Digestion and Absorption

Question 1

What is the role of the small intestine?

Answer 1

Receives
- chyme from stomach (via the pyloric sphincter)
- pancreatic juice from pancreas (via the sphincter of Oddi)
- bile from gall bladder (via the sphincter of Oddi)
Secretes intestinal juice (succus entericus)

Moves remaining residues to the large intestine via the ileocaecal valve (opens in response to proximal pressure and in response to gastrin)

Question 2

Where does most absorption in the small intestine occur?

Answer 2

duodenum

Question 3

What increases the surface area of the small intestine?

Answer 3

circular folds (of Kerckring) - 3 fold
villi - 30 fold
microvilli (the brush border) - 600 fold

Question 4

Which peptide hormones does the small intestine secrete into the blood?

Answer 4

Gastrin – from G cells of gastric antrum (mainly) and duodenum
Cholecystokinin (CCK) – from I cells of duodenum and jejunum
Secretin – from S cells of duodenum
Motilin – from M cells of duodenum and jejunum
Glucagon-like insulinotropic peptide (GIP, aka gastric inhibitory peptide) – an incretin from K cells of duodenum and jejunum
Glucagon-like peptide-1 (GLP-1) – an incretin* from L cells of the gut
Ghrelin – from Gr cells of the gastric antrum, small intestine and elsewhere (e.g. pancreas)

Question 5

What is the role of the peptide hormones released by the small intestine?

Answer 5

act on G-protein coupled receptors

Question 6

What are secretions of succus enttericus initiated by?

Answer 6

feeding

Question 7

What do small intestine secretions include?

Answer 7

mucus – for protection/lubrication (from goblet cells)
aqueous salt - for enzymatic digestion (mostly from the crypts of Lieberkühn)
no digestive enzymes

Question 8

What is released from the secretory cell?

Answer 8

Chlorine

Question 9

Why do Cystic Fibrosis patients also have problems with the secretion of enzymes and electrolytes in the GI tract?

Answer 9

The CFTR is used to secrete chlorine and in CF this is damaged

Question 10

What is segmentation?

Answer 10

chopping” moves chyme back and forth – very vigorous after a meal
Alternating contraction and relaxation of segments of circular muscle
Initiated by small intestine pacemaker cells causing the BER which is continuous
Segmentation in the empty ileum is triggered by gastrin from the stomach (gastroileal reflex)

Question 11

What is the rate of segmentation in the duodeneum and ileum?

Answer 11

12 per min in duodenum

9 per min in the ileum

Question 12

Describe Peristalsis?

Answer 12

A few localised waves of contractions

Involves the migrating motor complex

Question 13

Describe the Migrating Motor Complex

Answer 13

occurs between meals every 90 – 120 min
strong peristaltic contraction passing length of the intestine (stomach ileocaecal valve)
clears small intestine of debris, mucus and sloughed epithelial cells between meals – ‘housekeeper function’
inhibited by feeding and vagal activity
triggered by motilin, suppressed by gastrin and CCK

Question 14

What are the exocrine and endocrine pancreatic secretions?

Answer 14

Endocrine – insulin and glucagon – secreted to blood
Exocrine – digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) – secreted to the duodenum collectively as pancreatic juice

Question 15

What neutralises acidic chyme when it enters the duodenum?

Answer 15

Bicarbonate

Question 16

What is the reason for chyme being neutralised?

Answer 16

provides optimum pH for pancreatic enzyme function

protects the mucosa from erosion by acid

Question 17

What are the 3 major types of digestive enzyme?

Answer 17

Proteases
Amylases
Lipasases

Question 18

When do proteases become activated?

Answer 18

when they reach the duodenum

Question 19

What are the 3 phases of pancreatic secretion?

Answer 19

Cephalic – mediated by the vagal stimulation of mainly the acinar cells (20% total secretion)
Gastric – gastric distension evokes a vagovagal reflex resulting in parasympathetic stimulation of acinar and duct cells (5-10% total secretion)
Intestinal - neutralises acid and digests fat and proteins in the duodenal lumen (70-80% of total secretion)

Question 20

Define digestion?

Answer 20

enzymatic conversion of complex dietary substances to a form that can be absorbed

Question 21

What is luminal digestion mediated by?

Answer 21

by pancreatic enzymes secreted into the duodenum

Question 22

What is membrane digestion mediated by?

Answer 22

by enzymes situated at the brush border of epithelial cells

Question 23

What is assimilation?

Answer 23

Overall process of digestion and absorption

Question 24

Define absorption?

Answer 24

processes by which the absorbable products of digestion are transferred across both the apical and basolateral membranes of enterocytes (absorptive cells of the intestinal epithelium)