Physiology - Digestive System
- Created by: Meloddymaybe
- Created on: 29-04-17 17:52
Mucosa and Submucosa
MUCOSA - lines luminal surface with highly folded SA
Mucous membrane
- Protective surface modified for secretion/absorption with exo/endocrine & epithelial cells
Lamina propria
- Gut-associated lymphoid tissue (defence against pathogenic intestinal bacteria)
Muscularis externa
- Sparse layer of smooth muscle
SUBMUCOSA
- Meissner's plexus - nerve network and
- Larger blood/lymph vessels
- Distensibility and elasticity with thick layer of connective tissue
Muscularis externa and Serosa
MUSCULARIS EXTERNA
- Circular - inner layer, contraction decreases lumen diameter
- Longitudinal - outer layer, contraction shortens tube
- Contractile activity produces propulsive and mixing movements
- Auerbach's myenteric plexus between 2 layers
SEROSA
- Secretory serous fluid - lubricates, preventing friction between viscera and organs
- Continuous with mesentery attachment - provides relative fixation, supports digestive organs whilst allowing freedom
Nervous Innervation
Enteric
- Intrinsic - within
- Short reflex
- Via ENS
- Myenteric and submucous plexus - interconnected, tract wall
- Motor, secretory and sensory neurones
Extrinsic
- Extrinsic - autonomic, from outside
Long reflex - Via CNS
- Parasympathetic (cholinergic) and sympathetic (achenergic)
Secretion
Functions of different components
- Salivary glands - 3: sublingual, parotid and submandibular pairs, secrete salt, water, mucus (lubrication) and amylase (polysaccharide digestion)
- Tongue - lingual lipase (starts fat digestion), LgA and lysozyme (protect from bacteria)
- Oesophagus and pharynx - mucus (lubrication)
- Stomach - HCL (solubilisation), pepsinogen (protein digestion), mucus (protection) and HCO3- (raises pH)
- Pancreas - trypsinogen phospholipase (activated into trypsin, which digests fats and proteins), lipases, carboxypeptidase, elastase, HCO3- (neutralises chyme)
- Liver - bile salts (solubilises fats), HCO3- (pH adjust), waste products removed
- Gallbladder - stores and concentrates bile
- Small intestine - digestive enzymes, salt, water, mucus
- Rectum - defecation
Gastrointestinal Facts and Functions
Functions
1. Motility
2. Secretion
3. Digestion
4. Absorption
"Doughnut" - not accessible directly by rest of body
Facts
- Drink average 1.2L per day and eat 800g solid
- Produce 1.5L saliva, 1.5/2L gastric, pancreatic and intestinal secretion, 0.5L bile
- Reabsorb 6.7L secretions
- Excrete 0.1L water and 50g solids
Carbohydrate
Types
- Polysaccharides/starches e.g. glycogen (1:4 and 1:6 alpha linkages), amylopection (1:4 and 1:6), amylose (1:4) and cellulose (fibre, 1:4 beta linkages)
- Disaccharides e.g. lactose, maltose, sucrose
- Monosaccharides e.g. glucose
Enzymes
- Salivary/pancreatic amylase - hydrolysis of 1:4 alpha, produces oligosaccharides
- Oligosaccharidases e.g. alpha limit dextrinase, glucosamylase
- Disaccharidases e.g. lactase, sucrase
Absorption
- Intestinal lumen
- Into capillaries via cotransporters with Na+
- K/Na pump maintains Na+ gradient
Protein
Digestion
- Stomach - pepsin hydrolyses bonds between aromatic amino acids
- Small Intestine - many secreted inactively so don't damage other areas of body, endopeptidases e.g. pancreatic chymotrypsin, exopeptidases e.g. pancreatic carboxypeptidase and brush border (microvilli-covered) aminopeptidases, dipeptidases
Absorption
- Proteins broken down into peptides
- Di/tripeptides by diffusion or cotransport with H+
- L-Amino acids by secondary active transport with Na+ cotransporters
- Small peptides by endocytosis
Fats
Digestion
- Mouth - lingual lipase (found in stomach too) digests up to 30% triglycerides
- Stomach - unimportant gastric lipase
- Duodenum - pancreatic lipase, most important
- Very slow process - helped by fat forming large lipid droplets, water soluble lipases divide droplets and prevent re-joining, bile emulsification
Absorption
