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GI lectures > Physiology of Digestion and Absorption > Flashcards

Physiology of Digestion and Absorption Flashcards

1
Q

what are the respective lengths of the 3 parts of the small intestine?

A

duodenum = 30cm
jejunum = 3.5m
ileum = 2.5m
(6m in total)

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2
Q

what 3 substances does the small intestine receive?

A

chyme from stomach (via pyloric sphincter)
pancreatic juice from pancreas (via sphincter of oddi)
from gall bladder (via sphinter of oddi)

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3
Q

what is succus tericus?

A

intestinal juice secreted by small intestine (2L per day)

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4
Q

how does the small intestine move residues to the large intestine?

A

via the ileocaecal valve which opens in response to proximal pressure and gastrin

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5
Q

what 3 features of the small intestine increase its surface area and therefore absorptive ability?

A

circular folds of mucosa (folds of kerckring) increases surface area 3X
villi increase it 30X
microvilli increase it 600X

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6
Q

what are the 7 hormones secreted by the endocrine cells of the small intestine mucosa?

A

GLP-1 (incretin from L cells)
Gastrin (from G cells of gastric antrum and duodenum)
CCK (from I cells of duodenum/jejunum)
Secretin (from S cells of duodenum)
Motilin (from M cells of duodenum/jejunum)
GIP (incretin from K cells of duodenum/jejunum)
Ghrelin (from Gr cells of gastric antrum, small intestine etc)

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7
Q

How do the hormonal secretions of the small intestine work?

A

All act on G-protein coupled receptors
Incretins act upon the ß-cells of the pancreas in essentially feed-forward manner to stimulate the release of insulin (stimulate insulin production before insulin reaches blood stream)

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8
Q

what 2 mechanisms control the secretion of succus entericus?

A

feeding and all activity associated with it (gastrin, secretin, CCK, parasympathetic activity) enhance secretion
fasting and all activity associated with it (sympathetic nerve activity) inhibit secretion

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9
Q

what substances are included in succus entericus?

A

Mucus = protection and lubrication (produced by goblet)
Aqueous salts = enzymatic digestion (from crypts of Leiberkuhn)
NOT DIGESTIVE ENZYMES but they are present

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10
Q

what are crypts of Leiberkuhn?

A

grooves in between microvilli

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11
Q

How does secretion from the crypts of leiberkuhn occur?

A

Na/K ATPase at basal membrane provides electrochemical gradient for Na to leave the cell
Na leaving helps Na, 2Cl and K to enter cell via co-transporter
Na recycled, Cl leaves cell via CFTR
This creates a gradient that allows Na and H2O to move paracellular into lumen

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12
Q

what are the 2 methods of mixing and propulsion of chyme?

A

segmentation (mixing)

Peristalsis

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13
Q

describe the process of segmentation

A

Alternating contraction and relaxation of segments of circular muscle
Initiated by small intestine pacemaker cells causing the BER which is continuous. At threshold activates segmentation which in the duodenum is primarily due to distension by entering chyme
Vigorous after meal and barely there when fasting
Tkes 3-5 hours

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14
Q

how frequent are contractions in segmentation?

A

12 per min in duodenum
9 per min in ileum
net movement is therefore aboral

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15
Q

what are the 2 patterns of peristalsis?

A

1) a few localised contractions

2) the migrating motor complex (MMC)

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16
Q

what is the action of the MMC?

A

strong peristaltic contraction between meals passing from stomach to the ileocaecal valve clearing intestine of debris, mucus and dead epithelial cells

17
Q

what triggers and inhibits the MMC?

A

triggered by motolin

inhibited by feeding (gastrin, CCK, vagal activity)

18
Q

what are the endocrine and exocrine secretions of the pancreas?

A 
endocrine = insulin and glucagon - secreted into blood
Exocrine = digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) – secreted to the duodenum collectively as pancreatic juice
19
Q

What do pancreatic duct cells secrete and how much? What is its function?

A

Duct cells secrete 1-2 litres of alkaline (HCO3 rich) fluid into duodenum per day.
Neutralises acidic chyme entering duodenum, provides optimum pH for pancreatic enzyme, protects mucosa from damage by acid

20
Q

how is bicarbonate produced in pancreatic duct cells?

A

Chloride secreted at apical membrane where it is recycled by antiporter and it is pulled back in as bicarbonate is pushed out
Water and sodium moves into duct

21
Q

what are the 3 pancreatic enzymes?

A

Protease
Lipase
Amylase

22
Q

what triggers the release of pancreatic enzymes?

A

rise in intracellular calcium

23
Q

Of the 3 pancreatic enzymes, only proteases are secreted in an inactive form, true or false?

A

true
They only become activated when they reach the duodenum as activated ones in the granules would digest the pancreas
Enterokinase converts trypsinogen to trypsin which converts the other 2 proteases
Other 2 are secreted as active

24
Q

What are the 3 phases of pancreatic secretion?

A

cephalic (vagal stimulation of acinar cells)
Gastric (parasympathetic stimulation of acinar and duct cells as a result of vagovagal reflex of gastric distension)
Intestinal (Most important)

25
Q

what are the 2 feedback systems of intestinal phase of pancreatic secretion?

A

1) acid in duodenum > increased secretin released from S cells > carried in blood to pancreatic duct cells > secretion of NaHCO3 into duodenum > neutralises acid
2)Fat/protein in duodenum > CCK released from I cells > carried in blood to pancreas acinar cells > secretion of digestive enzymes into duodenum > digests fat/protein
Cycles turn themselves off until more acid/fat and protein enters duodenum

26
Q

what are the 3 main constituents of food?

A

carbohydrates
Lipids
Protein

27
Q

what is digestion?

A

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

28
Q

what are the 2 types of digestion that occur in the small intestine?

A

Luminal digestion – mediated by pancreatic enzymes secreted into the duodenum
Membrane digestion – mediated by enzymes situated at the brush border of epithelial cells

29
Q

what is the definition of absorption?

A

the 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)

30
Q

what is the overall process of digestion and absorption known as?

A

assimilation

31
Q

How are carbohydrates, peptides and triglycerides absorbed across the membranes?

A 
Carbohydrates = broken down outside cell into monomers (glucose)
Peptides = absorbed whole and broken down inside cell
Triglycerides = broken down outside cell, then put back together inside cell

GI lectures (75 decks)

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