Gastrointestinal Tract 6 - Small Intestine

Question 1

Small intestine divided into three sections

Answer 1

• Duodenum (closest to stomach) - 30cm
• Jejunum-~1 m
• Ileum-~1.7 m (closest to large intestine)

Question 2

Does the small intestine do absorption AND digestion?

Answer 2

YES

Question 3

Duodenum functions

Answer 3

• Mixing of pancreatic digestive enzymes and bile with food
• Absorption of nutrients, iron and calcium.
• Release of secretin and CCK

Question 4

Jejunum function

Answer 4

• Digestion and absorption

Question 5

Ileum function

Answer 5

• Digestion and absorption

- Bile acids, vitamin B12

Question 6

How is SA increased in the small intestine?

Answer 6

By Folds of Kerckring/ Circular folds

Question 7

Structure of a villus in the small intestine contain:

Answer 7

• cells enterocyte (absorptive cell)
• goblet cell (mucus)
• endocrine cell
• paneth cell

Question 8

Potential cell types derived from stem cells

Answer 8

• Absorptive cells: absorption, brush-border enzymes
• goblet cell: secrete mucus
• endocrine cell: release hormones
• paneth cell: secrete antibacterial proteins

Question 9

Brush border

Answer 9

small projections (microvilli) of epithelial cells covering the villi of the small intestine, major absorptive surface of the small intestine

Question 10

Brush border enzyme

Answer 10

enzyme anchored to the brush border with catalytic activity in the lumen

Question 11

Brush border enzymes are Important for breaking down

Answer 11

carbohydrates and peptides into sugars and amino acids prior to transport across the enterocyte

Question 12

Digestion of carbohydrates

Answer 12

• Starch (amylose and amylopectin) is broken down into maltose, maltotriose, and α-limit dextrins by salivary and pancreatic amylases
• These are broken down to the monosaccharide glucose by brush border enzymes

Question 13

Other sugars broken down by brush border enzymes

Answer 13

• Sucrose broken by sucrase into (glucose + fructose)

- lactose broken by lactase into
glucose +galactose

Question 14

Intestinal absorption of glucose/galactose

Answer 14

Glucose/galactose from intestinal lumen goes into epithelial cells (low Na+, high K+) via transport protein (SGLT) and leaves through another one (GLUT2) into the blood (high Na+, low K+)

Question 15

Intestinal absorption of fructose

Answer 15

Fructose from intestinal lumen goes into epithelial cells (low Na+, high K+) via transport protein (GLUT5) and leaves through another one (GLUT2) into the blood (high Na+, low K+)

Question 16

When lactose cannot be completely digested:

Answer 16

Results in decreased water absorption in the gut (presence of lactose produces an osmotic gradient)

Lactose containing fluid passes on to large intestine and bacteria digest lactose
- Gas (colon distension and pain), diarrhea

Question 17

Proteins are broken down by

Answer 17

pepsin in the stomach, and in the small intestine by pancreatic proteases (major ones are trypsin and chymotrypsin)

Question 18

Once proteins are broken down in the stomach or small intestine, they are then further broken down by:

Answer 18

• Carboxypeptidase (pancreatic protease)
• Aminopeptidase (brush border enzyme in the small intestine)
• Other brush border enzymes (>20 different peptidases)

Question 19

Once digested, free amino acids are absorbed by

Answer 19

secondary active transport coupled to Na+ (multiple transport proteins)

Question 20

Small peptides can also be absorbed by different secondary active transport proteins coupled to H+, after …

Answer 20

Peptidases within the cytosol then hydrolyze peptides to amino acids

Question 21

After the Peptidases hydrolyze peptides to amino acids, the amino acids then undergo

Answer 21

facilitated diffusion across the basolateral surface of the enterocyte

• different facilities transporters between the two surfaces, each for a specific amino acid.

Question 22

Products of lipase are incorporated into

Answer 22

micelles which are in a dynamic state- breaking down and reforming

Question 23

As micelles breakdown they will release

Answer 23

fatty acids and monoglycerides that can then diffuse across the small intestine epithelium

Question 24

Once fatty acids and monoglycerides are absorbed by the enterocyte, they are

Answer 24

Processed by the ER back into triglycerides.

