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Alimentary system > Small Intestine > Flashcards

Small Intestine Flashcards

1
Q

What is the key function of the small intestine

A

To absorb nutrients, salt & water

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2
Q

Describe the dimensions of the small intestine

A

§ Is approx. 6m long (duodenum=25cm, jejunum=2.5m, Ileum=3.75m) and 3.5cm in diameter – there is no clear transition between sections.

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3
Q

What is key to remember about the duodenum, jejunum and ileum

A

No suddentransition between duodenum (25cm), jejunum (2.5m), ileum (3.75).
All have same basic histological organisation.

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4
Q

Describe the mesentery

A

§ The small intestine is lined on the inside by fan-shaped mesentery which increases the surface area of the intestine with folds and supports the blood supply.
blood supply provides substrate for S.I
Nerve supply to target small intestine

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5
Q

Describe the start and end of the small intestine

A

The small intestine originates immediately after the stomach, and coils around the abdomen (held in place by the mesentery) until it meets the large intestine in the lower right quadrant. It is functionally separated from these structures by two sphincters; the pyloric sphincter at the proximal end, and the ileocaecal valve at the distal end.

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6
Q

What are the 3 noteworthy features of the small intestine

A

The small intestine has the same general structure as the rest of the gut tube, with three noteworthy features:
The epithelial layer has finger-like projections called villi which enormously increase the internal surface area. These villi are motile and have a rich blood supply and lymph drainage
The mucosa is arranged in folds (like a concertina)
It has invaginations called crypts of Lieberkühn

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7
Q

Describe the characteristics of the digestive epithelia in the small intestine

A

External wall has longitudinal & circular muscles (important for motility).
Internal mucosa arranged in circular folds.
Mucosa covered in villi (~1mm tall).
Invaginations known as Crypts of Lieberkühn.

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8
Q

What is the purpose of the villi

A

to increase the surface are for absorption

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9
Q

Describe the villi

A

only occur in the small intestine
motile, have a rich blood supply & lymph drainage for absorption of digested nutrients (fats)
have good innervation from the submucosal plexus.
- have simple epithelium (1 cell thick, like the rest of the intestine), dominated by enterocytes (columnar absorptive cells)

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10
Q

What is the mucosa of the small intestine lined with

A

simple columnar epithelium consisting of
primarily enterocytes (absorptive cells)
scattered goblet cells
enteroendocrine cells

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11
Q

What do the crypts of liberkhun consist of

A

In Crypts of Lieberkühn, epithelium includes
Paneth cells
stem cells

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12
Q

Describe the enterocytes

A

Most abundant cells in small intestine.
Tall columnar cells with microvilli & a basal nucleus.
Specialised for absorption & transport of substances.
Short lifespan of 1-6 days.

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13
Q

Describe the ultrastructure of enterocytes

A

The apical region of the cell is populated with rough endoplasmic reticulum.
They are connected to each other by tight junctions
Each cell has approximately 2000 microvilli (about 1 μm in length, 1/1000 of the size of a villus) on its apical surface, and is hence referred to as the ‘brush border’.

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14
Q

What is the brush border of enterocytes covered in and what is its function

A

The surface of the microvilli are covered with glycocalyx
Glycocalyx
rich carbohydrate layer on apical membrane that serves as protection from the digestional lumen, yet it allows for absorption.
traps a layer of water & mucous known as the “unstirred layer” which regulates rate of absorption from intestinal lumen- also protects against damage (especially enzymes and pH of lumen which may damage the cell/membrane).

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15
Q

Describe the microvilli

A

Microvilli (~0.5-1.5 microm high) make up the “brush border”.
There are several thousand microvilli per cell
attach to cytoskeleton and stick out of cell

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16
Q

What is the surface area of the small intestine

A

The cylindrical internal surface area of the small intestine is 0.4m2.
The folds, villi and microvilli increase the surface area to ~200m2 (size of a tennis court!)
This is at least a 500 fold increase

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17
Q

Describe the goblet cells

A

2nd most abundant epithelial cell type
Mucous containing granules accumulate at the apical end of the cell, causing ‘goblet’ shape.
Mucous = large glycoprotein that facilitates passage of material through the bowel.

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18
Q

Describe the features of goblet cells

A

he apical part of the cytosol is packed with mucin granules, which distorts the shape of the cell (like a goblet).
smaller in diameter than a RBC

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19
Q

What is the role of mucous

A

Mucous consists of water and glycoproteins, and serves as a lubricant to facilitate gut passage

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20
Q

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

A

increase in abundance of goblet cells along the entire length of the bowel ie relatively few in duodenum, very many in the colon.
This is especially useful as water is constantly being absorbed from the lumen, which steadily dehydrates the contents- becomes more solid and thus more lubrication is needed

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21
Q

Summarise the enteroendocrine cells

A

Columnar epithelial cells, scattered among the absorptive cells
In the intestine, most often found in the lower part of the crypts.
Hormone secreting - eg to influence gut motility (see Regulation of function lecture).
In older text books might find them referred to as chromaffin cells (affinity for chromium/silver salts).

