9.5 Digestion in the Small and Large Intestines

Pages 412-420

Small intestine

• Site of digestion of carbohydrates, lipids and protein

• Food passes through in 3-5 hours

Dimensions of Small Intestine

• Diameter 2.5 cm• 7 m long

Sections

Duodenum

• First section• 25-30 cm• Digestive enzymes added• Acid is neutralized

Jejunum

• Digestion and absorption

ileum

• absorption

Villi and microvilli

• Finger-like projections increase the surface area for absorption

Where the food goes

• Sugars, amino acids, water-soluble vitamins and minerals go into capillaries

• Fats go into lacteal lymphatic system bloodstream

Chemical Digestion

• Pyloric sphincter regulates movement of chyme into the small intestine

Pancreas

• Produces digestive enzymes which pass through pancreatic duct.

Ex. more amylase for carbs

• Produces insulin

Cholesystokinin, CCK, from duodenum lining prompts pancreas to secrete

Secretin

Prosecretin secretinacid

Functions - stimulates liver to secrete bile- stimulates pancreas to secrete enzymes-stimulates pancreas to secrete HCO3

-

- change in pH inactivates pepsin-regulates rate

trypsinogen

enterokinasetrypsinogen trypsin

Further digests long-chain protein into short-chain protein

Enzmes: carboxypeptidase and erepsin break protein segments down into amino acids

Fats

• Too big for enzyme lipase• Bile emulsifies fats…micelles

Liver

• Liver detoxifies blood,• Glycogen manufacture and storage, vitamins• Produces bile

Absorption

• Simple Diffusion• Fascilitated Diffusion• Osmosis• Active Transport

Large intestine

• 1.5 m in length• 7.6 cm diameterCecum, colon, rectum and anusColon: ascendingtransversedescending sigmoid

Rectum: last 20 cm

Functions of large intestine

• Absorbs water (20L per day)• Vitamins B and K, and Na+ and Cl-

• 4-72 hours• 500 species of bacteria (gas)

Cellulose and Fibre

• Bulk• Full-feeling lasts longer

Anus

• Defecation reflex-eliminated feces• 1st sphincter, the internal anal sphincter is a

smooth muscle (involuntary)• 2nd sphincter, the external anal sphincter is

made of skeletal muscle and is voluntary• Both muscles must be relaxed for defecation

to happen

Endoscopy

• Light, lens and camera• Through anus, mouth or opening in the

abdomen• Diagnosis: biopsy• Treatment: surgery