Small intestine

Question 1

what is the function of the small intestine?

Answer 1

To absorb nutrients, salt & water

Question 2

What are the lengths of the duodenum, jejunum and ileum?

what is the length of the small intestine and the diameter

Answer 2

25 cm
2.5 m
3.75 m
All have same basic histological organisation.

approx 6m long & 3.5cm in diameter

Question 3

what is the mesentery and what does it do?

Answer 3

A fold of the peritoneum which attaches the stomach, small intestine, pancreas, spleen, and other organs to the posterior wall of the abdomen.

Fan shaped mesentery;

• throws the small intestine into folds
• supports the blood supply

Question 4

How tall are villi?

Answer 4

1 mm

Question 5

What are the invaginations formed by the folding of the submucosa called?

Answer 5

Crypts of Lieberkuhn

Question 6

Describe the digestive epithelium?

Answer 6

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, this increases the surface area for absorption.

Question 7

what are the features of the villi?

Answer 7

• only occur in the small intestine
• motile, have a rich blood supply & lymph drainage for absorption of digested nutrients
• 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)

Question 8

what is the mucosa lined with?

Answer 8

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

Question 9

what does the Crypts of Lieberkühn, epithelium includes

Answer 9

Paneth cells

stem cells

Question 10

what are the features of the enterocytes( absorptive cells)

Answer 10

• 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.

Question 11

what is the surface of the microvilli covered in and what are the features of this?

Answer 11

-0.5-1.5 micrimetres
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

Question 12

by what factor does the villi and microvilli increase the surface area of the small intestine/

Answer 12

500 fold

Question 13

what does the goblet cells produce and what are the features of this?

Answer 13

-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.

Question 14

Describe the features of the paneth cells?

Answer 14

-Found only in the bases of the crypts.
-Contain large, acidophilic granules ( responding to acid)
-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.

Question 15

what is the epithelial lifespan and what is the cycle of the turnover?

Answer 15

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.

• Undifferentiated cells which remain capable of cell division to replace cells which die
• Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
• Continually divide by mitosis
• Migrate up to tip of villus, replacing older cells that die by apoptosis
• Differentiate into various cell types (pluripotent)

Question 16

what happens at the villus to the newly made cells?

Answer 16

-at villus tips, cells become senescent -> sloughed into the lumen of the intestine -> digested and reabsorbed.
rapid turnover contrasts with lifespan of weeks/months for other epithelial cell types e.g. lung, blood vessels.

Question 17

why is there such a rapid turnover>

Answer 17

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

Question 18

what does cholera enterotoxin cause?

Answer 18

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.

Question 19

what are the distinctive features of the duodenum?

Answer 19

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

Question 20

what are some distincitve features of the jejunum?

Answer 20

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.

Question 21

what are some distinctive features of the Ileum?

Answer 21

• 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

Question 22

what are the 5 cell types and summarise the functions?

Answer 22

Enterocytes
Absorptive (covered by microvilli), most abundant, frequent renewal

Goblet cells
mucous secreting

Enteroendocrine cells
hormone secreting

Paneth cells
antibacterial, protect stem cells

Stem cells
migrate up villus ‘escalator’, pluripotent

Question 23

what are the functions of the small intestine motility?

Answer 23

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

Question 24

what does segmentation do?

Answer 24

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.

Question 25

what does peristalsis do?

Answer 25

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). Segmentation and peristalsis result in chyme being segmented, mixed and propelled towards the colon.

Question 26

what happens in migrating motor complex

Answer 26

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

Question 27

Summarise the motility process and what happens?