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22
Q

List some enteroendocrine cells

A

G-cells secrete gastrin
I-cells secrete cholecystokinin
S-cells secrete secretin
D-cells secrete somatostatin

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23
Q

Describe the features of the enteroendocrine cells

A

These are hormone secreting epithelial cells, most commonly found in the bottom of the crypts. They are roughly columnar in shape. Their sensory apparatus is in the apical portion, and collections of manufactured hormones are kept near the basolateral membrane ready to secrete into the local blood supply.

Some enteroendocrine cells can also be closed, which means they are not exposed to the gut lumen at all.

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24
Q

Summarise the Paneth cells

A

Found only in the bases of the crypts.
Contain large, acidophilic granules
Granules contain:
antibacterial enzyme lysozyme (protects stem cells),
glycoproteins, and zinc (essential trace metal for a no.of enzymes).
Also engulf some bacteria and protozoa.
May have a role in regulating intestinal flora.
Protect the stem cells from damage

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25
Q

What are the functions of the acidophilic granules found in Paneth cells

A

lysozyme: an antibacterial enzyme
glycoproteins: to protect local cells from enzymes
zinc: a common cofactor for lysozymes

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26
Q

Describe epithelial lifespan

A

Cell proliferation, differentiation, & death are continuous processes in gut epithelium.
Enterocytes and goblet cells of the small intestine have a short life span (about 36 hrs).
Continually replaced by dividing stem cells in the crypts.

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27
Q

Describe the turnover of the intestinal epithelia

A

The stem cells are PLURIPOTENT (differentiate into any cell) and divide by mitosis in the crypt. Dividing stem cells in crypt shunted up - at tip, cells become senescent - apoptosed cells sloughed into lumen - apoptosed cells digested and reabsorbed by intestine.

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28
Q

Why is this rapid turnover important

A

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

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

Any lesions will be short-lived.

If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur

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29
Q

What have studies on intestinal stem cells shown

A

The stem cells have memory- if transferred to large intestine they don’t change- hard to change their programme
if the stem cells are damaged (i.e radiation)- the gut will disintegrate

30
Q

Describe the actions of the cholera endotoxin

A

Cholera enterotoxin =
results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water.

Bodily fluid moves freely into the lumen & hence out through the intestine, leading to rapid, massive dehydration & death.
Treatment is rehydration. Cholera bacteria will clear - epithelium will be replaced.

People die in the first few days- if you survive the first few days- cell turnover- symptoms stopped

31
Q

How can we distinguish the duodenum

A

Distinguished by the presence of Brunner’s glands
Submucosal coiled tubular mucous glands secreting alkaline fluid.
Open into the base of the crypts

32
Q

What is the purpose of these alkaline secretions secreted by Brunner’s glands

A

Alkaline secretions of Brunner’s glands
Neutralizes acidic chyme from the stomach, protecting the proximal small intestine
Help optimise pH for action of pancreatic digestive enzymes.

33
Q

How can we easily identify the jejenum

A

Characterised by the presence of numerous, large folds in the submucosa, called plicae circulares (or valves of Kerckring).

Also present in the duodenum and ileum, but plicae in the jejunum tend to be taller, thinner and more frequent (larger and closer together- giving a frilly appearance)

34
Q

How can we easily identify the ileum

A

Shares some features with the large intestine
The ileum has a lot of Peyer’s patches- large clusters of lymph nodules in the submucosa.
Prime immune system against intestinal bacteria (other mechanisms for defence = bactericidal Paneth cells, rapid cell turnover).
Well positioned to prevent bacteria from colon migrating up into small intestine

35
Q

Describe the M cells

A

The ileum has up to 100 Peyer’s patches, which are aggregates of specialised lymphoid tissue that are key to the immune response to gut-borne pathogens. They are able to initiate leukocyte and immunoglobulin responses to pathogens. These patches contain M-cells (key for immunity, see later lesson) which do not have microvilli like the neighbouring enterocytes.

36
Q

What are the key functions of motility in the small intestine

A

To mix ingested food with digestive secretions & enzymes

To facilitate contact between contents of intestine & the intestinal mucosa

To propel intestinal contents along alimentary tract

37
Q

Why are the Peyer’s patches in the ileum particularly important

A

Lots of bacteria in the L.I- prevents them from colonising the S.I

38
Q

Describe segmentation in the small intestine

A

Mixes the contents of the lumen
Segmentation occurs by stationary contraction of circular muscles at intervals.
More frequent contractions in duodenum compared to ileum – allow pancreatic enzymes & bile to mix with chyme
Although chyme moves in both directions, net effect is movement towards the colon.

