Session 6 - The Intestines Flashcards
How does the small intestine give great surface area for absorption?
- Plicae Circulares - large folds
- Mucosa folded into Villae.
- Microvilli - cell surface covered in these (brush border)
Slow movement! for increased absorption.
What are the main types of cells in the small intestine?
- Epithelial cells: enterocytes
- Goblet cells: produce mucus
Intestinal Crypts
- Stem cells at base
- Enteroendocrine cells
- Paneth cells (immune function)
What is digested and absorbed in the small intestine?
Carbohydrates, Proteins and Lipids
How are carbohydrates digested and absorbed?
- Poly/di-saccharides broken down to monosaccharides (only molecules that can be absorbed)
- Final breakdown is at brush border (brush border hydrolases)
- Glucose can only enter with Na+ (cotransport)
What are the main monosaccharides?
Glucose, Fructose, Galactose
What are common dietary carbohydrates?
Starch, Lactose, Sucrose.
Which forms is starch found in?
Amylose (straight a 1-4 bonds)
Amylopectin (branches - a 1-6)
Which enzymes can break down starch?
- Amylase - breaks down amylose (to glucose or maltose)
- Alpha dextrins - break down amylopectin into smaller molecules, retain branching though.
- Isomaltase - break down the bonds between branches.
What is maltose, what breaks it down?
Disaccharide composed of two glucose. Maltase.
What is Lactose made up of, what breaks it down?
Disaccharide of glucose and galactose. Lactase.
What is Sucrose made up of? What breaks it down?
Disaccharide containing glucose and fructose. Sucrase.
How are monosaccharides absorbed in the intestine?
Glucose/ Galactose via SGLT-1 on apical membrane with Na+
Fructose via Glut5 on apical membrane.
Move into blood via GLUT2 on basolateral membrane.
What drives the absorption of monosaccharides into the enterocytes?
Na/K ATPase on basolateral membrane establishes Na gradient, which drives Na and thus glucose/ galactose into the cell via SGLT-1.
How does oral rehydration therapy work?
Mixture of glucose and salt. Glucose uptake stimulates Na+ uptake.
Uptake of Na+ generates an osmotic gradient, thus water follows.
The mixture stimulates maximum water uptake.
How are proteins broken down at the stomach and small intestine?
In Stomach:
- Pepsinogen released from chief cells, converted to pepsin by HCL in stomach.
Small Intestine:
- Pancreas releases proteases as zymogens.
- Trypsinogen: is converted to trypsin by enteropeptidase.
- Trypsin then activates other proteases.
What are exo/ endopeptidases?
- Exopeptidases break bonds at the ends of the polypeptide to produce dipeptides/ amino acids.
- Endopeptidases break bonds in the middle of the polypeptide producing shorter polypeptides.
What are the major proteases in protein digestion?
Endopeptidases:
- Trypsin
- Chymotrypsin (activated by trypsin)
- Elastase
Exopeptidases:
- Carboxypeptidase (A+B)
Which protein products are absorbed in adults?
Only amino acids, dipeptides, and tripeptides are absorbed.
How are amino acids transported into the cells in the intestines?
- Via Na+/ amino acid co-transporters (similar to glucose).
In what form are most protein products ingested? How are they transported?
Most moved as di/tri-peptides. (not AA)
- Di/tri-peptides moved by H+ co-transporter: Peptide transporter 1 (PepT 1).
- Inside cell these are converted to amino acids.
What is the difference between electrolyte/water uptake in the small and large intestine?
- Both have basolateral Na/K ATPase.
Small Intestine: Na+ is co-transported.
Large Intestine: Has Na+ Channels, induced by aldosterone.
How is calcium uptake orchestrated in the small intestine?
(when Ca intake low)
- Active transcellular absorption - Ca+ ATPase at basolateral membrane.
- The process requires Vitamin-D. (calbindin)
- This is stimulated by parathyroid hormone.
How does calcium move into the body when calcium intake is normal/high?
It moves passively, by paracellular absorption.
How is iron absorbed in the intestine?
- Mostly in haem/Fe2+ form.
- Iron is absorbed across apical membrane, co-transported with H+.
What happens to iron absorption and storage when iron levels are low or high?
Low
- Iron binds to transferrin (transported to stores: mainly ferriting complexes in bone marrow, liver and spleen. Half of iron is in haemoglobin!)
High
- Iron contained in ferritin complexes (trapped in cell).
This is lost when the enterocyte is replaced.
How is vitamin B12 absorbed in the gut?
- It is absorbed in the terminal ileum, bound to intrinsic factor. (IF)
- IF is secreted by gastric parietal cells.
What may cause a B12 deficiency?
Reduced secretion of Intrinsic Factor by gastric parietal cells, due to gastritis.
Or terminal ileal removal.
What are the symptoms of coeliac disease?
- Diarrhoea
- Flatulance
- Weight loss
etc.
What is coeliac disease?
- Intolerance of the gliadin fraction of gluten.
This is found in wheat, rye and barley.
The intolerance results in an immune response which damages the mucosa of the intestines.
What damage does coeliac disease do to the intestines?
- Absence of intestinal villi.
- Lengthening of intestinal crypts.
- Lymphocytes infiltrate epithelium.
All causes impaired digestion/ malabsorption.
What investigations would be done to rule out/ confirm coeliac disease?
- Upper GI endoscopy/ biopsies (duodenum)
(look for pathology/ villi structure) - Blood tests: serology, electrolyte imbalance, anaemia.
- Treatment: diet
