3.5 Digestion in the Small and Large Intestines

Pages 185-188

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• Secretions from the liver and pancreas enter

the duodenum through a common duct

Jejunum

• Digestion and absorption

ileum

• absorption

Chemical Digestion

• Pyloric sphincter regulates movement of chyme into the small intestine

Chyme

• Water• Food• Digestive enzymes

Pancreas

Produces:1)digestive enzymes which pass through pancreatic duct.• Lipase fat glycerol and fatty acids• pancreatic amylase• Trypsin2) insulin

Pancreas

• Produces sodium bicarbonate which increases the pH of chyme from pH 2.5 to pH 9.

Small Intestine

• Secretes maltase maltoseglucose

• Peptidases, in the small intestine, complete the digestion of protein

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

• Mass is 1.5 kg• Liver detoxifies blood,• Glycogen manufacture and storage, vitamins• Produces bile

Bile

• Bile contains bile salts that help to emulsify fats. Large fat molecules globules are broken down into tiny globules.

• Bile is made in the liver and stored in the gall bladder

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

Large intestine

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

descending 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