Small bowel and large intestine Flashcards
Small intestine anatomy; Digestive epithelium; Large intestine anatomy; Large intestine function; Intestinal microbiome
What are the 3 main divisions of the small intestine and what are their approximate lengths?
Duodenum - 0.25m
Jejunum - 2.5m
ileum - 3.75m
What are 3 noteworthy features of the small intestine?
Villi that increase the internal sfa
Mucosa arranged in folds
Invaginations called cypts of Lieberkühn
How is the small intestine functionally separated?
Pyloric sphincter at proximal end
Ileocaecal valve at distal end
What are the 4 sections of the duodenum?
1) Duodenal cap - superior
2) Major duodenal papilla - descending
3) Crosses IVC - inferior
4) Duodenojejunal flexure -ascending
What is secreted in the duodenum and why?
Bicarbonate rich alkaline solution is secreted to:
- neutralise acid chyme
- Protect lining
- provide optimum pH
What differentiates the jejunum from the ileum?
Perfusion
jejunum has longer vasa recta and less prominent arterial arcades
ileum has prominent arterial arcades and shorter vasa recta
What are features of the jejunum?
plicae circularis - large submucosal folds like frills
found in upper left quadrant
long vasa recta
less prominant arterial arcades
What are features of the ileum?
Up to 100 Peyer’s patches - GALT that contain M cells that have no microvilli (initiate leukocyte and immunoglobulin responses to gut borne pathogens)
found in lower right quadrant
Shorter vasa recta
prominent arterial arcades
What are the roles associated with each section of the small intestine?
Duodenum - digestion and gut regulation
Jejunum - absorption
Ileum - absorption
Where do bile and pancreatic juice enter the small intestine?
Ampulla of Vater→Sphincter of Oddi→Major duodenal papilla
Where are digestive secretions in the small intestine derived from?
Liver
Pancreas
Small intestine wall
What are the two sites where enzymes are effective in the small intestine?
Gut lumen - enzymes that cleave big nutrients to small nutrients
Brush border of enterocytes - enzymes that cleave dimeric nutrients into monomers
What are the 3 main functions of intestinal motility?
mix ingested food with digestive secretions and enzymes
facilitate contact between contents and mucosal surface
propel contents along small intestine
What are the 3 main mechanisms of motility in the small intestine?
Segmentation
Peristalsis
Migrating motor complex
Describe the main features and effects of segmentation
Alternate contraction and relaxation of non-adjacent segments of circular muscle in the gut wall
Food pushed forwards then backwards
Major effect is mixing and mechanical breakdown
Minor effect is some propulsion
Describe the main features and effects of peristalsis
Sequential contraction of adjacent segments of circular muscle in the gut wall, combined with contractions of longitudinal muscle
Food propelled distally along GI tract
Major effect is propulsion
Minor effect is some mixing
Describe the main features and effects of the migrating motor complex
Periodic contractions from stomach to distal ileum during fasted state
Restart once complete
Present in fed state but less ordered
Effects are to prevent colonic flora travelling backwards and to help cleanse small intestine of residual fluid
What are the 5 main cell types in the small intestinal mucosa?
Goblet cells Enterocytes Enteroendocrine cells Paneth cells Stem cells
What are the main features and responsibilities of enterocytes in the small intestine?
Most abundant in gut
Microvilli on apical surface
Connected to each other by tight junctions
Lifespan 1-6 days
Rich network of carbohydrates called the glycocalyx
Responsible for absorption
What is the purpose of the glycocalyx?
Traps H20, mucous (and enzymes in small intestine) on cell surface
Protects cell from luminal contents
Regulates digestion and absorption
What are the main features and responsibilities of goblet cells in the small intestine?
2nd most abundant cell in gut
Apical cytosol packed with mucin granules
(distorts cell shape giving it goblet shape)
Secretes mucous
Number increases along length of small intestine
What is mucous?
Water and glycoproteins
What is the purpose of mucous in the small intestine?
Lubricate intestine to facilitate gut passage
What are the main features and responsibilities of enteroendocrine cells in the small intestine?