- This maintains diffusion gradient

Question 25

Within the ER triglycerides aggregate into

Answer 25

lipid droplets coated with amphipathic proteins

Question 26

The lipid droplets are packaged into the ______ and secreted via exocytosis

Answer 26

Golgi

Question 27

Extracellular fat droplets secreted are known as

Answer 27

“chylomicrons”

Question 28

Chylomicron contains

Answer 28

triglycerides, phospholipids, fat soluble vitamins and cholesterol

Question 29

The large chylomicrons enter into the

Answer 29

``` lymphatic system (rather than the capillaries in the intestine - Lacteals are leakier ``` Lymphatics eventually enter into the systemic circulation through the thoracic duct

Question 30

Lipoprotein lipase on endothelial cells of blood vessels release

Answer 30

triglycerides from chylomicrons as monoglycerides and free fatty acids which can be taken up by tissues

Question 31

Iron (as Fe2+) is actively transported into the enterocyte and incorporated into the protein

Answer 31

ferritin, a protein iron complex that acts as a storage form of iron

Question 32

Iron that is not stored is released on the blood side of the enterocyte and transported through the blood attached to the plasma protein

Answer 32

transferrin

Question 33

Iron that remains in the enterocyte bound to ferritin is excreted from the body when the enterocytes

Answer 33

slough off of the villi tips

Question 34

When iron stores are ample in the body expression of ferritin protein is

Answer 34

upregulated resulting in a reduced absorption of iron

Question 35

When iron stores are depleted the production of

Answer 35

intestinal ferritin decreases resulting in increased absorption

Question 36

Is there a mechanism for the excretion of iron, if so what’s it called?

Answer 36

NO, it accumulates in the tissue = toxicity = skin pigmentation and hear failure.

Question 37

Causes for iron accumulating in tissue

Answer 37

- Genetic defects in absorption control pathways - Adult males/post-menopausal women excessively supplementing - Poisoning-often children accidentally consuming supplements

Question 38

Iron-deficiency anemia

Answer 38

- Reduced number and/or size of red blood cells - Symptoms: tiredness, light-headedness, headaches, - Causes: not enough iron in diet, iron loss via blood loss, poor iron absorption, intestinal disease

Question 39

Water and electrolyte absorption and | secretion in the intestine is critical for GI function

Answer 39

- Permits contact between food and digestive enzymes - Diffusion of digested nutrients to absorption site - Fluidity provides for transit without damage to the epithelium

Question 40

Intestine handles large quantities of fluids each day

Answer 40

- Some from diet - Majority from intestine and organs that drain into the intestine (e.g., liver and pancreas) - Majority is “reclaimed” or reabsorbed by the small intestine

Question 41

How much fluid is handled by the GI tract each day ?

Answer 41

- 8-9 L - Majority is reabsorbed in the small intestine. - 100 ml lost in feces.

Question 42

Villi does what and crypts do what?

Answer 42

absorption at the villi, secretion from the crypts

Question 43

Intestinal epithelium establishes an osmotic gradient and water

Answer 43

follows through tight junctions (paracellular transport).

Question 44

Na+, Cl-, HCO3- are important electrolytes for

Answer 44

water transport

Question 45

Absorption of water in the small intestine Predominantly depends on

Answer 45

Na+ gradients generated during secondary active nutrient uptake (e.g., glucose and amino acids)

Question 46

Secretion of water in the small intestine Predominantly depends on

Answer 46

Cl- gradients generated by secondary active Na+/K+/2Cl- transporter (NKCC1)

Question 47

Cholera Occurs after eating food or drinking water contaminated with

Answer 47

Vibrio cholerae bacteria, which produces a toxin that INCREASES cAMP in crypts of small intestine = activation of the Cl- channel and excessive secretion of Cl- into the gut lumen, water follows (diarrhea)

Question 48

Treatment to cholera

Answer 48

- Consumption of clean water containing salt and glucose (fluid replacement) - Intravenous fluids

Question 49

Most common motion in small intestine during digestion is

Answer 49

“segmentation”

Question 50

Segmentation is continuous

Answer 50

division and subdivision of intestinal contents - Mechanical breakdown of food (increases surface area) - Mixing of food with digestive enzymes

Question 51

During segmentation, Frequency of contraction set by

Answer 51

basic electrical rhythm - 12 contractions/min in duodenum - 9 contractions/min in ileum

Question 52

Contraction force during segmentation is determined by

Answer 52

neurohormonal input - Slow net migration towards the large intestine - Allows digestion and absorption of food

Question 53

Segmentation occurs in the small intestine

Answer 53

during digestion

Question 54

After digestion, segmentation stops and is replaced by

Answer 54

a pattern of peristaltic activity called the migrating myoelectric complex (MMC)

Question 55

MMC Begins in the

Answer 55

lower portion of the stomach travels about 2 feet along the small intestine and then dies out, next wave starts further down SI, this is repeated (takes -2hrs, then repeats)

Question 56

Purpose of MMC

Answer 56

- Pushes any undigested material from the small to the large intestine - Prevents bacteria from remaining in the small intestine

Question 57

Regulation of MMC

Answer 57

- Intestinal hormone motilin (released by cells in the small intestine) thought to initiate MMC - Feeding inhibits the release of motilin - MMC ceases when next meal is consumed