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BRS Gastroenterology > Small Bowel > Flashcards

Small Bowel Flashcards

1
Q

What is the general function of the small bowel?

A

To absorb nutrients, salt and water.

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2
Q

Approximately, how long is the small bowel and what is its diameter?

A

6m long

3.5cm in diameter

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3
Q

What are the functions of the mesentery?

A

Suspends small and large bowel from the posterior abdominal wall anchoring them in place whilst still allowing some movement.
Provides a conduit for blood vessels, nerves and lymphatic vessels.

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4
Q

What are the 3 main regions of the small intestine/bowel?

A

Duodenum
Jejunum
Ileum

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5
Q

What are the 3 layers of the small intestine from superficial to deep?

A

Serosa, longitudinal muscle layer and circular muscle layer.

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6
Q

Are villi found in the large bowel?

A

No; only occur in small intestine

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7
Q

What innervates villi?

A

Submucosal plexus

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8
Q

How thick is the epithelium of villi?

A

1 cell thick (dominated by enterocytes - simple columnar absorptive cells)

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9
Q

What are villi (mucosa) lined with?

A

Simple columnar epithelium consisting of:
Primarily enterocytes
Scattered goblet cells
Enteroendocrine cells

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10
Q

What types of cells does the epithelium of the Crypts of Leiberkuhn include?

A

Paneth and stem cells

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11
Q

What is the most abundant type of cell in the small bowel?
What are they specialised for?
Lifespan?

A

Enterocytes (tall columnar cells with microvilli and a basal nucleus)

Absorption and transport of substances

Lifespan: 1-6 days (relatively short)

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12
Q

What is the cylindrical internal surface area of the small bowel?

A

0.4m^2

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13
Q

What increases the surface are of the small bowel and by how much?

A

Villi and microvilli

~500 fold ^

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14
Q

What structure makes up the ‘brush border’?

A

Microvilli - several thousand microvilli per cell

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15
Q

What is glycocalyx?

A

Rich carbohydrate layer on apical membrane.

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16
Q

What is the purpose of glycoalyx?

A

Covers surface of microvilli

Protection from digestional lumen yet allows for absorption. Traps a layer of water and mucous known as ‘unstirred layer’ → regulates rate of absorption from intestinal lumen

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17
Q

What is the second most abundant epithelial cell type?

A

Goblet cells

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18
Q

What is mucous?

A

Large glycoprotein that facilitates passage of material through bowel.

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19
Q

What happens to the number of goblet cells along the entire length of bowel?

A

Abundance increases along entire length of bowel

Low in duodenum and high in colon.

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20
Q

What type of epithelial cells are enteroendocrine cells and where are they most often found?

A

Columnar epithelial cells

Lower parts of crypts

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21
Q

What is the function of enteroendocrine cells?

A

Secrete hormones e.g. to influence gut motility.

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22
Q

What do enteroendocrine cells have a high affinity for in terms of staining?

A

Chromium/silver salts

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23
Q

Where are Paneth cells exclusively found?

A

Bases of crypts

24
Q

What do Paneth cells contain that are important for their function?

A

Large, acidophilic granules containing anti-bacterial lysozyme (protects stem cells) and GPs and zinc (essential for tracing metal for a number of enzymes)

25
Q

What are the other functions of Paneth cells?

A

Engulf some bacteria and protozoa.

May have a role in regulating intestinal flora.

26
Q

Why are epithelial stem cells essential in the GI tract?

A

To continually replenish the surface epithelium. They continually divide by mitosis and migrate to the top of villus, replacing older cells that die by apoptosis → digested and reabsorbed.

27
Q

How long is the lifespan of enterocytes and goblet cells present in the small bowel?

A

~36 hours (relatively short)

28
Q

List reasons why enterocytes and goblet cells in the small bowel have this lifespan.

A

Enterocytes are first line of defence against GI pathogens and may be directly affected by toxic substances in diet.

Effects of agents which interfere with cell function, metabolic rate will be diminished.

Any lesions → short-lived

If escalator-like transit of enterocytes is interrupted through impaired production of new cells, severe intestinal dysfunction will occur.

29
Q

What structure distinguishes the duodenum from the jejunum and ileum?

A

Brunner’s glands

30
Q

What do Brunner’s glands secrete?

A

Alkaline fluid to neutralise chyme from stomach, protecting proximal small bowel and to help optimise pH for action of pancreatic digestive enzymes.

31
Q

Compare the jejunal and ileal mesentery.

A

Wall of jejunal mesentery is wider, thicker and redder than ileum because jejunal mesentery is larger and has more cells.
Jejunal mesentery forms 1-2 arterial arcades whereas ileal forms 3-4, but these 3-4 are shorter.
Jejunal mesentery is above and to left of aorta whereas ileal mesentery is attached below and to the right of the aorta.

