Physiology - Digestive System

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

• 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

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)

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 

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

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

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

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

• 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)

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

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 

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" 

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

• 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