Hormone secreting cells
Located at the bottom of crypts
Sensory apparatus in apical portion
Secretory granules are stored and kept ready for secretion into blood supply
Can be closed- not exposed to the gut lumen
What are common examples of enteroendocrine cells and what do they secrete?
G cells - gastrin
I cells - cholecystokinin
S cells -secretin
D cells - somatostatin
What are the main features and responsibilities of paneth cells in the small intestine?
Immunological cells
Located at bottom of crypts
Contain high number of acidophilic granules
Located near stem cells
Priority to help protect progenitor cells
Engulf bateria
Regulate intestinal flora
What do acidophilic granules in paneth cells contain and what is their purpose?
Lysozyme - antibacterial enzyme
Glycoproteins - protect local cells from enzymes
Zinc - common co-factor for lysozymes
What are the main features and responsibilities of stem cells?
Continually replace enterocytes and goblet cells
Located at bottom of crypts
Proliferate in crypts them move up sides of villus
Reach top, become sentient and naturally slough off and are broken down and reabsorbed
Process extremely energy intensive
Essential to normal gut operation
What two things does the process of replacing enterocytes and goblet cells allow?
Effects of gut borne toxins/drugs to eventually run their course
Leisons are short lived and can be repaired quickly
How does radiation affect GI function?
Impairs stem cell activity
Usually results in severe GI dysfunction
What is the process of carbohydrate digestion?
Amylase in mouth and pancreatic amylase in duodenum break down chains into dimers of sucrose/maltose/lactose
Dimers broken down by brush border enzymes sucrase/maltase/lactase to glucose + fructose/glucose/galactose
How are the products of carbohydrate digestion absorbed?
Glucose + Galactose absorbed by 2° active transport via Sodium Glucose Linked Transporter 1 SGLT1
Fructose abosorbed by facilitated diffusion through Glucose transporter 5 channels GLUT5
All 3 diffuse down conc gradient through basolateral membrane via GLUT2 channels
What is the process of protein digestion?
Pepsin in stomach breaks long peptides into smaller peptides
Proteases (e.g trypsin, chymotrypsin, carboxypeptidase) in pancreatic juice continues breaking down long peptides into tripeptides and dipeptides
Final stage of digestion occurs in brush border by enzymes secreted from epithelial cells - tripeptidases and dipeptidases (endopeptidase, dipeptidase, aminopolypeptidase and carboxypeptidase)
How are the products of protein digestion absorbed?
Single amino acids absorbed into enterocytes by 2° active transport via AA/Na+ symporters
Di/tripeptides absorbed via 2° active transport by AA/H+ symporters and are broken down to amino acids by cytoplasmic peptidase enzymes
Amino acids move into blood via facilitated diffusion
What is the process of lipid digestion?
Lingual lipase from mouth hydrolyses triglycerides
Gastric lipase in stomach cleaves single fatty acid chains from free triglycerides
Mechanical churning of stomach slightly emulsifies fats to increase their sfa
Bile in duodenum provides chemical emulsification to increase sfa even more
Pancreatic juice cleaves 2 fatty acid chains from triglycerides to form monoglycerides and free fatty acids (pancreatic lipase and colipase)
As digestion products reach brush border, combine with bile salts to form micelles that can cross aqueous unstirred layer
How are the products of lipid digestion absorbed?
Lipolytic products diffuse through apical membrane (bile salts remain in lumen)
Monoglycerides and free fatty acids are resynthesised into triglycerides via 2 separate pathways:
-Monoglyceride acylation pathway
-Phosphatidic pathway
Triglycerides then packaged with proteins, phospholipisds and cholesterols = lipoprotein chylomicrons
Chylomicrons exocytosed and enter lymphatic system
What are the divisions of the large intestine?
Caecum
appendix
Ascending/transverse/descending/sigmoid colon
rectum
How is the large intestine innervated?
PNS-vagus nerve innervates ascending and transverse colon, pelvic nerves innervate distal colon
SNS-lower thoracic and upper lumbar spinal cord
How is the large intestine perfused?
Middle colic artery - ascending and 2/3 transverse colon
Inferior mesenteric artery - last 1/3 of transverse, descending, sigmoid and rectum
What are the dimensions of the large intestine?
