Lecture 22: The Small Intestine

Question 1

What are the 3 main regions of the small intestine?

Answer 1

• duodenum (25cm)
• jejunum (2.5m)
• ileum (3.6m)

Question 2

what is the position of the small intestine?

Answer 2

• initial segment (duodenum) is retroperitoneal
• jejunum and ileum are intraperitoneal and are held in place by mesentery. the mesentery allows movement, but prevents getting tangled

Question 3

what does the mesentery contain?

Answer 3

• arteries
• veins
• nerves
• lymphatics

Question 4

what is delivered to the small intestine? what is done with it?

Answer 4

acidic chyme is delivered.

because it is acidic the epithelium needs to be protected and neutralise pH.

• mucus secreting cells and glands in submucosa of duodenum protect epithelium
• bicarbonate from the pancreas neutralises pH

Question 5

what are the functions of the small intestine?

Answer 5

• needs to carry out further digeston (mechanical digestion is complete by now) and then absorption

Question 6

what structures are needed to carry out the functions of the small intestine?

Answer 6

• needs large surface area for absorption

Question 7

how is the small intestine submucosa modified for absorption?

Answer 7

Plicae circulares increase surface area
- permanent folds of the small intestine
- core of submucosa with overlying mucosa
- mucosa has villi to further increase surface area

Question 8

where are villi? how do they function?

Answer 8

• numerous villi found on surface of plicae circulares to increase surface area
• muscularis mucosae allows villi to wiggle back and forth

Question 9

what is the structure of villi?

Answer 9

• core of each villi is made of FCT (lamina propria)
• contains lymph lacteal (product of fat digestion)
• capillary network (products of protein and carbohydrate digestion)

Question 10

what are the blood vessels and lymphatic drainage of the small intestine?

Answer 10

• nutrient rich deoxygenated blood is absorbed into the small intestine and enters into mesenteric veins
• these drain into the hepatic portal vein
• lymph lacteal drain into cisterna chyli, thoracic duct and left subclavian vein

Question 11

what structures increase the surface area of the small intestine?

Answer 11

• length (6m)
• permanent folds of the submucosa - plicae circulares
• folds of the mucosa - villi
• folds of apical plasma membrane of epithelium - microvilli

Question 12

what is the epithelium of the small intestine?

Answer 12

simple columnar

Question 13

what is on the apical surface of the simple columnar epithelium?

Answer 13

microvilli to further increase surface area

Question 14

what are features of the microvilli?

Answer 14

has glycocalyx:
- glycoproteins (branched filaments) that tether enzymes

involved in contact digestion:
- needs enzymes attached to the brush border (brush border enzymes)

Question 15

what are the junctions between epithelial cells of the small intestine?

Answer 15

plasma membrane
- selectively permeable barrier
- has protein channels and transporters

tight junctions
- very tight
- small molecules can diffuse through

Question 16

what are the different types of epithelial cells of the small intestine and their function?

Answer 16

enterocytes - absorptive cells
goblet cells - secrete mucus
stem cells - make all cell types
paneth cells - granules, antibacterial enzymes
endocrine cells - secrete hormones