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Year 1: Alimentary System > 3. Small Intestine > Flashcards

3. Small Intestine Flashcards

1
Q

What is the function of the small intestine?

A

To absorb nutrients, salt & water

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2
Q

What is the size of the small intestine?

A

6m longs and 3.5 cm in diameter

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3
Q

What is the size of the duodenum, jejunum and ileum?

A

25cm, 2.5m and 3.75m

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4
Q

What is the mesentary?

A

A layer of fan shaped tissue which holds the small intestine in place and supports the blood supply.

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5
Q

Describe the mucosa

A

The internal mucosa is arranged into folds where it is covered in villi (1mm tall)

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6
Q

What are the invaginations of the mucosa?

A

The crypts of Lieberkuhn

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7
Q

Where are villi found?

A

Only in the small intestine

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8
Q

Describe a villi

A

1) Motile, rich blood supply, rich lymph drainage
2) Good innervation from submucosal plexus
3) Simple 1 cell thick epithelium like the rest of the intestine

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9
Q

Cells of the small intestine?

A

Enterocytes, goblet cells, enteroendocrine cells, paneth cells and stem cells.

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10
Q

What are the cells of the mucosa?

A

Enterocytes, goblet cells and enteroendocrine cells

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11
Q

What are the cells in the crypts of lieberkuhn?

A

Paneth cells and stem cells

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12
Q

What are the most abundant cells of the small intestine?

A

Enterocytes

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13
Q

Describe the structure of enterocytes?

A

Tall columnar cells with microvilli and a basal nucleus

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14
Q

What is the function of enterocytes?

A

Absorption & transport of substances

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15
Q

What is the life span of enterocytes?

A

1-6 days

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16
Q

What is the size of the micro villi?

A

About 1 um high

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17
Q

What is the number of microvilli per cell?

A

Several thousand

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18
Q

What is the surface of the microvilli covered with and what is its function?

A

Glycocalyx - Carbohydrate rich layer that protects the digestional lumen and traps a layer of water (unstirred layer) and mucous which regulates the rate of absorption.

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19
Q

How many fold increase is the combination of the cylindrical internal surface area, with the folds, villi and microvilli?

A

500 fold increase

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20
Q

What is the 2nd most abundant epithelial cell type

A

Goblet cell, there is an increase in the abundance of goblet cells along the entire length of the bowel

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21
Q

What is mucous?

A

A glycoprotein that protects and facilitates the passage of material through the bowel

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22
Q

What are enteroendocrine cells?

A

Hormone secreting, to influence gut motility

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23
Q

What are paneth cells?

A

Found in the bases of the crypts they contain large acidophilic granules. They engulf some bacteria and protozoa. Have a role in regulating intestinal flora. Protect cells

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24
Q

What do the acidophilic granules contain?

A

Antibacterial enzyme lysozyme and glycoproteins and zinc.

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25
Q

What cells replace dying enterocytes?

A

Stem cells

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26
Q

Define stem cells

A

Undifferentiated cells which remain capable of cell division to replace cells which die. Epithelial stem cells are essential in the GI tract.

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27
Q

How do stem cells divide?

A

They divide by mitosis and differentiate into various cell types (pluripotent)

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28
Q

What happens when an escalator-like transit of enterocytes is interrupted through impaired production of new cells?

A

Severe intestinal dysfunction will occur

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29
Q

Why is there a rapid turn over of cells in gut lining?

A

Enterocytes are the first line of defence against GI pathogens. Any negative effects that infect these epithelial cells will be short lived as they can be replaced

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30
Q

What is Cholera enterotoxin?

A

Results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water.

31
Q

How do you treat cholera?

A

Rehydration. Since the epithelium will be replaced the cholera bacteria will clear

32
Q

What is a defining feature of the duodenum?

A

Brunner’s glands which are submucosal coiled tubular mucous glands secreting alkaline fluid (HCO3-)

33
Q

What is the function of the alkaline secretions of Brunner’s glands?

A

1) Neutralises acidic chyme from the stomach, protecting the proximal small intestine
2) Help optimise pH for action of pancreatic digestive enzymes

34
Q

What is a defining feature of the jejunum?

A

Presence of numerous large folds in the submucosa called the plicae circulares (or valves of kerckring) which are taller, thinner and more frequent in the jejunum.

35
Q

What is the defining feature of the ileum?

A

Lots of Peyer’s patches which are large clusters of lymph nodules in the submucosa. They prime the immune system against intestinal bacteria and prevent bacteria from colon migrating up into the small intestine.

36
Q

List the 3 functions of small intestine motility

A

1) To mix ingested food with digestive secretions & enzymes
2) To facilitate contact between contents of intestine & the intestinal mucosa
3) To propel intestinal contents along alimentary tract

37
Q

What is segmentation?

