10. The Intestines Flashcards
What size of carbohydrates can be absorbed and what are the main ones that are absorbed?
Only monosaccharides (glucose, fructose, galactose)
What happens to carbohydrates of plant origin that cannot be digested in SI?
• These are utilised and partially digested by bacteria in the colon (providing nutrients for colonic mucosa)
Which ion does glucose enter with?
Na+
Give 3 examples of common dietary carbohydrates
Starch, lactose and sucrose
What chains does starch consist of and what bonds are present?
- Straight chains of glucose- Amylose (alpha 1-4)
* Branched chains of glucose-Amylopectin (alpha 1-6)
Which enzymes are required to break the bonds in amylose and amylopectin?
Amylose (alpha 1-4): salivary and pancreatic amylase
Amylopectin (alpha 1-6): isomaltase (brush border)
What is produced from amylopectin when the alpha 1-4 bonds are broken?
alpha Dextrin (shorter but still branched chains of glucose)
What are the products from digestion of starch?
- Glucose (fine)
- Maltose (maltase) = Glucose + Glucose
- Alpha dextrins (isomaltase) = Glucose
Which enzymes digests the disaccharides?
Lactose (lactase) = glucose + galactose
Sucrose (sucrase) = glucose + fructose
maltose (maltase) = glucose + glucose
(brush border enzymes)
What maintains the Na+ ions concentration gradient in the enterocytes?
Na+/K+ ATPase (3Na+ out, 2K+ in) on basolateral membrane
• Maintains low intracellular Na+
How do glucose enter enterocytes?
SGLT -1 binds Na+
• This allows glucose binding
• Na+ & glucose moves into cell
How does glucose get from the enterocytes into the blood?
GLUT2 transports glucose out of enterocyte
• Diffuses down gradient into capillary blood
How does fructose enter enterocytes?
Fructose uses GLUT5 transporter to enter enterocyte
• Facilitated diffusion
Which 4 places do proteins get digested and by what?
- Stomach- (H+/pepsin)
- Intestinal lumen- (Trypsin…)
- Brush border
- Cytosol (cytosolic peptidases)
Where is pepsinogen released from and what causes its activation?
Chief cells in the stomach, converted to pepsin by HCL
What are the products of protein digestion by pepsin?
- Oligopeptides/amino acids
* Move to small intestine
In what form does the pancreas release proteases?
Pancreas releases proteases as zymogens –(move into intestinal lumen to be activated)
• Trypsinogen is important
What activates trypsinogen?
Enteropeptidase (enterokinase)
What does trypsin lead to activation of?
Various other proenzymes:
- Trypsinogen
- chymotrypsinogen
- proelastase
- procarboxypeptidase A
- procarboxypeptidase B
What is the difference between endo and exopeptidase?
Endo: breaks bonds in the middle of the polypeptide to produce shorter polypeptides
Exo: Breaks bonds at the end of polypeptides to produce dipeptides and amino acids
Give examples of endo and exopeptidases.
Endo: trypsin, chymotrypsin, elastase
Exo: carboxypeptidase A&B
What does the brush border contain that aids protein digestion?
Brush border- also contains proteases
•The enterocytes express peptidases in their brush border
•Sometimes cannot completely digest proteins down to amino acids
What size peptides can be absorbed?
short peptides and amino acidds
Which transporter absorbs di/tripeptides?
PepT1 (peptide transporter 1)
How are amino acids transported into enterocytes?
• Na+-amino acid co-transporters
What happens to the small peptides in enterocytes?
- The small peptides are then acted on by cytosolic peptidases (broken down to amino acids)
- Certain di- and tri-peptides can also be absorbed into blood
How is water absorbed?
Absorption driven by movement of sodium into enterocytes
•Na+ moved by active transport out of cell on basolateral membrane
• Na+ diffuses into epithelial cells
• Water then moves by osmosis
• Movement occurs through para-cellular and transcellular paths
*Osmotic gradient from all absorption leads to uptake of water
◦ Fluid absorbed is isosmotic
What is the difference in Sodium uptake between small and large intestine?
• Both have Na+-k+ ATPase on basolateral membrane
• Apical membrane
- Small intestine- Na+ is co-transported with (glucose, amino acids…)
- Large intestine- Na+ channels
What hormone induces Na+ channels in the large intestine?
Aldosterone, increases Na+ uptake when blood pressure is low
What is the principle of oral rehydration therapy?
Mixture of glucose and salt will stimulate maximum water uptake:
- glucose stimulates Na+ uptake
- Na+ uptake creates osmotic gradient so water follows
What is secretion of water driven by?
Chloride movement (predominantly)
Describe the mechanism for Chloride ion secretion.
- Chloride enters crypt epithelial cell, Co-transported with Na and K
- Cyclic AMP levels increase inside cells
- Increased cAMP activate CFTR
- Cl ions are secreted
- Na is drawn into lumen across tight junctions
- NaCl secretion creates osmotic gradient
- Water moves into lumen
What does B12 deficiency cause?
Megaloblastic anaemia and neurological symptoms
What are the causes of B12 deficiency?
•Lack of intrinsic factor (IF is released by parietal cells)
- B12 is bound to IF in the small intestine and transports this to distal ileum
- Most reabsorbed
•Hypochlorhydria (inadequate stomach acid)
- Acid is important in initially releasing cobalamin
- Gastric atrophy, PPIs
•Inadequate intake in food (strict vegetarians)
•Inflammatory disorders of ileum (where it is absorbed)
- Crohn’s disease
What is lactose intolerance?
Caused by deficiency of the enzyme lactase (brush border enzyme)
•After the age of 2 years the enzyme is expressed less
Why do symptoms of lactose intolerance occur?
When lactose is consumed in quantity it cannot be absorbed
• Remains in gut lumen created high osmotic effect
• Water is not absorbed resulting in diarrhoea
• Lactose is fermented in gut producing flatus/bloating
What are the symptoms of irritable bowel syndrome?
- Abdominal pain (often cramping, sometimes relieved by defaecation)
- Bloating
- Flatulence
- Diarrhoea/constipation (sometimes alternating)
- Rectal urgency
Who are IBS more common in?
- More common in females vs males (2:1)
- 20s-40s most affected age range
- More common in association with psychological disorders
What is the cause of IBS?
Causes are multifactoria
What is coeliac disease?
Immunological response to the gliadin fraction of gluten (found in wheat, rye, barley)
What damage does coeliac disease cause?
Damages mucosa of intestines
• Absence of intestinal villi
• Hypertrophy/Lengthening of intestinal crypts
• Lymphocytes infiltrate epithelium and lamina propria
• Impaired digestion/malabsorption
What are the symptoms of coeliac disease?
- Majority related to malabsorption (diarrhoea, weight loss, flatulence, abdo pain)
- Anaemia (impaired iron absorption), neurological symptoms (hypocalcaemia)
How do genetic factors affect coeliac disease?
Genetic factors (high concordance monozygotic twins (75%) • Very common (underdiagnosed) estimates of 1% of western population affected
What are the investigations for coeliac disease?
• Bloods- Immunoglobulin A (IgA) antibodies to smooth muscle endomysium and tissue
transglutaminase
• Upper GI endoscopy + biopsies (duodenum)
• Mucosal pathology
• Villi are reduced or absent
What is the treatment for coeliac disease?
- Diet- strict gluten free diet
- Clinical improvement quite quick (days/ weeks)
- Histological improvement (weeks/months
