Week 8 Gastrointestinal System Flashcards
Anatomical features of the GI system
GI tract plus accessory organs, the liver and exocrine pancreas
Characteristic 4 layered structure of GI tract (uniquely conserved)
Mucosa: absorbative surface, blood capillaries, metabolic active part (below thin muscular layer to maintain integral structure
Submucosa: larger blood vessels, heading of to the liver, branches of lymphatic system and ANS
Muscularis: double muscle layer, different types of movements and constractions, circular and longitudinal
Serosa: thick layer of connected tissue (outer covering, bag is called serosa) for separation of stomach and not the stomach
Stomach
Muscular bag;
Vary in size from 75ml to 1L Expands to 2L of substrate Basolateral foldes (Rugae) relevant to stretch out and increase surface
Highly adapted for muscular contraction (antrum) & Secretion (pits in body)
Esophcgus spincter relaxes when food enters
Ragae = folding sheets allows us to eat big meals
Greater curvature
Cells secreting mucus, protects the cells in the stomach
Small Intestine
(6m) same 4 layered structure; fully adapted for absorption
Folding of the microvilli = increase SA for absorption
Lacteal=part of lymphatic system
Long chained fatty acids can’t enter the blood stream so enters lacteal as its wider in diameter
Small intestinal structure (duodenum/jejunum)
Surface area is everything!
2 functional segments of the sI
Duodenum Jejunum
Lumen is greater towards stomach Highly vascularised; appears dark red (more absorption) and more muscular Brunners glands (mucus & bicarbonate) Long villi absorption increased
Large Surface Area absorption increased
Ileum
Smaller food volume, less muscles Paler appearance (less vessels, less absorption)
Peyers patches (lympatics) protection from bacteria
Short villi lower level of absorption Smaller Surface Area
Other functions of the gut
(in addition to digestion and absorption)
Gut Barrier function varies between intestinal segments
Outside world
Tight junction proteins (“cement”) prevent molecules & bacteria passing through the intercellular space
Inside the body
Leaky gut: increased permeability
Change in expression of TJ proteins Claudin and Occludin
More TJ proteins in ileum (more bact.) Ileum membrane less permeable (prevent bacteria to move across)
Peyers patches form part of GALT (gut-assoc. lymphoid tissue)
Immune Functions
Located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum.
• Peyer’s patches catch microorganisms and other antigens entering the intestinal tract by dendritic cells, B-lymphocytes, and T-lymphocytes
• Peyer’s patches react to dietary proteins
• Peyer’s patches are covered by a special epithelium that contains
microfold cells (M cells) direct contact to lumen, linking it to the IS
• M cells (1) sample antigen directly from the lumen (2) deliver it to antigen-presenting cells
Large Intestine - the Colon
• 1.5 m long, but large lumen size
• decreases in lumen diameter
(water absorbed, bacteria function)
4-layer structure, but longitudinal muscle in 3 bands, the taeniae coli, gap only circular muscle, evolutionary disadvantage
• Skeletal and smooth muscle, faecal continent due to anus sphincter
Internal and external sphincters, one you can crono rol with motor neurone and one you can controlled and is just reflex’s
Internal spincter = smooth muscle, innervated by enteric neurons
External sphincter
Innevertaed by somatic efferent neurons
Crypts are no pits, but wholes (defined as crypt rather than gland)
Increased surface area, but colon has no villi (no nutrient absorption)
• Columnar cells with short microvilli
• Absorptive cells are most numerous (water absorption)
• Mucus producing cells for protection
Liver & Pancreas
Liver is the largest of internal organs
Closely associated with other accessory organs of GI system
Secretion of bile into gall bladder
Pancreas endocrine (hormones in blood) and exocrine (substrates in GI)
Enter in the duodenum
Motility and its regulation within the GI Tract
Function:
1. Controlled movement of food from the mouth to the anus 2. Mixing of food with digestive enzymes and exposure to
absorptive surface (main function is digestion & absorption)
• Different types of contraction in different segments of the gut
• Migrating motor complex (unusual pattern when gut is empty)
• Regulation; neural and hormonal inputs (switch on and off)
Stomach
Reservoir for food (up to 2L) Receptive relaxation reflex (funduns expands)
increase the volume without increasing the pressure
Basic electrical
Rhythm
(Interstitial cells
of cajal); start of contractions
Regular peristaltic Contractions push food in antrum
Vigorous contractions
circular movement /turning & mixing
Tonically closed
Oesophageal peristalsis causes relaxation of LES
Primary Stimulus after a meal is gastric distension
Food is pushed from fundus
Relaxes pylorus
Mechanical actions of the stomach
2 different muscle layers lead to 3 different pattern:
Propulsion Construction starts in body moves to antrum
Circular muscle at pylorus closes with each gastric contraction but pylorus always 1-2mm open (liquids)
Grinding within antrum to reduce particle size
Retropulsion backwards to mix into body
Facilitates mixing & reduces particle size
Small intestinal motility
Up to 5-6 h transit
Controlled movements from duodenum to end of ileum (allows absorption, not too slow not too fast and mixed well)
• 2 types of motility (periods of segmentation & peristalsis)
• frequency gradient of rhythmic contractions in the SI (stronger and more frequent in duodenum, larger food bolus)
• Regional motility: construction only where food bolus is (energy efficiency!) constriction follow distention
• Distension is primary stimulus via enteric NS
