Session 15 - Digestive System 3 – Small Intestine & Large Intestine

Question 1

Review: Digestion of carbohydrates

Answer 1

Question 2

Review: protein digestion

Answer 2

Question 3

Review: Lipid digestion

Answer 3

oMouth: Lingual lipase (activated in stomach)

oStomach: Gastric lipase(has a limited role in the adult) (kids digesting fats in breast milk)

oSmall Intestine:

• Bile salts
• Pancreatic lipase
• No enzymes in brush border

oTriglycerides are broken down into long/short chain fatty acids and monoglyceridesfor absorption

Question 4

Digestion of Nucleic Acids (basically sugars joined together)

Answer 4

Question 5

How does absorption occur in the small intestine?

Answer 5

oAbsorption: The passage of the end products of digestion from the GI tract (the gut) into the blood or lymph

oOccurs by:

• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport

Question 6

Essentially all carbohydrates are absorbed as monosaccharides.

How are they absorbed into the epithelial cell and how do they move out of the epithelial cell into the bloodstream?

Answer 6

oAbsorption into epithelial cell

• Glucose and Galactose -sodium glucose symporter (active transport)
• Fructose -facilitated diffusion

oMovement out of epithelial cell into bloodstream

•by facilitated diffusion

Question 7

How does Absorption occur of Amino Acids and Dipeptides into epithelial cell (of small intestine) and how do they move out into blood?

Answer 7

oAbsorption into epithelial cell

•active transport with Na+ or H+ ions (symporters)

oMovement out of epithelial cell into blood

•diffusion

Question 8

Absorption of lipids

How are they absorbed into epithelial cell ?

How are they moved out?

Answer 8

oDietary lipids are all absorbed by simple diffusion.

oAbsorption into epithelial cell:

• Long-chain fatty acids and monoglycerides: absorbed as part of micelles
• Small fatty acids: simple diffusion

oMovement out of epithelial cell

• Long-chain fatty acids and monoglycerides: chylomicrons into the lacteal of a villus.
• Small fatty acids: simple diffusion into blood

Extra info ** They are absorbed as part of micelles - when the lipids are in the gut and they have been emulsified by the bile they form these little structures called micelles and when they move into the epithelial cell by simple diffusion, they then reassemble themselves into chylomicrons. That’s how they move out of the epithelial cell.

Question 9

How are electrolytes abosrbed by the small intestine?

Answer 9

There are specific process on the epithelial cells that are involved in absorbing electrolytes

electrolytes such as..

sodium

chloride, iodide and nitrate

Iron, magnesium, potassium and phosphate ions

Calcium

Are absorbed by diffusion and secondary active transport

Question 10

Define how vitamins such as fat soluble, water soluble and B12 are absorbed in the small intestine?

Answer 10

oFat-soluble vitamins (A, D, E, and K):

•travel in micelles and are absorbed by simple diffusion

oWater-soluble vitamins (B and C):

•absorbed by diffusion

oB12 combines with intrinsic factor before it is transported into the cells

•receptor mediated endocytosis

** intrinsic factor Produced in the stomach to help absorb B12 in the small intestine

Question 11

Explain water absorption in the small intestine

Answer 11

Absorption is by Osmosis

Osmosis from the lumen of the intestines through epithelial cells and into blood capillaries.

o Small intestine reabsorbs 8.3 liters

Question 12

Functions of the large intestine

Answer 12

• Haustral churning, peristalsis, and mass peristalsis drive contents of colon into rectum.
• Bacteria in large intestine convert proteins to amino acids, break down amino acids, and produce some B vitamins and vitamin K.
• Absorbing some water, ions, and vitamins.
• Forming faeces and defecating (emptying rectum).

Question 13

Anatomy of the large intestine - Name the divisions

Answer 13

Anatomical subdivisons :

• caecum
• Colon
• Rectum
• Anal canal

Appendix

Question 14

Mechanical digestion in the large intestine

Answer 14

oMechanical movements:

•Haustral churning - Slow contracting pockets that mixes the contents locally: Slow haustral contractions that occur about every 30 minutes and last approximately 1 minute.

