1. Digestive system - Functions
Chapter 14

2. General functions Ingestion Propulsion
Placement of food into mouth
Propulsion
Peristalsis – involuntary – alternating waves of contraction/relaxation of muscle – longitudinal then circular
Squeezes food along
Food breakdown: mechanical digestion
Prepares food for breakdown by enzymes by breaking it into smaller pieces

3. General functions cont.
Food breakdown: chemical digestion
Enzymes break food down into building blocks
Hydrolysis reactions – add water to break apart bonds
Carbs
Monosaccharides – glucose, fructose, galactose
Disaccharides – sucrose (sugar), lactose (milk)
Polysaccharides – starch
Proteins – broken into amino acids
Lipids – broken into fatty acids and glycerol
Absorption – movement of nutrients from lumen to blood
Defecation – removal of indigestible residue

4. Activities of mouth, pharynx, & esophagus
Mastication - chewing – mechanical digestion
Mixing w/ saliva – begins starch digestion
Continuously released – more released when food enters
Deglutition – swallowing
Buccal phase – voluntary
Bolus is forced into pharynx by tongue

5. Mouth cont. Pharyngeal-esophageal phase – involuntary
Transport of food through pharynx and esophagus
Food moved by peristaltic contractions
Cardioesophageal sphincter – separates esophagus and stomach – pushed open by food
Gravity has nothing to do with food moving down.
Epiglottis – blocks trachea ; soft palate – blocks nasal passages; tongue – blocks mouth
No absorption – exceptions: some
heart medications

6. Activities of stomach Food breakdown
Gastrin – triggers stomach glands to release pepsinogens, mucus, and HCl acid
2-3 liters of gastric juices/day
Hydrochloric acid – changes pepsinogen into pepsin to break down proteins
In infants – also have rennin to break down proteins in milk – conversion
resembles sour milk

7. Stomach cont. Stretching of walls triggers mixing/churning = chyme
No absorption – exceptions: aspirin and alcohol
Heartburn – cardioesophogeal sphincter doesn’t close tightly and stomach acid enters esophagus
Ulcers – insufficient mucus – hydrochloric acid and pepsin eat away mucosa

8. Stomach cont. Food propulsion
Peristalsis begins in lower stomach – increasing toward pyloric valve
Pylorus allows only liquids and small enough particles to pass through pyloric sphincter
Each contraction only pushes about 3ml of chyme through to small intestine
Triggers enterogastric reflex – slows emptying of stomach to allow intestinal digestion to keep up
Takes 4 hours to empty stomach (well-balanced meal) – if high in fat = 6 hours or more
Emetic center in medulla of brain registers irritation in stomach and may trigger emesis (vomiting)

9. Activities of small intestine
3-6 hours
Food breakdown
Brush border enzymes – complete protein breakdown
Mucosa cells release hormones secretin (pancreas/liver) and cholecystokinin (CCK) (pancreas/gallbladder) – trigger release of pancreatic juice and bile

10. Small intestine cont. Pancreatic juice
Pancreatic amylase – completes starch digestion
Trypsin, chymotrypsin, carboxypeptidase – help complete protein digestion
Lipases – lipid digestion
Nucleases – nucleic acid digestion
Biocarbonate – neutralizes the acid from stomach
Bile – emulsifies (breaks down) large fat molecules into smaller ones
Food absorption – occurs mainly by active transport
All along small intestine
Food propulsion - peristalsis

11. Activities of large intestine
12-24 hours
Food breakdown
Bacteria breakdown few remaining nutrients
Produce methane and hydrogen sulfide
Bacteria make vitamin K and some vitamin B
Food absorption
Vitamins, water
Feces – what remains –
undigested material,
bacteria, small amount
of water

12. Large intestine cont. Propulsion
Peristalsis – doesn’t work as well
Mass movements – long, slow-moving waves of contraction – moves contents toward rectum
Defecation reflex – triggered by stretching of rectum walls
Feces are moved into anal canal
Message is sent to brain about use of external voluntary sphincter – brain can tell sphincter to close and the reflex is put on hold
Diarrhea – food is rushed through large intestine and there isn’t time for water to be absorbed – may result in dehydration
Constipation – food moves too slowly and too much water is absorbed