150 cm long
6cm wide
What are the 3 main functions of the colon?
Reabsorption of water and electrolytes
Elimination of waste
Microenvironment for gut bacteria
What is the purpose of the ileocaecal valve?
Separate ileum from caecum
Tonically active and only relaxes to allow passage of fluid chyme
Prevents microbiota from migrating into ileum
What is the right hand bend of the large intestine called?
Hepatic flexure
What is the left hand bend of the large intestine called?
Splenic flexure
What is the purpose of the rectum?
Store faeces on ‘shelves’ formed by transverse folds in rectal submucosa
What are the main differences between the structure of the rectum and the rest of the colon?
No taeniae colis
transverse rectal folds in submucosa
What is the role and main features of the anal canal?
Controls movement of things out of the GI tract
Surrounded by 2 anal sphincters
External sphincter under voluntary control by pudendal nerves
What are 4 unique features of the large intestine?
Appendices epiploicae -Fatty tags that arise from serosa Longitudinal muscle (taenia coli) -3 bands of longitudinal muscle -shorter than the intestine∴causes colon to form pouches called hastra Circular muscle -segmentally thickened -held together by taenia coli Nodules of lymphoid tissue
How does the large intestine reabsorb ions and water?
Na+ and Cl- absorbed by exchange mechanisms
Water absorbed by osmosis
Potassium moves passively into lumen via gap junctions
How much can the large intestine absorb and how much does it usually absorb per day?
can absorb 4500mL per day
usually absorbs 1500mL per day
if water volume in colon >4500 = diarrhoea
What are the main cells found in the colon?
Abundant enterocytes
Abundant goblet cells
Stem cells @bottom of colonic crypts
few enteroendocrine cells
What is the purpose of enterocytes in the large intestine?
Re-absorption of salt creates osmotic gradient
Glycocalyx present but no brush border enzymes present
What is the purpose of goblet cells in the large intestine?
Secrete mucous to facilitate passage of luminal contents
Cover bacteria and particulates to protect from infection/abraision
Abundance increases along length
Stimulated by ACh
What is the main difference between the large intestine and the small intestine?
No villi ∴ much smaller sfa due to no glucose or amino acid absorption in colon
Why are paneth cells not needed in the large intestine?
The presence of commensal bacteria
What are basic colonic contractions?
Kneading process
Provides minimal propulsion to allow chyme to stay in colon for longer time to promote absorption
Short propulsive movements every 30 mins
What are anti-propulsive contractions?
Impede propulsion in proximal colon to keep food in that region for longer
What are haustral contractions?
In transverse and descending colon
Shuffle contents backwards and forwards
What is a mass movement?
1-3 times a day
Co-ordinated contractions propel contents uip to 3/4 of the length of the large intestine in a few seconds
What kind of food promotes mass movements?
high fibre food
What is defecation?
expulsion of faeces
What are the steps of defecation?
Folowing mass movement, afferent sensation of needing to defecate
Rectal wall distension detected by pressure receptors
Signals sent via the myenteric plexus to initiate peristaltic waves in the descending&sigmoid colon and the rectum
Internal anal sphincter relaxes
Person makes conscious decision to relax external sphincter
What is faeces?
Indigestible waste of chyme
How can an individual perceive whether to let things pass or not?
Last centimetres of the rectum can distinguish solids, liquids and gases
How much faeces does a healthy adult produce per day?
150 g
What gives faeces its brown colour?
Bile pigments
What is the most prevalent species of bacteria in the gut?
Bacteriodetes
gram -ve
anaerobic non spore forming
What are the physiological roles of the intestinal microbiome?
Synthesise and excrete vitamin K
Prevent pathogen colonisation by acting as competition
Antagonise other bacteria (inhibit and kill non indigenous species)
stimulate cross-reactive body production
stimulate tissue development
Fibre can be broken down
Links to lipid metabolism, insulin resistance, bile acid metabolism, drug metabolism and obesity
What are the possible pathophysiological roles of the intestinal microbiome?
Gut bacteria populations help maintain and prime the immunological system
Inappropriate population or loss of commensal bacteria can predispose to infection and illness throughout body
Faecal transplant to re-instate bacteria
