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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
What cells replace dying enterocytes?
Stem cells
26
Define stem cells
Undifferentiated cells which remain capable of cell division to replace cells which die. Epithelial stem cells are essential in the GI tract.
27
How do stem cells divide?
They divide by mitosis and differentiate into various cell types (pluripotent)
28
What happens when an escalator-like transit of enterocytes is interrupted through impaired production of new cells?
Severe intestinal dysfunction will occur
29
Why is there a rapid turn over of cells in gut lining?
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
30
What is Cholera enterotoxin?
Results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water.
31
How do you treat cholera?
Rehydration. Since the epithelium will be replaced the cholera bacteria will clear
32
What is a defining feature of the duodenum?
Brunner's glands which are submucosal coiled tubular mucous glands secreting alkaline fluid (HCO3-)
33
What is the function of the alkaline secretions of Brunner's glands?
1) Neutralises acidic chyme from the stomach, protecting the proximal small intestine 2) Help optimise pH for action of pancreatic digestive enzymes
34
What is a defining feature of the jejunum?
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
What is the defining feature of the ileum?
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
List the 3 functions of small intestine motility
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
What is segmentation?
Mixes the contents of the lumen driven by the enteric nervous system. Segmentation occurs by the stationary contraction of circular muscles at intervals
38
Where is segmentation contractions more frequent?
Duodenum is more frequent compared to the ileum
39
What is peristalsis?
Involves the sequential contraction of adjacent rings of smooth muscle, propelling chyme towards the colon. 10 cm
40
Define migrating motor complex
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
What is function of the migrating motor complex?
This occurs during fasting; it prevents the migration of colonic bacteria into the ileum and may 'clean' the intestine of residual food
42
What kind of environment does digestion take place in the small intestine?
Alkaline
43
Where does digestive enzymes & bile enter the duodenum from?
From the pancreatic and bile duct
44
What does the duodenal epithelium produce?
Digestive enzymes; digestion occurs both in the lumen and in contact with the membrane.
45
What enzyme in the mouth begins digestion?
a-amylase, destroyed in the stomach
46
Where does most of the digestion of carbohydrates occur in?
Small intestine
47
What enzyme is used to digest starch & glycogen in the small intestine?
Pancreatic a-amylase
48
Where is pancreatic a-amylase secreted?
Into the duodenum in response to a meal
49
What are the optimum requirements for pancreatic a-amylase?
Cl- and a neutral/slightly alkaline pH (Brunner's glands)
50
How are carbohydrates absorbed?
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
What carrier protein facilitates the removal of glucose from the basolateral membrane?
GLUT-2
52
How are pancreatic proteases secreted as?
Precursors
53
Where is trypsin located?
Duodenal brush border
54
What enzyme activates trypsin?
Enterokinase
55
What can trypsin do?
It can activate other proteases
56
How are amino acids absorbed?
Facilitated diffusion and secondary active transport | Di and tri-peptides use distinct carrier proteins from single amino acids
57
What happens to di and tripeptides before they cross the basolaterel membrane
They are broken down by cytoplasmic peptidases
58
What are the four stages of the digestion of lipids?
1) Secretion of bile and lipases 2) Emulsification 3) Enzymatic hydrolysis of ester linkages 4) Solubilization of lipolytic products in bile salt micelles
59
Where are bile and lipases secreted?
duodenum
60
What do bile salts do?
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
What do pancreatic lipases break triglycerides down into?
Two fatty acids and a monoglyceride
62
Describe the structure of bile salts
Steroid nucleus with two faces making it amphipathic
63
What is the function of colipase?
Prevents bile salts from displacing lipase from the fat droplet
64
What is the function of Phospholipase A2
Hydrolyses fatty acids at the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids
65
What is the function of pancreatic cholesterol esterase?
Hydrolyses cholesterol ester to free cholesterol and fatty acid
66
Function of micelles?
They allow transport across the unstirred layer, and present the fatty acids and monoglycerides to the brush border
67
Where are bile salts absorbed?
Ileum
68
Where are lipids absorbed?
Middle of the jejunum
69
Where are bile salts transported to for recycling?
The liver
70
How are tri-glycerides resynthesized?
``` Monoglyceride acylation (major) Phosphatidic acid pathway (minor) ```
71
Describe monoglyceride acylation
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
How are triglycerides synthesised via the phosphatidic acid pathway?
CoA fatty acid and a-glycerophosphate
73
Where do chylomicrons go?
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
What is ileum and the colon separated by?
Ileocaecal sphincter

Year 1: Alimentary System (15 decks)

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