Lecture 22- The small intestine

Question 1

What does the small in small intestine refer to?

Answer 1

The diameter, it is just 3-4 cm

Question 2

What are the three main parts of the small intestine + the key function of each?

Answer 2

1. Duodenum: receiving chamber after stomach
2. Jejunum: most absorption takes place
3. Ileum: is the longest and empties into the large intestine

Question 3

What is the approximate total length of the small intestine?

Answer 3

6m

Question 4

Why is the small intestine coiled?

Answer 4

So we are not freakishly tall

Question 5

Where is the duodenum (1st segment of small intestine) located in terms of the peritoneal cavity? How does this differ from the other two segments?

Answer 5

• Duodenum is retroperitoneal (behind +outside fixed to posterior wall)
• Jejunum + Ileum need to move, they are therefore intraperitoneal

Question 6

What are the Jejunum and ileum anchored by? What does this do?

Answer 6

A mesentery (double layer of visceral peritoneum), prevents tube getting tangled.

Question 7

What does the mesentery contain?

Answer 7

• Arteries
• Veins
• Nerves
• Lymphatics

Question 8

What’s delivered to the small intestine?

Answer 8

Partially digested food material (chyme)

Question 9

What is required in the small intestine due to the acidity of chyme?

Answer 9

Need to…

Protect epithelium via

• Mucous secreting cells
• Glands in submucosa (specifically in duodenum)

Neutralize pH
-Bicarbonate (HCO3-) from pancreas duct cells

Question 10

What is the small intestine specialized for and what does this mean it needs?

Answer 10

• Absorption

- Large surface area

Question 11

What are the permanent large folds in the small intestine called? What is their structure?

Answer 11

• Plicae Circulares (Circular Folds)

- Core of submucosa (with overlying mucosa)

Question 12

Aside from increase surface area what is an additional function of the Plicae Circulares?

Answer 12

Slow passage of material through as causes it to

spiral around giving more time for digestion+ absorption

Question 13

What is found on the surface of each plicae circulares?

Answer 13

• The mucosa is folded to form finger like projections called villi
• These further increase surface area for absorption

Question 14

What does the Muscularis mucosae do in regards to the villi?

Answer 14

Contract to cause villi to move backwards and forwards and thus exposing them to more of the luminal contents

Question 15

What is the structure of villi?

Answer 15

• Core of each villi is made of FCT (lamina propria)

- Contains vessels

Question 16

What are the two types of vessels found in the villi and their respective functions?

Answer 16

• Lymph lacteal (products of fat digestion)

- Capillary network (products of protein and carbohydrate digestion e.g. amino acids, monosaccharides)

Question 17

Where does nutrient rich, deoxygenated from the small intestine go to?

Answer 17

• Absorbed into the mesenteric veins

- These drain to the hepatic portal vein (bring nutrients to the liver and detox blood)

Question 18

Where do lymph lacteals drain?

Answer 18

Into the cisterna chyli, thoracic duct and then left subclavian vein (reentering blood vascular space)

Question 19

What is a special feature of the epithelium in the small intestine?

Answer 19

• It is simple columnar

- Microvilli on the apical surface increase surface area further forming the brush border.

Question 20

What is found at the brush border giving it a fuzzy appearance?

Answer 20

Glycocalyx:
Network of Glycoproteins forming branched filaments
which tether enzymes to the brush border

Question 21

What function do the enzymes tethered to the brush border perform?

Answer 21

Contact digestion

Question 22

How does the plasma membrane of the small intestine act as a barrier for absorption?

Answer 22

• Is selectively permeable

- For things that can’t just pass through has protein channels which act as transporters

Question 23

What are the two pathways for absorption in the small intestine?

Answer 23

• Transcellular pathway= from lumen of small intestine across the epithelial cell
• Paracellular pathway= goes between the cells

Question 24

What dictates what can pass through the paracellular pathway for absorption?

Answer 24

• Tight junctions

- These only allow the passing through of very small molecules

Question 25

What are the 5 cell types found in the epithelium of the small intestine + basic functions?

Answer 25

• Enterocytes: Absorptive cells
• Goblet cells: Secrete mucous for protection
• Stem cells: Make all cell types by dividing and replacing cells further up
• Paneth cells: granules, antibacterial enzymes (lysozyme, defensins)
• Endocrine cells: secrete hormones into blood supply not the lumen

Question 26

What is the intestinal Crypt? What cell types does it contain?

Answer 26

• Little glands at base of the villi

- Contain the stem cells, Paneth cells

Question 27

What are the four ways that surface area for absorption is increased in the small intestine?

Answer 27

• 1: Long (~6m in length!)
• 2: Fold the submucosa – permanent folds = plicae circulares
• 3: Fold the mucosa (not the Muscularis mucosa) = villi
• 4. Fold the apical plasma membrane of the epithelium = microvilli