39
Q

What are the effects of segmentation

A

major effect is mixing and mechanical breakdown of food

minor effect is some propulsion

40
Q

Describe peristalsis

A

Involves sequential contraction of adjacent rings of smooth muscle
Propels chyme towards the colon
Most waves of peristalsis travel about 10cm (not the full length of the intestine).

41
Q

Which muscles contract in peristalsis

A

sequential contraction of adjacent circular muscle (combined with longitudinal muscle) propels food distally (minor effect = mixing)

42
Q

Describe the migrating motor complex

A

In fasting = cycles of smooth muscle contractions
Each cycle = contraction of adjacent segments of small intestine
Begin in stomach, migrate through small intestine towards colon. On reaching terminal ileum, next contraction starts in the duodenum
Prevents migration of colonic bacteria into the ileum & may ‘clean’ the intestine of residual food
Also occur in fed state – but less ordered & less frequent
centrally regulated
prevents accumulation of residue

43
Q

Summarise digestion in the small intestine

A

In the small intestine digestion occurs in an alkaline environment.
Digestive enzymes & bile enter the duodenum from the pancreatic duct and bile duct.

Enzymes come from pancreas, liver and small intestinal wall

44
Q

Where are the sites of actions of the digestive enzymes in the small intestine

A

Enzymes that cleave big nutrients into smaller nutrients operate in the gut lumen (usually released from glandular organs)
Enzymes that cleave dimeric nutrients into monomers for absorption are in the brush border of the enterocytes (usually released from the gut epithelium)

45
Q

Describe the sphincter of oddi

A

Sphincter of Oddi controls digestive enzyme and bile secretion.
Point of entry of pancreatic and bile duct into the duodenum

46
Q

What is the key difference between primary and secondary active transport

A

Primary- energy from hydrolysis of ATP

Secondary- energy from electrochemical gradient

47
Q

Summarise the digestion of carbohydrates

A

Carbohydrates contain ~50% of ingested calories in western diet.
Digestion begins in the mouth by salivary -amylase, but is destroyed in the stomach (acid pH)
Most of the digestion of carbohydrates occurs in the small intestine.
No digestion in the stomach

48
Q

What is the aim of carbohydrate digestion

A

The goal of digestion is to break the long polymers into simple monomers that can be absorbed by the gut wall.

49
Q

Describe the difference between simple and complex carbohydrates

A

Complex carbohydrates: long polymers in basic chains or branched chains
Simple carbohydrates: disaccharides and monosaccharides

Simple carbohydrates e.g. monosaccharides - glucose & fructose, disaccharides - sucrose & maltose

Complex carbohydrates (starch, cellulose, pectins) = sugars bonded together to form a chain

50
Q

Summarise pancreatic alpha-amylase

A

Secreted into duodenum in response to a meal
Continues digestion of starch & glycogen in the small intestine (started by salivary amylase)
Needs Cl- for optimum activity & neutral/slightly alkaline pH (Brunner’s glands in duodenum = alkaline secretion)

51
Q

Describe the activity of pancreatic alpha-amylases

A

Pancreatic  amylase - acts mainly in the lumen (some also adsorbs to the brush border)

Pancreatic amylase can break polysaccharides into three different disaccharides (sucrose, maltose and lactose).

52
Q

Describe the digestion of the amylase products

A

Digestion of amylase products and simple carbohydrates occurs at the membrane.
These are broken down by disaccharidase enzymes of similar names (sucrase, maltase and lactase). Maltose is converted into two glucose monomers, sucrose to one glucose and one fructose, and lactose into one glucose and one galactose (note that they all produce at least one glucose, but LACtose also gives gaLACtose and sUCrose also gives frUCtose)

53
Q

Describe the absorption of carbohydrates

A

Absorption of glucose & galactose is by 2o active transport (carrier protein & electrochemical gradient). Carrier protein = SGLT-1 on apical membrane.
Absorption of fructose is by facilitated diffusion. Carrier protein = GLUT-5 on apical membrane.
GLUT-2 facilitates exit at the basolateral membrane.
The human small intestine can absorb 10kg of simple sugars per day!

54
Q

Summarise the digestion of proteins

A

Ingested proteins are not broken down at all in the mouth, but do receive significant degradation in the stomach, where pepsin breaks long protein chains into smaller peptides.
Pepsin relies on an acidic environment to function, so as the gastric contents make their way into the duodenum pepsin stops working effectively.

55
Q

How are pancreatic proteases secreted

A

Pancreatic proteases are secreted as precursors.
Trypsin is activated by enterokinase, an enzyme located on the duodenal brush border.
Trypsin then activates the other proteases.