A

Mixes the contents of the lumen driven by the enteric nervous system. Segmentation occurs by the stationary contraction of circular muscles at intervals

38
Q

Where is segmentation contractions more frequent?

A

Duodenum is more frequent compared to the ileum

39
Q

What is peristalsis?

A

Involves the sequential contraction of adjacent rings of smooth muscle, propelling chyme towards the colon. 10 cm

40
Q

Define migrating motor complex

A

This is a wave of contraction starting in the stomach migrating through the small intestine towards the colon. On reaching the terminal ileum, the next contraction starts in the duodenum. Cycles of smooth muscle contraction

41
Q

What is function of the migrating motor complex?

A

This occurs during fasting; it prevents the migration of colonic bacteria into the ileum and may ‘clean’ the intestine of residual food

42
Q

What kind of environment does digestion take place in the small intestine?

A

Alkaline

43
Q

Where does digestive enzymes & bile enter the duodenum from?

A

From the pancreatic and bile duct

44
Q

What does the duodenal epithelium produce?

A

Digestive enzymes; digestion occurs both in the lumen and in contact with the membrane.

45
Q

What enzyme in the mouth begins digestion?

A

a-amylase, destroyed in the stomach

46
Q

Where does most of the digestion of carbohydrates occur in?

A

Small intestine

47
Q

What enzyme is used to digest starch & glycogen in the small intestine?

A

Pancreatic a-amylase

48
Q

Where is pancreatic a-amylase secreted?

A

Into the duodenum in response to a meal

49
Q

What are the optimum requirements for pancreatic a-amylase?

A

Cl- and a neutral/slightly alkaline pH (Brunner’s glands)

50
Q

How are carbohydrates absorbed?

A

Glucose & galactose are pumped into the cell via secondary active transport. Carrier protein = SGLT-1
fructose is pumped by facilitated diffusion. Carrier protein = GLUT-5

51
Q

What carrier protein facilitates the removal of glucose from the basolateral membrane?

A

GLUT-2

52
Q

How are pancreatic proteases secreted as?

A

Precursors

53
Q

Where is trypsin located?

A

Duodenal brush border

54
Q

What enzyme activates trypsin?

A

Enterokinase

55
Q

What can trypsin do?

A

It can activate other proteases

56
Q

How are amino acids absorbed?

A

Facilitated diffusion and secondary active transport

Di and tri-peptides use distinct carrier proteins from single amino acids

57
Q

What happens to di and tripeptides before they cross the basolaterel membrane

A

They are broken down by cytoplasmic peptidases

58
Q

What are the four stages of the digestion of lipids?

A

1) Secretion of bile and lipases
2) Emulsification
3) Enzymatic hydrolysis of ester linkages
4) Solubilization of lipolytic products in bile salt micelles

59
Q

Where are bile and lipases secreted?

A

duodenum

60
Q

What do bile salts do?

A

Facilitate the emulsification of fat into a suspension of lipid droplets. This increases the surface area for digestion for pancreatic lipases to split triglycerides

61
Q

What do pancreatic lipases break triglycerides down into?

A

Two fatty acids and a monoglyceride

62
Q

Describe the structure of bile salts

A

Steroid nucleus with two faces making it amphipathic

63
Q

What is the function of colipase?

A

Prevents bile salts from displacing lipase from the fat droplet

64
Q

What is the function of Phospholipase A2

A

Hydrolyses fatty acids at the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids

65
Q

What is the function of pancreatic cholesterol esterase?

A

Hydrolyses cholesterol ester to free cholesterol and fatty acid

66
Q

Function of micelles?

A

They allow transport across the unstirred layer, and present the fatty acids and monoglycerides to the brush border

67
Q

Where are bile salts absorbed?

A

Ileum

68
Q

Where are lipids absorbed?

A

Middle of the jejunum

69
Q

Where are bile salts transported to for recycling?

A

The liver

70
Q

How are tri-glycerides resynthesized?

A 
Monoglyceride acylation (major)
Phosphatidic acid pathway (minor)
71
Q

Describe monoglyceride acylation

A

1) Fatty acids bind to the apical membrane
2) Fatty acid binding proteins (FABP) facilitate transfer of fatty acids from apical membrane to the smooth ER
3) In the smooth ER - fatty acids esterified into diglycerides and triglycerides

72
Q

How are triglycerides synthesised via the phosphatidic acid pathway?

A

CoA fatty acid and a-glycerophosphate

73
Q

Where do chylomicrons go?

A

Transported to the golgi and secreted across the basement membrane by exocytosis. They are too big to enter blood capillaries of villi and enter the lacteals instead

74
Q

What is ileum and the colon separated by?

A

Ileocaecal sphincter

Year 1: Alimentary System (15 decks)

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