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• Amylase (ptyalin) - breaks down starch into oligosaccharides

Saliva (under PNS control)

• Hypotonic

Phases of swallowing

1. Oral (voluntary)
   • Bolus progressively moved upwards and backwards by pressure of tongue
2. Pharyngeal
   • Contraction of constrictors
   • Larynx pulled upwards/forwards against epiglottis (protects airway)
   • Upper oesophageal sphincter relaxes, superior constrictor contracts - food enters oesophagus
   • Inhibition of medullary respiratory centre
3. Oesophageal
   • Swallowing centre initiates primary peristaltic wave
   • Relaxation of LOS (normal pressure 30mmHg)

Endocrine

• Gastrin (gastric G-cells) of fundus: stimulate acid section, stomach contraction, pancreatic secretions

Exocrine

• Pepsinogen (precursor for protein digestion)
• Intrinsic factor (gastric parietal cells): Aids resorption of B12Water
• HCl GPC (Fundus predominantly) - activates pepsin
• Mucous - necks of gastric glands in pylorus

Emptying hormones

1. gastrin (GPC)
2. CCK + secretin (duodenum) 

Output = 2l/day

Gastric Innervation

• Sympathetic: coeliac plexus
• PNS: vagus nerve (increased motility)

1. H+ generated from CO2 dissolving in cytoplasm
2. Exchanged with K via H/K ATPase
3. HCO3- generated via dissociation and goes back into plasma

Acid secretion control

+ ACh (M2): vagus
+ Gastrin : G-cells (fundus)
+ Histamine: Mast cells (Rx ranitidine)
- Somatostatin
- Secretin
- CCK

Phases of gastric acid secretion

• Cephalic phase: thought/smell/taste - vagal activity stimulates gastrin secretion/HCl secretion
• Gastric phase: presence of food - stimulates gastrin and HCl
• Intestinal phase: presence of amino acid and food (later inhibited by release of secretin and CCK from duodenum)

Types of contraction

• Peristalsis:
• Retropulsion - passes food boluses back
• Vomiting [Pyloric stenosis]

• Dumping: early (osmotic sucking effect) / late (pancreatic insulin secretion following food)
• B12 deficiency (no IF)
• Achlorhydia (no Fe absorption)

• Iron absorption (acidic environment)

• CCK: stimulates GB contraction, stomach emptying, stimulates pancreatic lipase secretion
• Secretin: stimulates stomach emptying, stimulates pancreatic secretion

Prinicple site to absorption of carbs, fats, protein, water, electrolytes, vitamins, minerals

Output = 1.5l/day

Absorbs 8.5l/day

Type of contraction

• Segmentation
• Peristalsis (localised contraction)
• Pendular movements (contraction of longitudinal muscles)

• Folate absorption

• B12 absorption
• Bile salt uptake
• Water resorption

• B12 deficiency, macrocytic anaemia
• Increased bile salt production + increased incidence of gallstones (Cf Crohn's disease)
• Loose/frequent stools (reduced water absorption)
• Reduced Gamma-globulin:

• Endocrine
• Exocrine

• Diabetes mellitus
• Insulin sensitivity - due to additional loss of glucagon
• Reduced fat absorption (leads to steatorrhoea)
• Reduced protein absorption - negative nitrogen balance
• Reduced absorption of Fe and Ca - due to loss of alkalinisation of chyme in stomach

• Water absorption
• Mineral absorption
• Expulsion of faeces
• (Bacterial synthesis of vitK

Gastro-Colic reflex

Meal leads to increased activity of colon, with increase in mass contraction movements

Defecation

1. Distension of rectal walls (from faeces) >18mmHg intra-rectal pressure
2. Afferent impulse pass to sacral segments (S234)
3. Stimulates efferent reflex + stimulation of thalamus/cortical sensory areas producing consicous desire to defecate
4. Efferent impulses back to myenteric plexus activating PNS
5. Leads to contraction and expulsion of faeces + relaxation of internal anal sphincter
6. Augmentation with voluntary contractions of pelvic floor muscles

Resistance to defecation - mediated by pudendal nerve
Involuntary defecation occurs when rectal pressure > 55mmHg due to contents or spasm.

[Rectal prolapse]

Types of contraction

• Segmentation (mixes contents)
• Peristalsis (propels contents)
• Mass contraction - propels contents along entire lenght of colo; 1-3times/day

Frequency of wave of contraction increases along lenght of colon (caecum 2/min, sigmoid 6/min)