56
Q

Describe the actions of proteases in protein digestion

A

Digestion continues in the duodenal lumen, where protease-rich pancreatic juice continues to break longer peptides into tripeptides and dipeptides. The pancreatic enzymes include trypsin, chymotrypsin and carboxypeptidase.
The final stage of digestion occurs in the brush border by enzymes secreted from the epithelial cells. These enzymes are typically tripeptidases and dipeptidases (i.e. they break down proteins 3 or 2 amino acids in length). Specifically, these enzymes are endopeptidase, dipeptidase, aminopolypeptidase and carboxypeptidase.

57
Q

Describe the absorption of proteins

A

apically, amino acid use an AA/Na+ symporter whereas di/tripeptides use an AA/H+ symporter; only amino acids enter the blood and do so using facilitated diffusion
Cytoplasmic peptidases break down most di- and tri-peptides before excretion past the baso-lateral membrane.

58
Q

Summarise the digestion of lipids

A

Lipids are poorly soluble in water, which makes them more complicated to digest.
Four stage process in the small intestine
Secretion of bile and lipases,
Emulsification
Enzymatic hydrolysis of ester linkages,
Solubilization of lipolytic products in bile salt micelles.

59
Q

Where does the digestion of fats begin

A

The first stage of digestion is in the mouth where lingual lipase hydrolyses triglycerides. After swallowing, digestion continues in the stomach by ingested lingual lipase and secreted gastric lipase, which cleaves single fatty acid chains from free triglycerides. The mechanical churning of the stomach slightly emulsifies the fats to increase their surface area.

60
Q

Describe emulsification

A

Water and fat don’t mix.
Bile and lipases are secreted into the duodenum.
Bile salts facilitate the emulsification of fat into a suspension of lipid droplets (~1m diam).
The function of emulsification is to increase the surface area for digestion.
Allows pancreatic lipase to split triglycerides
A triglyceride is broken down into two fatty acids and a monoglyceride at fat/water interface.

61
Q

Summarise luminal digestion of lipids

A

As acid chyme leaves the stomach, fats are still largely intact (undigested), and as bile enters the duodenum it provides chemical emulsification with huge increases in surface area by creating small fat droplets. Pancreatic juice is secreted simultaneously, which acts on this large surface area to cleave two fatty acid chains from the triglycerides to form monoglycerides and free fatty acids (via pancreatic lipase & colipase). This is luminal digestion, and as the digestion products reach the brush border they are combined with bile salts to form micelles, which are soluble enough to cross the aqueous unstirred layer.

62
Q

Describe the roles of pancreatic lipases in the digestion of lipids

A

Lipase breaks down triglycerides into monoglycerides and free fatty acids.
Pancreatic lipase complexes with colipase.
Colipase prevents bile salts from displacing lipase from the fat droplet.

63
Q

Describe the other enzymes involved in 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.

64
Q

Describe the structure of the bile salt molecule

A

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

65
Q

Describe bile salt micelles

A

Micelles = hydrophilic “head” regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle centre
Mixed micelles in small intestine = water insoluble monoglycerides from lipolysis are solubilised by forming a core, stabilised by bile salts

66
Q

Summarise the absorption of lipids

A

Micelles are important in absorption… they are absorbed much quicker than emulsion.
Micelles allow transport across the unstirred layer, and present the fatty acids and monoglycerides to the brush border.
The whole micelle is not absorbed together… bile salts are absorbed in the ileum, but lipid absorption is usually complete by the middle of the jejunum.
Bile salts are transported back to the liver for recycling (enterohepatic circulation)

67
Q

Where are the bile salts and lipids absorbed

A

The whole micelle is then not absorbed – bile salt absorbed in ileum, lipids absorbed by middle jejunum.

68
Q

What happens to the lipolytic products absorbed

A 
Monoglycerides & free fatty acids absorbed by enterocytes are resynthesized into tri-glycerides by 2 different pathways:
	Monoglyceride acylation (major),
	Phosphatidic acid pathway (minor)
69
Q

Describe monoglyceride acetylation

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.

70
Q

Describe the phosphatidic acid pathway

A

secondary - triglycerides synthesised form fatty acid CoA and alpha-glycerophosphate

71
Q

Describe chylomicrons

A

Lipoprotein particles synthesised in enterocytes as an emulsion.
80-90% triglycerides, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate.
Chylomicrons are transported to the Golgi and secreted across the basement membrane by exocytosis.
Too big to enter blood capillaries of villi
Enter lacteals (lymph channels) instead

72
Q

Describe control of the ileum

A

The ileum is separated from the colon by the ileocaecal sphincter.
Relaxation and contraction controls the passage of material into the colon.
Also prevents the back flow of bacteria into the ileum.

colon should not be absorbing
lots of proteins may require absorption in ileum

Alimentary system (15 decks)

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