DIGESTIVE SYSTEM - FUNCTIONS

Chapter 14

GENERAL FUNCTIONS

Ingestion 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

GENERAL FUNCTIONS CONT. Food breakdown: chemical digestion

Enzymes break food down into building blocksHydrolysis reactions – add water to break apart

bondsCarbs

Monosaccharides – glucose, fructose, galactose

Disaccharides – sucrose (sugar), lactose (milk)Polysaccharides – starch

Proteins – broken into amino acidsLipids – broken into fatty acids and glycerol

Absorption – movement of nutrients from lumen to blood

Defecation – removal of indigestible residue

ACTIVITIES OF MOUTH, PHARYNX, & ESOPHAGUS

Mastication - chewing – mechanical digestion

Mixing w/ saliva – begins starch digestionContinuously released –

more released when food enters

Deglutition – swallowing Buccal phase – voluntary

Bolus is forced into pharynx by tongue

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

ACTIVITIES OF STOMACH

Food breakdownGastrin – triggers stomach glands to

release pepsinogens, mucus, and HCl acid2-3 liters of gastric juices/dayHydrochloric acid – changes pepsinogen

into pepsin to break down proteins In infants – also have rennin to break down

proteins in milk – conversion resembles sour milk

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

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)

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

SMALL INTESTINE CONT. Pancreatic juice

Pancreatic amylase – completes starch digestion

Trypsin, chymotrypsin, carboxypeptidase – help complete protein digestion

Lipases – lipid digestionNucleases – 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

ACTIVITIES OF LARGE INTESTINE 12-24 hours Food breakdown

Bacteria breakdown few remaining nutrientsProduce 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

LARGE INTESTINE CONT. Propulsion

Peristalsis – doesn’t work as wellMass movements – long, slow-moving waves of

contraction – moves contents toward rectumDefecation reflex – triggered by stretching of

rectum wallsFeces 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