprotein, lubricates food and protects mouth
  • Buffers and antibacterial agents (e.g. lysozyme)
• Tongue

  1. Manipulates food into a bolus and pushes into the pharynx (throat region)

  2. Oesophagus - stomach

  3. Windpipe - lungs

  4. Epiglottis - flap of cartilage that covers the glottis (vocal chords + opening between them)

  5. Larynx - upper part of respiratory tract, guides swallowing movement that directs bolus down oesophagus
• If mechanism fails = lack of airflow to lungs = fatal
• When not swallowing

  Oesophageal sphincter muscle is contracted, epiglottis up, glottis open and air flows to lungs via trachea
• Swallowing

  1. Oesphageal sphincter relaxes and allows bolus to enter

  2. Larynx moves up, epiglottis covers glottis and stops food entering trachea

  3. Swallowing triggered when bolus reaches pharynx

  4. Food enters oesophagus, larynx moves down, allowing breathing

  5. Peristalsis moves bolus down oesphagus to the stomach
• Stomach

  • Located below diaphragm in upper abdominal cavity
  • Stores food and continues digestion
  • Elastic walls can stretch to accommodate 2 litres of food/liquid
  • Gastric pits secrete gastric juice, which mixes with food through churning action of stomach, to produce digestive juice called chyme
• Chemical digestion

  • Gastric juice contains:
  • Hydrochloric Acid (HCl) - pH 2.0, strong enough acid to dissolve iron nails, kills bacteria, denatures (unfolds) proteins, exposing peptide bonds
  • Pepsin - a protease (protein-digesting enzyme), unusual in that it works well in strongly acidic pH
• Gastric juice kept inactive until reaches stomach lumen
• Parietal Cells

  Secrete H+ and Cl- ions, ATP-driven pumps move H+ into lumen to achieve very high conc'n; Cl- ions pass through specific channels in membranes
• Chief Cells

  Release pepsinogen (inactive form of protease), HCl in lumen clips off small part of pepsinogen molecule to expose enzyme's active site + hence produces pepsin, Pepsin itself also activates pepsinogen (i.e. an example of positive feedback)
• Cells lining stomach

  • Protects against self-digestion by secreting mucus - viscous, slippery mixture of glycoproteins, cells, salts and water
  • Cell division = new epithelial layer every 3 days, damaged cells replaced
• Gastric ulcers

  Damaged areas of stomach lining, originally thought to occur due to XS acid secretion (stress), but infection of acid tolerant bacterium Helicobacter pylori, hence antibiotics cure most ulcers
• Stomach dynamics

  • Chemical digestion accompanied by churning action
  • Co-ordinated contractions and relaxations mix stomach content every 20 secs
  • Food becomes acidic, nutrient-rich broth "chyme"
  • Most times sphincters at both ends closed but....Acid-reflux = backflow of chyme into oesphagus ("heartburn")
  • Sphincter at bottom of stomach regulates passage of chyme into small intestine; 2-6 hours after meal
• Small Intestine

  • Most enzymatic hydrolysis reactions occur in small intestine
  • Humans = 6 metres long
  • Small diameter (c.f. large intestine)
  • Duodenum = first 25 cm; chyme from stomach mixes with digestive juices from: pancreas, liver, gall bladder, + intestinal wall secretions
• Hormonal control of digestion

  • AAs & FAs trigger cholecystokinin (CCK) release, stimulates release of pancreatic enzymes and bile
  • Fats trigger secretin and CCK = inhibit peristalsis + stomach secretion
  • Gastrin, via bloodstream, stimulates gastric juice production
  • Secretin stimulates pancreas to release Na bicarbonate to neutralize chyme
• Pancreatic secretions

  • Alkaline solution rich in bicarbonate and some enzymes
  • Bicarbonate neutralizes acidity of chyme and acts as a buffer
  • Enzymes secreted into duodenum, including trypsin and chymotrypsin (both proteases)
  • Only activated when safely located in extracellular space in duodenal lumen
• Bile

  • Digestion of fats and lipids relies on bile
  • Bile = mixture of substances produced in liver and stored in gall bladder
  • Bile contains bile salts that act as detergents (emulsifiers) to aid digestion and absorption of lipids
  • In producing bile, liver incorporates some pigments (from breakdown of old red blood cells) which are eliminated with the faeces - giving its distinctive colour
• Secretions of the small intestine

  • Epithelial lining of duodenum secrete other enzymes involved in:
  • Carbohydrate digestion = Disaccharidases
  • Protein digestion = dipeptidases, carboxypeptidases and aminopeptidases
  • Nucleic acid digestion = Nucleosidases and phosphatases
  • Duodenum = most of digestion occurs here
  • Jejunum = mainly absorption of nutrients and water
  • Ileum = mainly absorption of nutrients and water
• Absorption in the Small Intestine

  • Huge surface area (300m2)
  • Large folds in lining have finger-like projections called villi
  • Each epithelial cell of a villus has microscopic appendages or microvilli on its apical surface (brush border appearance)
  • Microvilli = enormous surface area = greatly increases nutrient absorption
  • Nutrients absorbed across epithelial membrane pass into microscopic blood vessels (capillaries) and into bloodstream
  • Passive transport - occurs down conc'n gradient, via transporters (e.g. fructose)
  • Active transport - requires energy (ATP), uptake of nutrients (e.g. amino acids, peptides) can occur against conc'n, so much more absorbed
• PepT1 transporter


  • Expression along rat intestine: D = duodenum, J = jejunum, I = Ileum, C = colon
  • PepT1 localization to villi in rat jejunum
• Nutrient-rich blood from villi capillaries

  • Travels to the liver via hepatic portal vein
  • Liver controls distribution of nutrients to rest of body (e.g. blood exiting liver normally has glucose concentration = 90 mg/ml, regardless of time after meal)
  • Liver removes toxic substances before circulated to rest of body
  • Liver is major site of detoxification of organic molecules, including drugs
• Fats broken down by lipase


  • Into fatty acids and glycerol, which can be absorbed across epithelial cell membranes
  • Then recombine into triglycerides
  • Coated with phospholipids, cholesterol and proteins = water-soluble globules called chylomicrons
  • Transported into lacteal (vessel at core of villus) then into lymphatic system - large veins - heart
• Large Intestine

  • Consists of caecum, colon and rectum
  • Sphincter controls entry of material from small intestine
  • Caecum = fermentation (relatively minor role in humans)
  • Colon = 1.5m long, leads to rectum and anus
• Colon

  • Major function = water absorption
  • 7 litres secreted in alimentary canal per day, 90% of which reabsorbed
  • Na+ ions actively pumped out of colonic lumen and water follows via osmosis
  • Faeces (i.e. the waste) becomes more solid as moved along colon by peristalsis
  • Problems = diarrhoea (too little water absorbed), constipation (too much water absorbed)
• Faeces

  • Undigested material (e.g. cellulose fibre) and ~1/3 is bacteria (e.g. E.coli), which produce methane and hydrogen sulphide
  • Stored in terminal portion of large intestine, rectum, until can be eliminated
  • Between rectum and anus: 2 sphincters - inner one (involuntary), outer one (voluntary)
  • Periodically (~once a day), strong contractions of colon create urge to defecate