The small intestine. Flashcards
Describe the functions of villi and microvilli in the small intestine.
Amplify the surface area available for interaction with food.
Where would you find brush border enzymes?
Integral membrane proteins on the luminal surface of the intestinal microvilli.
What is the function of brush border enzymes?
To break down materials in contact with brush border.
Define peristalsis.
Waves of muscular contractions which move contents along the length of the GI tract.
Describe the action of circular and longitudinal muscles during peristalsis.
Circular muscles contract behind bolus, while circular muscles ahead of bolus relax. Longitudinal muscles ahead of bolus contract,shortening adjacent segments. Wave of contraction in circular muscles forces bolus forward
What is segmentation and what is its function?
Alternate contraction of neighbouring segments. Churn and fragment the bolus. Mix contents with intestinal secretions
What is the migrating motor complex and what is its function?
When the gut is relatively empty, MMCs are generated roughly every 90 minutes. Stimulated by motilin, secreted by M cells (and by erythromycin). Suppressed by feeding. For a few minutes, a series of strong, slow, peristaltic waves will sweep down, mostly from the stomach, and along the small bowel. Pyloric sphincter is relaxed, allowing larger things to pass These waves are thought to help keep the gut clean, prevent reflux, and reduce bacterial growth.
Which nutrients must be digested before they can be absorbed?
Carbohydrates, lipids and proteins.
Which bonds within the starch polymer can be hydrolysed by amylase?
Internal α1,4 bonds.
Following initial starch digestion by amylase, where are the remaining short-chain carbohydrates digested and how?
At the brush border by brush border enzymes.
How are glucose and galactose absorbed in the small intestine?
Actively via SGLT.
How is fructose absorbed in the small intestine?
Passively via GLUT5.
What percentage of proteolysis takes place in the stomach?
15%
From where is enterokinase secreted and what is its function?
Secreted from crypts of Lieberkühn. Converts trypsinogen to trypsin.
What are the functions of trypsin?
Endopeptidase (serine protease), activates further trypsinogen via autoactivation, activates other pancreatic zymogens via hydrolysis.
Name the pancreatic zymogens secreted into the small intestine actived by trypsin.
Chymotrypsinogen, proelastase, procarboxypeptidase A & B
Why is it important that these enzymes are produced as zymogens and only become active once entering the small intestine?
To prevent autodigestion of the pancreas - can occur in pancreatitis.
Which of the main peptidases are endopeptidases?
Trypsin, chymotrypsin, elastase .
Which of the main pepsidases are exopeptidases and what does this mean?
Carboxypeptidases A & B. They cut at the terminal peptide bond.
What happens to the small peptides produced by the soluble peptidases?
They diffuse to the brush border where they are digested by membrane-bound proteases to mono and dipeptides.
How are amino acids taken up by the small intestine?
Sodium-linked secondary active transporters.
How are di- and tri- peptides sometimes taken up?
By proton-linked secondary active transporters.
How is the proton gradient across small intestinal luminal cells created?
Luminal Na+/H+ antiporters are linked to the sodium gradient, maintained by the basal 3K+/2Na+ ATPase.
What is the function of bile salts?
The emulsification of lipids to increase digestive surface area.
How does pancreatic lipase work?
Cleaves the ‘outside’ fatty acids leaving 2 free fatty acids and one monoglyceride.
What name is given to the complex of bile salts and free fatty acids/monoglycerides?
Micelle
What happens to micelles?
They diffuse close to the brush border, delivering their contents to the cell membrane.
What happens to lipids once inside the cell?
Inside the cell, triglycerides are resynthesised and packaged into chylomicra, which are exocytosed into the interstitium.
What happens to chylomicra in the interstitium?
They can’t get into capillaries, but can get into lymphatic lacteals.
Where/how are conjugated bile salts reabsorbed?
Actively in the distal ileum.
What happens to bile salts that reach the colon?
They are deconjugated by bacteria, rendering them lipophilic and allowing passive reabsorption.
What percentage of bile salts are lost in the faeces?
5%
How is Fe3+ reduced to Fe2+?
Duodenal cytochrome B (Dcytb) at the luminal surface.
How does Fe2+ enter the luminal cell?
DMT co-transports Fe2+ with H+
What happens to non-heme Fe2+ once it has entered the luminal cell?
Tranfers to mobilferrin.
What happens to heme - Fe2+ once it has entered the luminal cell?
Heme oxygenase oxidises heme Fe2+ and releases Fe3+. Heme is broken down to bilirubin.
What happens to Fe2+ at the basal surface of the cell?
Leaves the cell via ferroportin. Fe2+ is then oxidised to Fe3+ by hephaestin. Iron binds to tranferrin in plasma.
How is Ca2+ absorbed in the gut?
Ca2+ is actively reabsorbed in the duodenum
Regulated by Vit. D
In the rest of the small bowel, it is reabsorbed paracellularly.
How are fat-soluble vitamins absorbed?
Via micelles and chylomicrons.
Which vitamins are fat soluble?
A, D, E and K
How are water-soluble vitamins absorbed from the gut lumen?
Require special transport proteins – usually Na+-linked
How is vitamin B12 absorbed?
Absorbed only when bound to intrinsic factor (secreted by gastric parietal cells).
In what part of the small intestine are glucose, galactose and fructose absorbed?
The duodenum.
In what part of the small intestine are calcium, magnesium and iron absorbed?
Duodenum.
Where in the small intestine are lipids and fat-soluble proteins absorbed?
Duodenum and ileum.
Where in the small intestine are Na+, K+ and Cl- absorbed?
Jejunum
In what part of the small intestine are amino acids and peptides absorbed?
Jejunum and ileum.
In what part of the gut are bile acids reabsorbed?
Colon