32
Q

List the functions of small intestine motility.

A

Mix ingested food with digestive secretions and enzymes.

Facilitate contact between contents of intestine and intestinal mucosa.

Propel intestinal contents along alimentary tract.

33
Q

Outline small bowel motility.

A

Segmentation (mixing) - Mixes contents of lumen. Occurs due to stationary contraction of circular muscles at intervals. More frequent contraction in duodenum. Contractions allow pancreatic enzymes and bile to mix with chyme. Although chyme moves in both directions, net effect is movement into the colon.

Peristalsis (propelling) - Involves sequential contraction of adjacent rings of smooth muscle. Propels chyme towards colon. Most waves of peristalsis only travel about 10cm. Segmentation and peristalsis result in chyme being segmented, mixed and propelled into the colon.

Migrating motor complex - Cycles of smooth muscle contractions sweeping through gut. Begin in the stomach → small intestine → colon → next wave starts in duodenum. Prevents migration of colonic bacteria into ileum.

34
Q

Describe the environment that digestion in the small bowel occurs in.

A

Alkaline environment

35
Q

How do bile and pancreatic digestive enzymes enter the duodenum?

A

Main pancreatic duct (MPD) and common bile duct (CBD)

36
Q

What does duodenal epithelium produce to aid digestion?

A

Digestive enzymes

37
Q

Where does digestion of carbohydrates begin and by what enzyme?

A

Mouth by salivary-alpha-amylase, which is destroyed in stomach by acid pH.

38
Q

Where does most of the digestion of carbohydrates occur?

A

Small intestine

39
Q

Give examples of simple and complex carbohydrates.

A

Simple: monosaccharides such as glucose and fructose; disaccharides such as sucrose and maltose.

Complex: starch, cellulose and pectin.

40
Q

What is secreted into the duodenum in response to a meal?

A

Pancreatic-alpha-amylase

41
Q

What does this enzyme need for optimum activity?

A

Cl- and neutral/slightly alkaline pH (enzyme acts mainly in lumen - some also adsorbs to brush border)

42
Q

Where does digestion of amylase products and simple carbohydrates occur?

A

Brush Border

43
Q

What carrier protein is used for absorption of glucose and galactose?

A

SGLT-1 on apical membrane (absorption is by secondary active transport)

44
Q

What carrier protein is used for absorption of fructose?

A

GLUT-5 on apical membrane (done by facilitated diffusion)

45
Q

What carrier protein facilitates exit of digestion products of carbohydrates into the intestinal space at the basolateral membrane?

A

GLUT-2

46
Q

How much simple sugar can the human small intestine absorb daily?

A

10kg

47
Q

Where does protein digestion begin and by what enzyme?

A

Lumen of stomach by pepsin (pepsin inactivated in alkaline duodenum)

48
Q

List the 5 pancreatic proteases secreted as processes into the the lumen of the small bowel.

A

Trypsinogen → Trypsin (via enterokinase)
Chymotrypsinogen → Chymotrypsin
Proelastase → Elastase
Procarboxypeptidase A - Carboxypeptidase A
Procarboxypeptidase B - Carboxypeptidase B

49
Q

Where is the enzyme enterokinase located?

A

Duodenal brush border

50
Q

What are oligopeptides?

A

Peptide whose molecules contain a relatively small number of amino-acid residues, but more than 2.

51
Q

How do enterocytes directly absorb some small amino acids?

A

Via action of H+/oligopeptide co-transporter → PepT1

These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes

52
Q

Why are lipids more complicated to digest than carbohydrates and proteins?

A

Lipids are poorly soluble in water.

53
Q

Outline the 4 stage process of lipid digestion in the small bowel.

A

Secretion of bile salts and pancreatic lipases.
Emulsification of lipids to increase SA for digestion.
Enzymatic hydrolysis of ester linkages via colipase-lipase complex.
Solubilisation of lipolytic products in bile salt micelles (Holding station)

54
Q

What is the purpose of colipases in lipid digestion?

A

Prevents bile salts from displacing lipase from fat droplet.

55
Q

Outline the absorption of lipids.

A 
Fatty acids (FAs) and monoglycerides (MGs) leave micelles and enter enterocytes. 
FAs and MGs resynthesised into triglycerides (TGs) via 2 pathways:
Monoglyceride acylation (major) 
Phosphatidic acid pathway (minor)

Chylomicrons synthesised and secreted across basement membrane by exocytosis. Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel.

56
Q

Where are chylomicrons synthesised and what describe their composition?

A

Golgi apparatus

80-90% Triglycerides
2% Cholesterol
2% Protein
Small traces of carbohydrate.

57
Q

What separates the ileum from the colon and what is its purpose?

A

Ileocaecal valve

Relaxation and contraction controls passage of material into colon and it prevents backflow of bacteria into ileum.

BRS Gastroenterology (11 decks)

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