Alimentary Systems 3 - Small Intestine Flashcards Preview

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Flashcards in Alimentary Systems 3 - Small Intestine Deck (36)
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1
Q

What is the function of the small intestine?

A

To absorb nutrients salt and water

2
Q

List the dimentions of the small intestine

A
  • 6m long, 3.5cm diameter
  • Duodenum 25cm
  • Jejunum 2.5m
  • Ileum 3.75m
  • No sudden transition between the 3 parts
3
Q

What is the function of mesentry in the small intestine?

A
  • Throws the small intestine into folds

- Supports the blood supply

4
Q

Describe the layout of the epithelium of the small intestine

A
  • External wall has longitudinal and circular muscles
  • Internal mucosa arranged in circular folds
  • Mucosa covered in villi
  • Invaginations (crypts of Lieberkuhn)
5
Q

List the characteristics of villi

A
  • Only present in the small intestine to increase the SA
  • Motile with rich blood supply and lymph drainage for absorption of digested nutrients
  • Good innervation from the submucosal plexus.
  • Simple epitelium dominated by enterocytes
6
Q

List the cell types in the mucosa

A
  • Simple columnar epithelium
  • Primarily enterocytes (absorptive)
  • Scattered goblet cells
  • Enteroendocrine cells
7
Q

List the cell types in the crypts of Lieberkuhn

A
  • Paneth cells

- Stem cells

8
Q

Where are villi located on the enterocytes?

A
  • Apical membrane

- Blood at the basolateral membrane

9
Q

What are microvilli

A
  • They make up the brush border, present on villi.

- Have glycocalyx, a carboydrate layer

10
Q

What are the functions of glycocalyx

A
  • Allows for absorption while protecting from the digestional lumen.
  • Also traps a layer of water and mucous called the unstirred layer which regulates the rate of absorption from the intestinal lumen
11
Q

What is the function of goblet cells?

A
  • Secrete mucous containing granules

- There are more the further down the digestive tract

12
Q

What is the function of enteroendocrine cells in the small intestine? Give examples

A
  • Found in the lower parts of the crypts of Lieberkuhn
  • Secrete hormones to influence gut motility
  • G-cells secrete gastrin
  • I-cells secrete cholecystokinin
  • S-cells secrete secretin
  • D-cells secrete somatostatin
13
Q

What is the function of paneth cells in the SI?

A
  • Found only in the bases of the crypts
  • Contain acidophilic granules containing lysozyme and glycoproteins
  • Engulf some bacteria and protozoa
14
Q

Why is the cell turnover in the epithelium of the gut so high?

A
  • Due to damage by toxins
  • Lesions will be sort lived
  • Allows people to recover from cholera if hydrated for 1-2 days
15
Q

How is the duodenum distinguished?

A
  • The presence of brunners glands
  • These glands are submucosal coiled tubular mucous glands that secrete alkaline fluid
  • Open into the base of the crypts
  • Alkaline secretions neutralise chyme from the stomach, and optimise pH for pancreatic digestion enzymes
16
Q

How is the jejunum distinguished?

A
  • Presence of numerous large folds in the submucosa called plicae circulares (valves of Kerckring)
  • Though present in all the small intestine, they are taller, thinner and more frequent here
17
Q

How is the ileum distinguished?

A
  • Lots of peyers patches (lymph nodules in the submucosa)

- Prevents bacteria from the colon migrating into the SI

18
Q

How are things moved through the small intestine?

A
  • Segmentation mixes contents of the lumen by contraction of circular muscles
  • Peristalsis moves chyme towards the colon
  • Migrating motor complex
19
Q

What is the migrating motor complex?

A
  • Seen in fasting

- Cycles of smooth muscle contraction to clean the intestine and move bacteria away from the ileum

20
Q

Where does digestion occur in the duodenum?

A
  • In the lumen and in contact with the membrane
21
Q

What is the difference between primary and secondary active transport?

A
  • Primary involves hydrolysis of ATP

- Secondary involves electrochemical gradients

22
Q

Describe the process of carbohydrate digestion

A
  • Starts with salivary amylase in the mouth, destroyed by the stomach pH
  • In the duodenum pancreatic a-amylase is secreted in response to a meal. Needs an alkaline pH and Cl-.
  • a-amylase acts on the lumen to break down starch and glycogen to smaller carbohydrates
  • Amylase products and simple carbohydrates are digested at the membrane
23
Q

How does absorption of carbohydrates occur in the SI?

A
  • Secondary active transport of glucose and galactose by SGLT-1
  • Fructose by facilitated diffusion using GLUT-5
  • GLUT-2 facilitates exit at the basolateral membrane
24
Q

Describe the digestion of proteins in the SI

A
  • Begins in the stomach by pepsin which is inactivated in the duodenum due to pH
  • Trypsin is activated by enterokinase at the duodenal brush border, and then activates other proteases
  • Brush border peptides break down larger peptides
25
Q

How are amino acids and di/tri peptides absorbed at the brush border?

A
  • Absorption of amino acids occurs by facilitated diffusion and secondary active transport
  • Di/tri-peptides require specific carrier proteins
  • Cytoplasmic peptidases break down di/tri-peptides before they cross the basolateral membrane
26
Q

How are lipids digested?

A
  • Secretion of bile and lipases
  • Emulsification (increase SA for digestion)
  • Enzymatic hydrolysis and ester linkages (lipase + colipase)
  • Solubilization of lipolytic products in bile salt micelles
27
Q

What is the function of colipase?

A

Prevents bile salts from displacing lipase from the fat droplet

28
Q

What are the other enzymes, not icluding lipase, important for lipid digestion?

A
  • Phospholipase A2 hydrolyses fatty acids at the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids.
  • Pancreatic cholesterol esterase hydrolyses cholesterol ester to free cholesterol and fatty acid.
29
Q

Describe the structure of a bile salt

A
  • Steroid nucleus planar- two faces.
  • Amphipathic.
  • Hydrophobic (nucleus and methyl) face dissolves in fat
  • Hydrophilic (hydroxyl and carboxyl) face dissolves in water
30
Q

What are bile salt micelles?

A
  • Hydrophilic head in contact with solvent, hydropobic tails in the middle with a lipid core
  • Water insolble in SI
31
Q

How are lipids absorbed?

A
  • Micelles important in absorption
  • Allow transport across the unstirred layer, present the fatty acids and monoglycerides to the brush border
  • Bile absorbed at the ileum, lipids by the middle of the jejunum
32
Q

How are lipids resynthesised into triglycerides?

A
  • Monoglyceride acylation (major),

- Phosphatidic acid pathway (minor)

33
Q

Describe the process of monoglyceride acylation

A
  • Fatty acids bind to the apical membrane.
  • Fatty acid binding proteins (FABP) facilitate transfer of fatty acids from apical membrane to the smooth ER.
  • In the smooth ER - fatty acids esterified into diglycerides and triglycerides.
34
Q

Describe the process that occurs in the phosphatidic acid pathway

A

Triglycerides synthesised from CoA fatty acid and a-glycerophosphate

35
Q

Describe the pathway taken by chylomicrons

A
  • Chylomicrons transported to the golgi and secreted across the basement membrane by exocytosis
  • Too big to enter blood capillaries of villi, so travel through lymph channels instead
36
Q

What is the function of the ileocaecal sphincter

A
  • Separates the ileum from the colon
  • Relaxation and contraction controls the passage of material into the colon
  • Prevents backflow of bacteria into the ileum