The Small Intestine Flashcards
What 3 parts can the SI be divided into?
- Duodenum
- Jejunum
- Ileum
What are villi?
Mucosa on inside of small intestine wrapped up into folds. Finger-like projections that cover the small intestine. Covered by microvilli. Designed to increase SA.
What are crypts?
An intestinal gland (also crypt of Lieberkühn and intestinal crypt) found in between villi in the intestinal epithelium lining of the small intestine and large intestine (or colon).
Secrete bicarbonate rich fluid
What are brush border enzymes?
Integral membrane proteins on surface of intestinal microvilli
What is function of brush border enzymes?
Break down materials in contact with brush border
What are the brush borders of the intestinal lining?
The site of terminal carbohydrate digestions. The microvilli that constitute the brush border have enzymes for this final part of digestion anchored into their apical plasma membrane as integral membrane proteins.
What is peristalsis?
Waves of muscular contractions which moves contents along the length of GI tract
What are the 2 layers of muscles that move contents along the GI tract?
- Circular muscles
2. Longitudinal muscles
How do circular muscles move food?
Circular muscles contract behind bolus, while circular muscles ahead of bolus relax (squeezing from behind)
Wave of contraction in circular muscles forces bolus forward
How do longitudinal muscles move food?
Longitudinal muscles ahead of bolus contract, shortening adjacent segments (effect of propelling food)
What is segmentation?
Alternate contraction of neighbouring segments that:
- Churns and fragments the bolus
- Mixes contents with intestinal secretions
What are MMCs?
Migrating Motor Complex
A series of strong, slow, peristaltic waves that sweep down (mostly from stomach) and along the SI
What is purpose of MMCs?
These waves are thought to help keep the gut clean, prevent reflux, and reduce bacterial growth.
How is the pyloric sphincter during MMCs?
Is relaxed, allowing larger things to pass
When the gut is relatively empty, how often are MMCs generated?
Roughly every 90 minutes
What are MMCs stimulated/suppressed by?
Stimulated by motilin, a 22 aa peptide secreted by M cells (and by erythromycin). Suppressed by feeding.
What are the Crypts of Lieberkuhn?
Glands found in the epithelial lining. They contain numerous cells such as stem cells to produce new cells to replenish the cells lost due to abrasion, as well as enteroendocrine cells to synthesise and secrete hormones.
What are the 4 main classes of APUD (endocrine) cells?
- I cells
- S cells
- G cells
- M cells
Where are enteroendocrine/APUD cells located?
In the Crypts of Lieberkuhn
What do I cells secrete?
CCK - in response to presence of fat in the SI
What does CCK stimulate?
The contraction of the gallbladder (which pushes bile out into the cystic duct) and the release of pancreatic enzymes.
What do S cells secrete?
Secretin - in response to the low pH of chyme in the small intestine
What is effect of secretin?
Induces HCO3– secretion from the pancreas and inhibits gastric emptying.
What do M cells secrete?
Motilin
What is function of motilin?
Motilin participates in controlling the pattern of smooth muscle contractions in the upper gastrointestinal tract.
What do G cells secrete?
Gastrin
What is bicarbonate secreted by?
Pancreatic duct cells in response to CCK and secretin
Where are goblet cells located?
In the epithelium
What do goblet cells secrete?
Mucin
What can be absorbed without processing?
Water, electrolytes, vitamins
How are carbohydrates digested?
Starts in mouth, proceeds with excretion of pancreatic enzymes into GI tract, along brush border with specific enzymes
Finally, monosaccharides are absorbed
What can soluble amylases only break? What then happens?
Integral alpha 1,4 bonds
Remaining short-chain carbohydrates are broken down by specific enzymes on the brush border membrane.
What is the inner wall/mucosa of the SI lined with?
Simple columnar epithelial tissue
What is close to the villus?
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries ( amino acids and carbohydrates) and lacteals (lipids)
Where does absorption of majority of nutrients take place?
In the jejunum
How is fructose absorbed?
Passive (facilitated) diffusion via Glut5 transporter
How are glucose and galactose absorbed?
Actively absorbed by SGLT1
What are examples of disaccharides?
Fructose, sucrose, lactose
What carbohydrates pass through the SI undigested?
Cellulose
How does Na+/K+ pump facilitate in absorption?
Pump moves Na+ from cell into bloodstream and brings K+ in
Creates low Na+ in cell
Causes Na+ to be taken into cell from GI tract (conc gradient)
As Na+ moves into cell, glucose or galactose comes with it
Where does proteolysis begin?
In the stomach - mix of pepsin and HCl
Where is pepsin inactivated?
In the duodenum
What are trypsin, elastase and chymotrypsin examples of?
Proteolytic enzymes
What are proteolytic enzymes secreted by? What is their function?
The pancreas
Cleave proteins into smaller peptides
What is function of pepsin?
Pepsin cuts proteins into smaller polypeptides and their constituent amino acids
How is the HCl neutralised once chyme enters the SI?
The pancreas releases sodium bicarbonate
How do secretin and CCK facilitate in protein break down?
Stimulate digestive processes to break down the proteins further.
Secretin also stimulates the pancreas to release sodium bicarbonate
How is breaking down the proteins that make up the pancreas and small intestine avoided?
Pancreatic enzymes are released as inactive proenzymes that are only activated in the small intestine
What are the proenzymes for:
- Trypsin
- Chymotrypsin
- Elastase
- Carboxypeptidase A
- Carboxypeptidase B
- Trypsinogen
- Chymotrypsinogen
- Proelastase
- Procarboxypeptidase A
- Procarboxypeptidase B
Once released into the SI, how is trypsinogen activated?
