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B1 > Physiology Of Intestines > Flashcards

Physiology Of Intestines Flashcards

1
Q

How is water secreted by the small intestine?

A

Intestinal epithelium secretes Na+ Cl- and HCO3-

H20 follows by osmosis

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2
Q

Where is mucous secreted from in the small intestine?

A

Brunners glands in the duodenal mucosa

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3
Q

What hormones does the small intestine secrete?

What do they do?

A

Secretin & CCK - inhibit gastric motility and secretion and control pancreatic/biliary secretion

Motilin - stimulates migrating motor complexes via ENS and ANS

VIP (vasoactive intestinal peptide) - increases blood flow to GIP tract

GIP (gastric inhibitory peptide) - inhibits gastric secretion and stimulates insulin secretion

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4
Q

The endocrine pancreas secretes insulin and glucagon. What does the exocrine pancreas secrete?

A

Ducts cells secrete alkali - isotonic solution rich in HCO3-

Acinar cells secrete digestive enzymes:
Trypsin
Chymotrypsin
Pancreatic amylase
Lipases
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5
Q

How are pancreatic enzymes activated?

A

Trypsinogen activates inactive enzymes (zymogens) by membrane bound molecule entereokinase.

Trypsin then made which can activate other enzymes in the intestinal lumen

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6
Q

Enzyme and alkali secretion by the pancreas are controlled separately, how?

A

Enzyme: stimulated by CCK and ACH (PNS Vagus)

Alkali: secretin and potentially by CCK and ACh

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7
Q

What molecules are digested in the lumen of the duodenum and proximal jejunum?
What enzymes are involved?

A

Fats > monoglyceride and fatty acids by pancreatic lipase

Starch > disaccharides > monosaccharides by pancreatic amylase and then brush border enzymes

Proteins > peptide fragments > amino acids by trypsin and chymotrypsin then carboxypeptidase and aminopeptidase

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8
Q

How are fats digested and absorbed in the small intestine?

A

Fats are emulsified by bile salts and phospholipids

Digested by pancreatic lipase and products (monoglyceride + fatty acids) held in micelles

Micelles diffuse into unstirred layer and fatty acids and monoglycerides diffuse into cell membrane - once in the cell they’re reassembled to fats

Triglyceride droplets packaged into chylomicrons and exported across basolateral membrane to lymph

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9
Q

How are carbohydrates digested/absorbed?

A

Broken down and transported across intestinal membrane via either SLGT1 (glucose/galactose) or GLUT5 (fructose)

Then across the basolateral membrane via facilitated diffusion, GLUT2

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10
Q

What regulates Ca2+ absorption?

A

Vitamin D

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11
Q

What are the two types of iron and how are they absorbed?

A

Haem - absorbed by receptor mediated endocytosis to release Fe3+ which is then reduced to Fe2+

Free Iron - reduced from Fe3+ to Fe2+ by duodenal cytochrome b ferric reductase in the brush border membrane and transported by DMT1

In both cases Fe2+ then binds to ferroportin 1 in basolateral membrane to be exported

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12
Q

How is Iron absorption regulated?

A

By demand: intracellular iron-ferritin pool LOS as cell is slouched in faeces

If iron is required by body then becomes absorbed into the blood across ferroportin 1 (receptors are unregulated)

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13
Q

How is vitamin B12 absorbed?

A

Binds to R protein in the stomach

Released from R protein and binds to intrinsic factor in the duodenum/jejunum

Vitamin B12-IF complex is absorbed by receptor mediated endocytosis in the terminal ileum

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14
Q

What can malabsorption arise from?

A 
Failure/deficiency in:
Pancreatic enzyme secretion
Brush border enzymes
Absorption
Bile secretion
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15
Q

Name some diseases of malabsorption and their site of pathology

A

Coeliac disease - duodenum/jejunum, fat absorption

Pancreatitis - pancreas, fat digestion

Crohn’s surgical resection - ileum, vitamin B12 and bile salt absorption

Lactase deficiency - small intestine, lactase hydrolysis

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16
Q

What are symptoms of malabsorption?

A 
Weight loss
Abdominal distension
Diarrhoea
Steatorrhoea
Pernicious anaemia
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17
Q

Outline lactose intolerance

A

In healthy individuals lactose is digested by lactase producing normal stools

In lactose intolerance there’s a lack of lactase enzyme so lactose is broken down by gut bacteria leading to fermentation producing gases, organic acids, osmotically active molecules and leading to bloating/increased motility so alter stools

18
Q

Outline what excessive vomiting or diarrhoea leads to with regards to acid-base balance in digestion

A

Vomiting: large loss of secreted acids = metabolic alkalosis

Diarrhoea: excessive loss of fluid and therefore NaHCO3 in faeces = metabolic acidosis

19
Q

What are the functions of the large intestine?

A

Extract Na+ and H20 from luminal contents
Make and store faeces
Move faeces towards the rectum

20
Q

What does the large intestine secrete and what is its role?

