The Small Intestine

Question 1

What gets delivered to the small intestine?

Answer 1

acidic chyme

Protect epithelium:
- goblet cells on surface
- mucous secreting glands in the submucosa (especially duodenum)

Neutralise pH:
- bicarbonate (HCO3-) from pancreas

Enzyme (pancreas) and bile (liber) for digestion

Question 2

what are the rough lengths of the threes sections of the small intestine and what are their roles?

Answer 2

1. duodenum: ~25cm
   - duodenum is retroperitoneal
   - receives chyme from stomach
   - enzymes and bicarbonate from pancreas
   - bile from liver
2. Jejunum: ~2.5m
3. Ileum: ~3.5m
   - the jejunum and ileum are longer and therefore do most of the absorption and digestion

Total length = ~6m

Question 3

Describe the mesentery’s role in the small intestine

Answer 3

• small intestine (except duodenum) is held in place my the mesentery. this allows movement but prevents tangling (anchors small intestine to body wall)
• mesentery can also have things running through it that we want to keep safe, such as blood vessels, nerves, lymphatics and fat in its tissue. the fat protects the vessels and nerves and things.

Question 4

describe mesenteric blood vessels and lymphatic drainage

Answer 4

• mesentery contains arteries, veins, nerves and lymphatics
• nutrient rich, deoxygenated blood from small intestine enters mesenteric veins
• these drain into hepatic portal vein
• lymphatic lacteals ultimately drain products of fat digestion into cisterns chill, thoracic duct then left subclavian vein

Question 5

how does the small intestine carry out further digestion and then absorption?

Answer 5

Needs a HUGE surface area

Achieved in four ways:
- length
- folding of intestinal all (plicae circulares - permanent folds!)
- extensions of mucosa (villi)
- extensions of epithelial cells (microvilli)

Question 6

describe the plicae circulares

Answer 6

• permanent large folds of small intestine
• core of submucosa with overlying mucosa
• have villi in the mucosal layer of these structures

Note: only two layers of muscularis in this organ

Question 7

describe the features of villi

Answer 7

• numerous villi found on the surface of plicae circulares
• part of the mucosa
• muscularis mucosae allows villi movement (so they can touch the chyme more and have a better chance of absorbing stuff)

Core of each villus is made of lamina propria which contains:
- lymph lacteal: takes in products of fat digestion
- capillary network: takes in products of protein and carbohydrate digestion (can be AA’s and sugars)

Question 8

describe the epithelium of the small intestine

Answer 8

• epithelium of the small intestine is simple columnar
• the absorptive cells (enterocytes) have microvilli on apical surface (which form the brush border)
• the microvilli have a branched filament network called glycocalyx around it and it holds proteins who’s role is contact digestion

Glycocalyx:
- glycoproteins - branched filaments
- tether enzymes

Contact digestion: involves enzymes - attached to brush border
- these enzymes are the final step of digestion where they cleave the final bonds between disaccharides and amino acids before allowing them to absorb through the cell

Question 9

describe the junctions between epithelial cells

Answer 9

The plasma membrane is a barrier:
- selectively permeable
- has proteins
- channels
- transporters

Tight junctions (the only one of the four junctions we need to know about)
- high up in the apical portion of the cell, hold cells together, sometimes tight and sometimes loose
- allow small molecules to diffuse through

Question 10

Describe the different cells in the epithelium of the small intestine

Answer 10

• Enterocytes - absorptive cells (make up most of the cells on villi)
• Goblet cells - secrete mucous for protection and lubrication
• Paneth cells - granules, antibacterial enzymes
• Endocrine cells - secrete hormones
• Stem cells - make all cell types. as the cells further up get damaged, these cells make more cells which push upwards and replenish the old ones

In the intestinal crypt (the part of villi that dips down into the lamina propria), there are epithelial cells, stem cells and then paneth cells. They are located here for protection.

Question 11

describe the histology of the wall of the small intestine from lumen to muscle layers

Answer 11

• lumen
• mucosa (forming villi, that have microvilli on their enterocytes)
• muscularis mucosa (for movement of the villi)
• submucosa (has lots of glands to make mucous)
• two muscle layers (longitudinal and circular)

Question 12

describe the junction between the small and large intestine

Answer 12

• the valve that separates these two structures is called the ileocecal valve and it is a sphincter
• it regulates the passage of material into the large intestine