•Peristalsis: If we want to move the contents along the large intestine - propels the contents along

3 to 12 contractions per minute

•Mass peristalsis: 3 or 4 times a day, during or immediately after a meal

Question 15

Chemical digestion in the large intestine

Answer 15

oBacterial fermentation:

• Undigested carbohydrates: into hydrogen, carbon dioxide and methane gas
• Flatulence
• Undigested proteins: into indole, skatole, hydrogen sulfide, and fatty acids
• Fecal odour and Urinary excretion
• Bilirubin (bile pigment): into simpler pigments-Stercobilin
• Fecal colour (brown poo)

•Vitamins K and B produced in the colon

Question 16

Feces is..

Answer 16

Solid or semisolid chyme

Question 17

Dietary fibers

Insoluble and soluble fibers and their benefits

Answer 17

oDietary fibers: indigestible plant carbohydrates

–cellulose, lignin, and pectin—found in fruits, vegetables, grains, and beans

–affect the speed of food passage through the GI tract

oInsoluble fiber: insoluble in water

–woody parts of plants (wheat bran, veggie skins)

–may help protect against colon cancer

o_Soluble fiber:_ soluble in water

–in beans, oats, barley, broccoli, prunes, apples, and citrus fruits.

–lowers blood cholesterol by preventing reabsorption of bile salts so liver has to use cholesterol to make more

Question 18

What are the three phases of digestion

Answer 18

oThree overlapping phases:

• Cephalic phase
• Gastric phase
• Intestinal phase

* They are not seperate, they overlap and can happen at the same time! *

Question 19

Explain the Cephalic phase of digestion

Answer 19

prepares the mouth and stomach for food that is about to be eaten.

oActivated by: The smell, sight, thought, or initial taste of food

oNeural centres: in the cerebral cortex, hypothalamus, and brain stem.

oThe facial and glossopharyngeal nerves:

•stimulate the salivary glands to secrete saliva

oVagus nerve: (stimulates the stomach)

ostimulate the gastric glands to secrete gastric juice

Question 20

Explain the gastric phase

Answer 20

Begins when food arrives in the stomach.

Promote gastric secretion and gastric motility

Two regulations:

• Neural control
• Hormonal control

Question 21

Explain neural control in the gastric phase of digestion

Answer 21

Food enters the stomach and disrupts PH of gastric juice and casues distension of the stomach (stretching of the stomach wall)

• Stretch receptors and chemoreceptors: in the wall of stomach detect this
• Nerve impulses are sent to Submucosal plexus

. Parasympatheitc nerve impulses are sent out releasing HCI

• In response = Vigorous peristalsis(mixing, churning and moving) and gastric juice secretions
• Gastric emptying

Question 22

Explain the role of hormonal control in the gastric phases of digestion

Answer 22

oGastrin: released from G cells secretion of gastrin into bloodstream

oStimuli:

• Distension of the stomach by chyme,
• Partially digested proteins in chyme,
• High pH of chyme due to the presence of food in the stomach*
• Acetylcholine released from parasympathetic neurons.

* Gastrin release inhibited when pH of gastric juice drops below 2.0

Question 23

The role of gastrin..

( gastrin is released as part of hormonal control in the gastric phase of digestion)

Answer 23

• Stimulates gastric glands to secrete large amounts of gastric juice.
• Strengthens the contraction of the lower oesophageal sphincter to prevent reflux of acid chyme into the oesophagus,
• Increases motility of the stomach, and relaxes the pyloric sphincter, which promotes gastric emptying.

Question 24

Explain the intestinal phase of digestion and how is it regulated ?

Answer 24

oThe intestinal phase: Begins when food enters the small intestine.

oSlow the exit of chyme from the stomach. Promote digestion of food in small intestine

oTwo regulations:

• Neural control
• Hormonal control

Question 25

Explain neural control of the intestinal phase

Answer 25

oStretch receptors, fatty acids or sugar signal Enteric NS and CNS (medulla)

o↑ Sympathetic nerves ↓Parasympathetic nerves: Slows stomach motility / emptying

oThe enterogastric reflex: stretch of duodenum causes inhibition of gastric motility and increases the contraction of the pyloric sphincter to decrease gastric emptying.

Question 26

Hormonal control of the intestinal phase of digestion releases what two hormones?

Answer 26

• Secretin:Stimulates the flow of pancreatic juice rich in bicarbonate, and inhibits the secretion of gastric juice.
• Cholecystokinin (CCK): decreases stomach emptying and stimulates the secretion of pancreatic juice rich in digestive enzymes, and increase the flow of bile.