Flashcards in AS Lecture 3 - Small Intestine Deck (64)
What is the structure of the digestive epithelium/SI?
External wall has longitudinal and circular muscles
How is the internal mucosa arranged?
Arranged in circular folds and covered in villi - has invaginations
What is the name for the intestinal invaginations?
Crypts of Lieberkuhn
What are the function of villi?
Increase SA in the si to allow for maximum absorption of nutrients
Motile and have a rich blood supply/lymph drainage for absorption of digested nutrients
Good innervation from submucosal plexus
What are the cell types of the SI?
Mucosa lined with: enterocytes (primarily), goblet cells (scattered), enteroendocrine
Paneth and stem cells - crypts
What are enterocytes?
Tall columnar with micro villi and basal nucleus
Short lifespan of 1-6 days
What are microvilli?
Make up brush border, several 1000 per cell
Surface covered in glycocalyx
How is surface area increased with villi and microvilli?
Villi and Microvilli increase SA from 0.4m^2 to 200m^2
What is the function of goblet cells?
Secrete mucus, 2nd most abundant cell in SI
Mucus secretion granules accumulate at apical end
What is mucus and what does it do?
Large glycoprotein that facilitates passage of material through the bowel
What are enteroendocrine cells and what is their function?
Columnar epithelial cells, scattered among absorptive cells in lower parts of the crypt
Hormone secreting to influence gut motility
What is the function of Paneth cells?
Contain large acidophilic granules (lysozyme and glycoproteins and zinc) which help to protect against bacteria - may be protecting stem cells
Engulf bacteria and Protozoa so may have roles in regulating intestinal flora
What is the epithelial lifespan?
Cell proliferation, differentiation and death is a continuous cycle in gut
Stem cells in crypts replace the other cells
Enterocytes and goblet cells have a lifespan of 36hrs
What are stem cells and how does this relate to the epithelial life span?
Pluripotent/undifferentiated cells, cells move up from crypts to replace dead cells (by apoptosis) from the surface of the epithelium. They continually divide by mitosis
Why is there such a rapid turnover in the small intestine as it takes up so much energy?
Enterocytes are first line of defence against pathogens and toxins, so anything that disturbs the cell function/metabolic rate/causes lesions will be short lived, as they will be rapidly displaced and replaced
Severe intestinal dysfunction can occur if it doesn't have a rapid turnover
How does the cholera enterotoxin cause cholera?
Symptoms: Extreme diarrhoea, massive dehydration and death
Cause: Results in opening of Cl- channels in SI allowing uncontrolled secretion of water
Treatment: rehydration until cholera bacteria will clear and epithelium will be replaced
What are some histological features of the Duodenum?
Have Brunner's glands, which open into base of crypts
What are some histological features of the Jejunum?
Plicae circulares/valves of Kerckring - numerous, large folds in submucosa
Also present in duodenum/ileum but tend to be taller, thinner and more frequent in JJM
What are some histological features of the Ileum?
Lots of Peyer's patches - helps prime immune system against intestinal bacteria. ALSO, have bactericidal Paneth cells and rapid cell turnover to defend the SI - prevent colon bacteria from moving up the gut
List 3 functions of SI motility
Mix ingested food w/digestive secretions and enzymes
Facilitate contact between intestine and intestinal mucosa
Propel intestinal contents along alimentary tract
What is Segmentation
Mixes contents of lumen, causing stationary contraction of circular muscles at intervals
More frequent in duodenum which allows pancreatic enzymes and bile to mix with chyme
Chyme moves in both directions but net movement is towards colon
Not very organised
What is Peristalsis in SI?
Sequential contraction of adjacent rings of smooth muscle
Propels chyme towards colon - waves travel about 10cm
Segmentation and peristalsis result in chyme being segmented, mixed and propelled towards colon
What is the Migrating motor complex?
Cycles of SM contractions (when fasting), cycle of contractions of adjacent segments of small intestine - sweeping to colon, and once it reaches ileum, it begins in duodenum again
Can occur during fed state but less ordered/frequent
How does digestion occur in duodenum?
SI digestion occurs in alkaline env - digestive enzymes and bile enter duodenum from pancreatic duct/bile duct (hepatopancreatic sphincter)
Duodenal epithelium also produces its own digestive enzymes
Which mechanisms of absorption are used in the SI?
Primary/secondary active transport (require energy from ATP hydrolysis/electrochemical gradient respectively)
All need carrier proteins apart from passive diffusion
How does digestion of carbohydrates occur?
Begins in mouth by salivary alpha-amylase, which is destroyed in stomach
Most occurs in small intestine
What are the types of carbohydrate structures?
Simple - monosaccharides/disaccharides
Complex - polysaccharides
How does pancreatic alpha amylase function?
Secreted into duodenum in response to meal
Continues starch and glycogen digestion in SI
Need Cl- for optimum activity and neutral/alkaline pH (Brunner's gland secrete alkaline)
Where does pancreatic alpha amylase act?
Mainly in lumen - some adsorbed onto brush border
Digestion of complex carbs (starch/glycogen) into simpler sugars in lumen, then disaccharides digested on membrane into glucose