L6: functions and secretions of the small intestine

Question 1

Functions of the SI

Answer 1

• site for most digestion and absorption takes place
• absorption of 95% water
• absorption of fat, carbs, proteins, ions etc
• moisten chyme arriving from stomach
• maintain digestive material in solution
• mixes food with pancreatic and biliary secretions

Question 2

Regions of the SI and their general function

Answer 2

Duodenum (12in): secretes bicarbonate rich mucus; bile/pancreatic juices enter via ampulla of Vater; continues process of chemical digestion
Jejunum (4-7ft): secretes additional enzymes and completes chemical breakdown of food.
Ileum (7-15ft): absorption; bile absorption too
Mesentery: blood to and from SI and neuronal supply

Question 3

Villi and microvilli

Answer 3

Finger-like projections are villi and microvilli (brushed border)
The many folds increase the SA for absorption
Enterocytes line the villi
Central capillary between enterocytes in the villi

Question 4

What is absorption?

Answer 4

The movement of food from the lumen to enterocytes into the central capillary

Question 5

How is food absorbed by the enterocytes?

Answer 5

After crossing the epithelial cells or enterocytes lining the villi, virtually all foodstuffs diffuse into the central capillary network
These drain into bigger blood vessels and are transported around the body
Except fats which enter the central lacteal

Question 6

What anchors the SI to the abdominal cavity?

Answer 6

Double layer of peritoneum anchors the SI to the back of the abdominal cavity, it contains and protects the blood vessels, lymph vessels and nerves feeding and draining the SI

Question 7

Why does the SI not get autodigested?

Answer 7

Auto-digestion of SI is prevented by:

• compartmentalisation of digestive enzymes in lumen
• tight junctions between epithelial cells
• mucus prevents access to epithelial cells

Ischaemic gut leads to auto-digestion since a lack of blood and oxygen would make the structure of the gut fall apart, and digestive enzymes would enter the bloodstream.

Question 8

Secretions of the SI

Answer 8

Water - lots secreted but almost all reabsorbed
Alkaline solution (bicarbonate, sodium, chloride)
Mucus
Digestive enzymes - success entericus
Hormones - CCK, secretin

Question 9

What do the Brunner’s glands do?

Answer 9

Duodenal Brunner’s glands serve to secrete bicarbonate:

• protects duodenum from acidic content of gastric chyme
• provide an alkaline condition for intestinal enzymes to be active
• lubricate intestinal walls

Question 10

What do crypt cells do?

Answer 10

Secrete chloride ions into the SI/gut lumen by the CFTR channel
Sodium ions secreted into lumen from crypts
Water follows by osmosis

So crypt cells allow water to be secreted into the gut lumen - 2L water/day to keep the intestines lubricated and liquefy chyme.

Question 11

How else is water drawn into the SI/gut lumen? (not crypt cells)

Answer 11

When the bigger structures of indigestion protein and starch first enter the SI, they have a lower osmolarity.
After enzymatic digestion, they get broken down into e.g. peptides and maltose, which have a higher osmolarity due to more molecules. This draws water into the intestinal lumen from nearby blood vessels,
Then when the nutrients are absorbed, water follows them out of the gut lumen and is reabsorbed.

Question 12

What hormones are secreted from the SI?

Answer 12

Secretin and CCK released by duodenum (discussed in pancreas cards)