5b.) The Intestines Flashcards
Define absorption
Movement of electrolytes, water and nutrients form the gut lumen to blood
Compare the jejunum and ileum in terms of:
- Where they are
- The thickness of their intestinal walls
- Length of vasa recta
- Number of arcades
Describe the blood supply to the intestines
SMA gives off following branches:
- Jejunal and ileal arteries: jejenum & ileum
- Ileocolic: terminal ileum, caecum
- Right colic: ascending colon
- Middle colic: transverse colon
Ileocoli, right colic and middle colic anastomose to form the marginal artery
Describe the venous drainage of the intestines
- Superior rectal vein becomes IMV when it comes out of pelvis
- Inferior mesenteric vein joins the splenic vein
- Splenic vein adn superior mesenteric vein join to form the portal vein which forms behind the neck of the pancreas
Describe the properties of chyme once it has left duodenum, include:
- Tonicity
- pH
- Digestion
- Isotonic
- Neutral
- Digestion nearly complete
Absorption requires a large surface area; describe 3 structural features of intestines that increase surface area
- Plicae circulares (permanent intestinal folds)
- Villi
- Microvilli
State the 3 main epithelial cell types in intestines and what each one does
- Enterocytes (most)= absorptive cells
- Goblet cells= mucus secreting
- Enteroendocrine= produce hormones
How often is the mucosa of intestinal glands shed?
Constantly shed evey 3-6 days
Describe the structure of intestinal glands (crypts of Lieberkuhn)
???
Only monosaccharides can be absorbed; true or false?
True
State the three monosaccharides that can be absorbed
- Glucose
- Fructose
- Galactose
Glucose can only be absorbed alongside Na+; true or false
True
State 3 common dietary carbohydrates
- Starch (polysaccharide)
- Lactose (disaccharide)
- Sucrose (disaccharide)
Describe the structure of starch
Consists of:
- Amylose: straight chains of glucose with alpha 1-4 bonds
- Amylopectin: branched chains of glucose with alpha 1-4 bonds on straight chains and alpha 1-6 bonds on branch points
Which bonds, in starch, can salivary and pancreatic amylase break?
What do you end up with once they have broke these bonds?
Salivary and pancreatic amylase can break alpha 1-4 bonds:
- In amylose: this produces the disaccharide MALTOSE
- In amylopectin: this produces shorer but still branched chains of glucose called ALPHA DEXTRINS
What enzyme is required to break alpha 1-6 bonds (in branches of amylopectin in starch)?
Isomaltase
State the monosaccharide components of maltose
2 x glucose
Summarise starch digestion
- Salivary and pancreatic amylase break alpha 1-4 bonds in produce:
- Maltose from amylose
- Alpha dextrins from amylopectin
- Isomaltase required to break alpha 1- bonds in amylopectin
State the monosaccharide components of lactose
Glucose and galactose
State the monosaccharide components of sucrose
Glucose and fructose
What enzyme breaks lactose into it’s monosacchardie components?
Lactase
(Lactose -> glucose + galactose)
What enzyme breaks sucrose into its monosaccharide components?
Sucrase
Glucose + fructose
What is starch?
Polysaccharide of glucose
Describe glucose and galactose absorption in enterocyte
- NA+/K+ ATPase on basolateral membrane creates low intracellular Na+
- SGLT-1 transports glucose or galactose coupled to Na+ into enterocytes (energy from dissipation of Na+ gradient)
- Glut2 transports glucose or galactose into plasma down concentration gradient
Describe absorption of fructose in enterocyte
- Facilitated diffusion of fructose via GLUT5 transporter into enterocyte
- Fructose can leave via GLUT2 or it’s own transporter
State two brush border enzymes involved in starch digestion
- Maltase (maltose -> glucose + glucose)
- Isomaltase (breaks alpha 1-6 bonds)
Briefly describe/state the 4 stages of protein digestion
- Stomach (H/pepsin)
- Intestinal lumen (Trynpsin…..)
- Brush border
- Cytosol (cystosolic peptidases
Describe stage 1 of protein digestion in the stomach
- Pepsinogen released from chief cell
- HCL converts pepsinogen -> pepsin
- Pepsin digests proteins into either oligopeptides or amino acids
- Oligopeptides or amino acids move to small intestine
Describe stage 2 of protein digestion in the small intestine
- Pancreas releases zymogens
- Zymogens acitvated in intestinal lumen
- Trypsinogen is activated by enteropeptidase (enterokinase) into trypsin
- Trypsin activates many other zymogens
Endopeptidases- produce shorter poplypeptides
Exopeptidases- produce dipeptides or amino acids
Describe stage 3 of protein digestion
- Enterocytes have proteases/peptidases in their brush border
- These enyzmes break down proteins further but can’t always break them down into aa
- HOWEVER, intestine can absorb short peptides (as well as aa) by Peptide transporter 1 (PepT1)
- AA transported in cell using amino acid transporters
Describe stage 4 of protein digestion
Cytosolic peptidases, in cytosol of enterocyte, break down small peptides (transported in by PepT1) into amino acids. NOTE: certain di- and tri- peptides can also be absorbed into the blood hence these cytoslic peptidases don’t have to break them down into single aa all the time
Explain the difference between exopeptidases and endopeptidases
Provide examples of each
- Endopeptidases: tyrpin, chymotrypsin, elastase
- Exopeptidases: carboxypeptidase (A&B)
Explain what drives the movement of water out of lumen of intestine into enteroctyes
- Na+/K+ ATPase in basolateral membrane
- Decreases intracellular [Na+]
- Na+ diffuses into enterocyte down conc gradient taking other stuff with it e.g. electrolytes, glucose….