Answer 27

``` Segmentation -mixing. Less ordered Peristalsis -propelling. MORE ordered Migratory Motor Complex -sweeps through gut, preventing accumulation of residue ```

Question 28

Describe the digestion in the duodenum?

Answer 28

-In the small intestine digestion occurs in an alkaline environment. -Digestive enzymes & bile enter the duodenum from the pancreatic duct and bile duct. -The duodenal epithelium also produces its own digestive enzymes. Digestion occurs both in the lumen and in contact with the membrane -in the lumen digestion is of large macromolecules to smaller molecules and as it goes to brush border, the microvilli and glycocalyx it is of the small molecules, there are a different set of enzymes here too.

Question 29

Describe the digestion of the carbohydrates?

Answer 29

-Digestion begins in the mouth by salivary alpha-amylase, but is destroyed in the stomach (acid pH) -Most of the digestion of carbohydrates occurs in the small intestine. - the carbohydrate is mainly digested by pancreatic alpha-amylase - release from the pancreas and released into duodenum. -Continues digestion of starch & glycogen in the small intestine (started by salivary amylase) -Needs Cl- for optimum activity & neutral/slightly alkaline pH this is supplied by (Brunner’s glands in duodenum = alkaline secretion). -the Pancreatic alpha amylase - acts mainly in the lumen (some also adsorbs to the brush border) and converts COMPLEX CARBOHYDRATES EG STARCH to DISACCHARIDES OLIGOSACCHARIDES which is then converted to monosaccharides - Digestion of amylase products and simple carbohydrates occurs at the membrane - Brush border enzymes eg maltase, lactase, sucrase

Question 30

how is carbohydrates absorbed?

Answer 30

Absorption of glucose & galactose is by secondary active transport (carrier protein & electrochemical gradient). Pumping against the gradient, which is created by using the sodium gradient. Low sodium inside, so if sodium is allowed to come in it releases energy which is used to pump glucose. 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. -uses the concentration of the sodium

Question 31

Describe the digestion of proteins

Answer 31

- Protein digestion begins in the stomach by pepsin, but pepsin is inactivated in the alkaline duodenum. - Pancreatic proteases are secreted as precursors. - Trypsin is secreted in trypsinogen which is inactive precursor - Trypsin is activated by enterokinase (only found in the gut), an enzyme located on the duodenal brush border. - Trypsin then activates the other proteases and can convert more trypsin from trypsinogen. - Brush border peptidases break down the larger peptides prior to absorption. - Amino acids are absorbed by facilitated diffusion and secondary active transport (similar to sugars). - Di- and tri-peptides are absorbed using carrier proteins distinct from single amino acids. - Cytoplasmic peptidases break down most of the di- and tri-peptides before they cross the basolateral membrane.

Question 32

describe the digestion of lipids?

Answer 32

-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, 1. Emulsification 2. Enzymatic 3. hydrolysis of ester linkages, 4. Solubilization of lipolytic products in bile salt micelles. 1. Bile and lipases are secreted into the duodenum. Bile salts facilitate the emulsification of fat into a suspension of lipid droplets (~1microm 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. 2. Lipase breaks down triglycerides into monoglycerides and free fatty acids. Pancreatic lipase complexes with colipase ( this is inactive and needs to be activated within the gut which is required for lipase to work effectively). Colipase prevents bile salts from displacing lipase from the fat droplet.

Question 33

what other important lipid enzymes

Answer 33

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.

Question 34

Describe bile salt molecule

Answer 34

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

Question 35

what do micelles allow?

Answer 35

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)

Question 36

what are the 2 ways in which triacylglycerols are resynthesised?

Answer 36

Monoglyceride acylation (major): 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. Phosphatidic acid pathway (minor): Triglycerides are synthesised from CoA fatty acid and alpha-glycerophosphate.

Question 37

How are fats transported?

Answer 37

Chylomicrons: 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

Question 38

describe the function of the ileum

Answer 38

- 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.

Question 39

Summarise digestion

Answer 39

Carbohydrate digestion Pancreatic  amylase & brush border enzymes Protein digestion Trypsin, which activates other proteases Lipid digestion Emulsification by bile, hydrolysis & solubilisation into bile salt micelles