4. Small intestine Flashcards

1
Q

What separates the small intestine from the stomach and large intestine?

A
  • Stomach - pyloric sphincter

* Large intestine - ileocecal valve

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2
Q

What are the 3 parts of the small intestine?

A
  • Duodenum - (0.25m) digestion and gut regulation
  • Jejunum - (2.5m) absorption
  • Ileum - (3.75m) absorption
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3
Q

Outline the structure of the lining of the small intestine

A
  • Epithelial layers - villi to increase SA, motile and rich blood supply/lymphoid drainage
  • Crypts of Lieberkühn
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4
Q

What 5 cell types are the mucosa made up of?

A
  • Enterocytes
  • Goblet cells
  • Enter-endocrine cells
  • Paneth cells
  • Stem cells
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5
Q

Describe the Enterocytes

A

• Most abundant
• Absorption
• Columnar epithelia with 2000 microvilli on apical surface (brush border)
• Nucleus and mitochondria at basolateral membrane
• RER at apical region
• Surface has a rich carbohydrate containing glycocalyx
- helps trap water, mucus and enzymes for protection
- regulates digestion and absorption
• Connected by tight junctions
• Life span = 1-6 days

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6
Q

Describe the Goblet cells

A

• 2nd most abundant
• Apical region - mucin granules that distort cell shape (goblet)
• Mucus = water + glycoproteins
- lubricant to facilitate gut passage
• Number increases down the lumen due to water being absorbed (dehydration)
• Life span < 36 hours

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7
Q

Describe the Enter-endocrine cells

A
• Hormone secreting epithelial cells
• Bottom of crypts
• Columnar
• Sensory apparatus in apical portion
• Hormones kept near basolateral membrane for secretion into blood
e.g.
- G-cells: gastrin
- I-cells: cholecystokinin 
- S-cells: selectin
- D-cells: somatostatin
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8
Q

Describe the Paneth cells

A

• Immunological cells
• Bottom of crypts
• Lots of acidophilic granules with:
- lysozyme
- glycoproteins - protect local cells from enzymes
- zinc - lysozyme cofactor
• Located near stem cells to protect progenitor cells
• Engulf bacteria and regulate intestinal flora

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9
Q

Describe the stem cells

A
  • Pluripotent
  • Proliferate in crypts and move up villus to the tip
  • Continuous escalator
  • Energy intensive, more energy efficient - lesions are rapidly repaired
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10
Q

What are the distinguishing features of the duodenum?

A
  • Coiled mucus secreting submucosal gland
  • Secretes bicarbonate-rich alkaline solution that open up into base of crypts
  • Help neutralise acid chyme
  • Protect lining of proximal small intestine
  • Provide optimum pH for enzymes
  • Contains specific Brunner’s glands in sub-mucosa - secretes alkaline substances
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11
Q

What are the distinguishing features of the jejunum?

A
  • Large submucosal folds = plicae circulares
  • Similar to folds of the rest of the small intestine
  • Larger and closer together
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12
Q

What are the distinguishing features of the ileum?

A
  • Peyer’s patches
  • Can initiate leucocyte and immunoglobulin responses to pathogens
  • Patches without microvilli contain M-cells
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13
Q

What are the 3 functions of ‘motility’?

A
  • Mix ingested food with digestive secretions and enzymes
  • Facilitate contact between content and mucosal surface
  • Propel content along
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14
Q

How is motility achieved (3 ways)?

A
• Segmentation 
- circular muscle 
- alternate contraction of non-adjacent segments
- food pushed forwards and backwards 
• Peristalsis 
- circular and longitudinal muscle 
- sequential contraction
- propulsion
• Migrating motor complexes 
- periodic contractions from stomach to distal ileum during fasted state
- prevents flora travelling backwards
- helps cleanse of food
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15
Q

Describe carbohydrate digestion in the GI tract

A
• Begins slightly in the mouth - amylase
• None in the stomach
• Resumes in duodenum - pancreatic amylase &amp; brush border enzymes
- complex => basic/branched chain
• Pancreatic amylase (and disaccharide enzymes) break polysaccharides into
- sucrose (to glucose &amp; fructose)
- maltose (to x2 glucose)
- lactose (to glucose &amp; galactose)
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16
Q

How are the monosaccharides absorbed?

A
  • Glucose and galactose - Sodium Glucose Linked Transporter 1 on apical surface (secondary active transport)
  • Fructose - Glucose transporter 5/GLUT-5 (facilitated diffusion)
  • All leave the basolateral membrane via GLUT-2
17
Q

Describe protein digestion in the small intestines

A

• Continues in duodenal lume post stomach
• Protease-rich pancreatic juice
- longer peptides => tri/dipeptides
- contains trypsin, chymotrypsin and carboxypeptidase
• Final stage at brush borders
- enzymes secreted from epithelial cells
- usually tri/dipeptidases: endopeptidase, dipeptidase, aminopolypeptidase, carboxypeptidase

18
Q

How are proteins absorbed?

A

• Amino acids
- transporters on enterocytes (secondary active transport)
• Short peptides (2-3 a.a.)
- absorbed via apical surface
- broken down via cytoplasmic peptidase enzymes
- move across basolateral membrane into blood (facilitated diffusion)

19
Q

Describe lipid digestion in the GI tract

A

• Begins in mouth via lingual lipase - hydrolyses triglycerides
• Stomach
- ingested lingual lipase + secreted gastric lipase
- free triglycerides => single fatty acid chains
- mechanical stomach churning - fat emulsion
• Duodenum - bile emulsifies undigested fat => fat droplets
• Pancreatic juice - triglycerides => monoglycerides + free fatty acids (luminal digestion)
• Digested products + bile salts (micelles) reach brush border

20
Q

How are fats absorbed?

A

• Lipolytic products diffuse through apical membrane
• Bile salts remain in lumen, travel through small intestines and reabsorbed in terminal ileum
• Monoglycerides and free fatty acids are resynthesized:
- primary mechanism: monoglyceride acylation pathway
- secondary mechanism: phosphatidic pathway
• Once resynthesized, packaged into chylomicrons (proteins, phospholipids and cholesterols)
• Exocytosed => lymphatic system