- Micelles - formed due to bile salts, much smaller than emulsion droplets so absorption accelerated
1. Monoglycerides and fatty acids diffuse into enterocyte
2. Triglyceride resynthesised in smooth ER
3. Droplets coated with emulsifying agent
4. Exocytosis as chylomicrons into interstitial water
5. Diffuse into lacteals of lymph
Sodium, Water and Vitamins
Sodium
- Na+ absorbed with glucose
- Cl- and HCO3- follow Na+
Water
- Passive absorption
- Follows solutes
- Maintains osmotic equilibrium
Vitamins
- Fat soluble - as in fat absorption e.g. A, D, E and K
- Water soluble - diffusion or mediated transport
- EXCEPT B12 - binds to intrinsic factor (produced by acid-secreting stomach cells), complex binds to specific sites on ileum epithelia, endocytosis follows, otherwise pernicious anaemia (B12 needed to produce healthy RBCs)
Control
Responds to
- distention of wall by luminal contents - mechanical receptors
- chyme osmolality - osmoreceptors, and acidity - chemoreceptors
- conc. digestive products e.g. fatty acids
Types
1. Nervous
2. Local muscle response
3. Hormonal - two families: a. gastrins (gastrin and cholecystokinin) and b. secretins (secretin, glucose-dependent insulinotropic peptide, some give greater combined effect than expected
Phases - names after site of initiation but have multiple action sites
- Cephalic - receptors in head (e.g. smell, chewing), efferent, mostly neuronal, increase HCL
- Gastric - volume/composition detected, efferent, short/long, gastrin, increase HCL
- Intestinal - ", secretin, CCK and GIP, decrease HCL
Mouth to Stomach
Mastication (chewing)
- somatic nerves to skeletal muscle of mouth/jaw
- mainly voluntary
- reflex - receptors in hard palette and gums, leads to saliva secretion and swallowing
Peristalsis
- Upper third of oesophagus - skeletal (voluntary) so can cough up something
- Lower third - smooth (involuntary)
- Longitudinal and circular contraction/relaxation
Stomach
Glands/Cells
- Oxyntic/fundic glands - fundus and body, HCL
- Pyloric glands - antrum, gastrin
- Cardiac glands - cardia (nearest to heart, encloses oesophagal opening), mucus
- Mucous, chief (pepsinogen) and parietal (HCL) cells
Events/Functions
- Filling - receptive relaxation (like elastic band, enhances ability to take extra vol., stimulated by eating), mediated by vagus nerve
- Storage - body
- Mixing - antrum
- Emptying - controlled by duodenum factors (fat - good indicator as digests slowly, acid - too much slows down, hypertonicity and distension), triggers a. neural "enterogastric reflex" (shuts off gastrin release, inhibiting motility, stimulated by parasympathetic), b. hormonal - duodenum produces enterogastrones (secretin, CCK) and 3. emotion - anxiety promotes, pain inhibits
NB: Pepsinogen - +ve feedback, HCL causes conversion to pepsin, "autocatalytic"
Small Intestine
Pancreatic secretions
- Secretin stimulates HCO3- flow (-ve feedback)
- CCK stimulates enzyme secretion, potentiatiation of effects
Bile secretions
- HCO3- from epithelial cells
- Bile salts - control secretion rate, sphincter of Oddi, CCK causes gallbladder contraction
Small intestinal motility
- Peristalsis - mixes food/chyme, slow movement forwards so nutrients are absorbed, to LI within hours
- Segmentation - result of "gastroileal reflex", movement in both directions allows greater mixing
- Migrating motility complex - short waves die away over 2ft, motilin speeds movement i.e. diarrhoea
Large Intestine and Anus
Large Intestine
- Ileocecal sphincter - relaxed by gastroileal reflex, chyme moves into SI, closure prevents reflux
- Reached by enteric nervous system
- Segmentation and mass movement (few times a day)
Anus
- Rectal distension - "defecation reflex"
- Internal (smooth/involuntary) sphincter relaxes, external (skeletal/voluntary) contracts
- Increased peristalsis in sigmoid column
- Increased rectal pressure
- External sphincter relaxation
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