- Creates osmotic gradient
- Water moves from area of high water potential to an area of low water potential
- Fluid absorbed is isosmotic
Compare the transporters on both the basolateral and apcial membrane in small and large intestine
Relate this to their ability to absorb water
Small Intestine
- Apcial membrane: Na+ cotransported wtih e.g. glucose, aa
Large Intestine
- Apical membrane: Na+ channels which are induced by aldosterone
Aldosterone can increase amount of Na+ body abosrobs- has greater effect on large intestine than small intestine
JUST CHECK THIS FC
Explain the principles behind oral rehydration fluid
Oral rehydration fluids contain a mix of salt and gluocse to stimulate maximum water uptake. Increased Na+, increases amount of Na+ absorbed and therefore the amount of glucose absorbed (as it is co transported with Na+) this generates greater osmotic gradient to stimulate water uptake
Describe water secretion into intestine
- Cl- enters crypt epithelial cell via NKCC1 (transported with Na+ and K+ adn 2Cl-)
- Cyclic AMP levels increase in cell
- Incrased cyclic AMP activates CFTR
- Cl- ions secreted
- Na+ is drawn into lumen across tight junctions
- NaCl secretion creates osmotic gradient to move water into lumen
JUST CHECK
Describe some causes of vitamin B12 (cobalamin) deficiency
State potential consequences of B12 deficiency
- Lack of intrinsic factor: e.g. if parietal cells damaged
- Hypochlorhydria: inadequete stomach acid possibly due to gastric atrophy or PPIs; acis is important in initial release of B12
- Inadequete intake: often vegetarians
- Inflammatory disorders of ileum: as this is where it is absorbed e.g. in Crohn’s disease
Consequences: megaoblastic anaemia, neurological symptoms (see MEH for more)
Describe the symptoms of lactose intolerance and why you get them
Lactose cannot be digested due to lack of lactase (different types of lactose intolerance, see BL) hence lactose remains in gut leading to:
- High osmotic effect- draw water in- diarrhoea
- Lactose is fermented- gas - bloating
After what age is the enzyme lactase less expressed?
2 years
State some symptoms of IBS
State who’s more affected in terms of gender, age, other morbidities
- Abdo pain (sometimes relieved by defaecation)
- Bloating
- Flactulence
- Diarrhoea/constipation
- Rectal urgency
More common in females (2:1), 20-40s most affected, more common in those with psychological disorders (maybe due to low serotonin and serotonin has role in enteric nervous system)
What is coeliac disease?
Immunological response to gliadin fraction of gluten which is found in wheat, rye and barley results in damage to mucosa of intestines e.g.:
- Absence of intestinal villi
- Hypertrophy/lengthening of intestinal crypts
- Lymphocytes infiltrate epithelium and lamina propria
- Impaired digestion/malabsorption
There is a high risk of coeliac disease in what type of twins?
Monozygotic
State some symptoms of coeliac disease
- Diarrhoea
- Weight loss
- Flactulence
- Abdo pain
- Anaemia (impaired Fe absorption)
- Neurological symptoms (hypocalcaemia)
Most related to malabsorption
State 2 investigations for coeliac disease and state which is “gold standard”
Bloods:
- IgA antibodies to smooth muscle endomysium and tissue transglutaminase
Upper GI endoscopy:
- Duodenal biopsy to look for mucosal pathology and absent villi *GOLD STANDARD
How do you treat coeliac disease?
Gluten free diet. Get clinical improvement quickly (days/weeks) and histological improvement (weeks/months)
Describe two benefits of fibre
- Fibre helps lower cholesterol: fibre binds bile salts, which are made of cholesterol, meaning we have to make more bile salts and hence use up more cholesterol
- Bacteria in large bowel can use fibre. Good diversity of colonic bacteria linked to whole body general health
Describe how stem cells in intestinal glands (crypts of Leiberkuhn) help to regenerate intestinal epithelia
Why are paneth cells useful to stem cells?
- Stem cells at base of gland migrate to surface
- Mature as they migrate into variety of cell types
Paneth cells produce antimicrobial peptides which protect stem cells against infection
What do we mean when we talk about segmentation in the intestines?
Idea that intestines can contract to push food along but then contract at a more distal part and cause almost a shunting effect; keeps food in intestines for longer to maximise absorption
Portal vein supplies what percentage of blood going to the liver
70-80%
Describe the pathophysiology of coeliac disease
Autoimmune response to the gliadin portion of gluten
What is gluten found in?
Wheate, rye, barley and oats
Describe symptoms of coeliac disease
- Diarrhoea
- Flatulence
- Weight loss
- Abdominal pain
- Steatorrhoea
- Fatigue
Explain why you get weight loss, diarrhoea and flatulence in coeliac disease
- Weight loss: autoimmune reaction to gliadin in gluten destroys villi so leads to malabsorption
- Diarrhoea: malabsorption increases osmolarity of lumen of gut hence less water leaves gut/more is drawn in leading to diarrhoea
- Flatulence: bacteria in gut ferment unabsorbed substances in bowel and produce gas
How is coeliac disease diagnosed?
- Blood test: serum antibodies to tTG
- Upper GI endoscopy and duodenal mucsoal biopsy (will often repeat this after gluten has been removed from diet)
How do you treat coeliac disease?
Gluten free diet