A

Alkaline mucous from goblet cells - protects epithelium from acid/abrasion/bacterial activity and provides adherent medium for holding faecal matter together

21
Q

How is water absorbed in the colon?

A

Na+/K+ ATPase pumps Na+ out across basolateral membrane

H20 follows transcellulary as gap junctions too tight

22
Q

What’s the role of the ileocaecal valve?

A

Separates terminal end of ileum from the caecum

One way valve/sphincter to prevent backflow of faecal contents from the colon and control the rate at which ideal chyme enter

23
Q

What’s the role of movements of the colon?

What are the two types of movement?

A

Facilitate efficient absorption of water and salts and permit orderly evacuation of faeces

Haustral contractions and mass movements

24
Q

Longitudinal muscle of the colon is aggregated into 3 bands cal taeniae coli - what is their role?

A

Combined contractions of taeniae coli and circular muscle layer cause colon to bulge into ovoid segments called haustrae

Haustral contractions mix contents to facilitate water absorption

25
Q

What are mass movements in the colon?

A

Propel luminal contents from beginning of transverse colon to sigmoid colon

Series of modified peristaltic events

Occur 1-3 times a day

26
Q

Outline the gastrocolic reflex and what is it mediated by?

A

Initiates mass movements in the colon to push contents into the rectum, triggering defecation reflex

Mediated by gastrin and autonomic nerves

27
Q

What is the defaecation reflex?

A

Reflex response to distension of rectal wall, mediated by mechanoreceptors:

Contraction of rectum
Internal anal sphincter relaxes
Initial contraction of external anal sphincter
Increased peristaltic activity in sigmoid colon
Relaxation of external anal sphincter
Expulsion of faeces

28
Q

What’s the valsalva manoeuvre?

A

Voluntary defaecation:
Full inspiration followed by forced expiration against closed glottis so diaphragm moves downwards
Abdominal and thoracic muscles contraction
Increased abdominal pressure forces faecal contents into rectum

29
Q

What’s the composition of faeces?

A 
3/4 water
1/4 solid matter:
Dead bacteria
Fat
Inorganic matter
Protein
Undirected materal - bile/sloughed cells
30
Q

Why is faeces brown?

A

Bilirubin derivatives

31
Q

What’s the role of anaerobic bacteria in the gut?

A 
Immunity against common bacteria
Converts bilirubin to urobilogens
Forms secondary bile acids
Degrade digestive enzymes
Digests mucous
Synthesises vitamin K

= all disrupted by oral antibiotics

32
Q

What do purgatives do?

A

Accelerate transit through the intestine and are used to alleviate constipation

33
Q 
Constipation:
Define
Causes
Treatments
Bristol stool chart?
What can be associated complications?
A

Infrequent/difficult evacuation of faeces (<3 times/week)

Diet, hormonal, side effect of medications or illness

Change in diet/exercise habits, purgatives

Types 1 & 2

Haemorrhoids & anal fissures

34
Q

Name the 4 classes of purgatives

A

Bulk laxatives
Osmotic laxatives
Faecal softeners
Stimulant purgatives

35
Q

What are the 4 types of diarrhoea?

A

Secretory - increased active secretion

Osmotic - loss of water

Motility related - abnormally high GI motility so reduced time for absorption

Inflammatory - damage to mucosal lining/brush border leading to passive loss of protein rich fluids

36
Q

What are the 2 ways diarrhoea is treated?

A

Maintenance of fluid/electrolyte balance (Na+ and K+)

Anti diarrhoea agents (anti motility)

37
Q

How are proteins digested/absorbed?

A

Proteins -> Polypeptides by pepsin
Polypeptides -> Di/Tripeptides by trypsin/chymotrypsin
Tripeptides -> Amino acids by aminopeptidases
Amino acids co-transported into cell with Na+ then diffuse across basolateral membrane into capillaries

38
Q

How are the exocrine and endocrine functions of the pancreas divided?

A

Exocrine (majority) by duct and acinar cells, secreting digestive enzymes and HCO3- (alkali)

Endocrine function by islets of langerhans: B cells secrete insulin, a cells secrete glucagon to regulate blood sugar levels

39
Q

Outline digestive enzyme secretion by the pancreas, including its stimulation

A

Acinar cells secrete zymogens (inactive enzymes: proteases, lipase, amylase) which are activated by trypsin (from trypsinogen) and enter the duodenum via the ampulla of vater

CCK and CN X stimulates zymogen secretion in response to taste, sight, smell of food or stomach stretching

40
Q

Outline alkaline secretion by the pancreas, including its stimulation

A

Inside pancreatic duct cells H2CO3 dislocates to H+ and HCO3-

H+ transported out of the cell into the blood by H+/Na+ antiporter
HCO3- exchanged via Cl- exchanger into intercalated ducts

B1 (19 decks)

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