Enterokinase binds to trypsinogen and converts it into its active form, trypsin
What does trypsin the bind to?
Binds to chymotrypsinogen to convert it into the active chymotrypsin
What is proteolysis?
Trypsin and chymotrypsin break down large proteins into smaller peptides, a process called proteolysis
What are enterokinases secreted by?
Crypt cells
What are endopeptidases vs exopeptidases?
Endopeptidases (endoproteinase) are proteolytic peptidases that break peptide bonds of nonterminal amino acids (i.e. within the molecule), in contrast to exopeptidases, which break peptide bonds from end-pieces of terminal amino acids
Can endopeptidases break down proteins into monomers?
No, only exopeptidases can break down proteins into monomers
What is a particular cause of endopeptidases?
The oligopeptidase, whose substrates are oligopeptides instead of proteins
Give 3 examples of endopeptidases
- Trypsin
- Chymotrypsin
- Elastase
Give an example of an exopeptidase
Carboxypeptidase
What are large peptides broken down by/into?
Chymotrypsin and elastase –> peptide with C-terminal neutral amino acid (e.g. Ser)
Trypsin –> peptide with C-terminal basic amino acid (e.g. Arg)
What are peptides with C-terminal neutral amino acid (e.g. Ser) broken down by/into?
Carboxypeptidase A
Into short peptides, free neutral and basic amino acids
What are peptides with C-terminal basic amino acid (e.g. Arg) broken down by/into?
Caboxypeptidase B
Into short peptides, free neutral and basic amino acids
What is carboxypeptidase?
A pancreatic brush border enzyme –> splits one amino acid at a time
Small peptides produced by soluble proteases diffuse to the brush border. What happens here?
Membrane-bound peptidases chop them up into amino acids and dipeptides
How are amino acids taken up?
by sodium-linked secondary active transporters.
How are di- (and sometimes tri-) peptides are taken up?
By proton-linked secondary active transporters
What does the proton gradient depend on?
Na+/H+ exchange, which depends on the sodium gradient
What is a dipeptide?
2 amino acids linked together
What are Peyer’s patches?
Patches of lymphoid tissue or lymphoid nodules on the walls of the ileum in the small intestine (differentiates the ileum from the duodenum and jejunum)
What do Peyer’s patches function as?
the immune surveillance system of the intestinal lumen and facilitate the generation of the immune response within the mucosa
What can M cells do?
Take in big chunks of proteins, phagocytosis, then sends out antigens (undigested big chunks of proteins)
Where are M cells?
Overlying Peyer’s patches
Where is protein not degraded?
M cells
Where is bile made/stored?
Liver / gall bladder
What are lipids broken down into?
Fatty acids and glycerol
What is a triglyceride?
An ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans
What are triglycerides broken down by/into?
Pancreatic lipase breaks down the triglycerides into 2 free fatty acids and 1 monoglyceride
How does bile aid pancreatic lipase?
Bile salts attach to triglycerides to help emulsify them and aid access by pancreatic lipase. This occurs because the lipase is water soluble, but the fatty triglycerides are hydrophobic. The bile salts are the main thing that holds the triglycerides in their watery surroundings until the lipase can break them into the smaller components that can enter the villi for absorption.
Describe structure of bile salts
Fat soluble on one side and water soluble on the other
Fat soluble side surrounds edge of fat globule
Water soluble side point outwards into solution
EMULSIFICATION
Once broken down, what do the monoglycerides and fatty acids complex with?
Bile salts which solubilise them –> ‘mixed micelles’
Where do these ‘mixed micelles’ then go?
Diffuse close to the brush border, delivering their contents to the membrane.
Inside the cell, what happens to triglycerides?
Triglycerides are resynthesised and packaged into chylomicra (as not fully water soluble), which are exocytosed into the interstitium
They can’t get into capillaries, but can get into lymphatic lacteals (not blood stream but into lymphatic system)
How are bile salts recycled?
Conjugated bile salts are actively absorbed in the distal (terminal) ileum.
How are some bile salts lost?
Some escape to the colon, and may be deconjugated by bacteria, making them lipophilic and allowing passive reabsorption. About 5% are lost in the faeces.
How is iron absorbed?
Starts in duodenom
Absorbed in Fe2+ state
How is calcium absorbed?
Ca2+ is actively reabsorbed in the duodenum
What is calcium absorption regulated by?
Vitamin D
In the rest of the SI, how is Ca reabsorbed?
Paracellularly
What are the fat-soluble vitamins?
A, D, E and K
How are fat-soluble vitamins absorbed?
Absorbed with lipids
dissolve in lipid droplets, micelles, chylomicrons
What are the water-soluble vitamins?
B vitamins and vitamin C
How are water-soluble vitamins absorbed?
Require special transport proteins – usually Na+-linked
How is vitamin B12 (Cobalamin) absorbed?
Absorbed only when bound to intrinsic factor (secreted by gastric parietal cells)
What is intrinsic factor secreted by?
Gastric parietal cells
What is Na+ and K+ secretion in the colon regulated by?
Aldosterone
Summary:
The small bowel secretes alkaline fluid into the lumen, and hormones into the blood
It receives a lot of secretions from the liver and pancreas
It is the main site of digestion of ingested macromolecules
It is the main site of absorption of nutrients, minerals, and vitamins.
