Basic GI Physiology

doughnutThe human body is doughnut shaped therefore the surface of the gut is continuous with the surface of the skin and composed of the same cell type i.e. epithelia (epi = outer). This is true for all organs which make direct contact with the outside world e.g. kidney, salivary glands, pancreas etc. however, the gut is the only one which connects through to other side to have an entrance and an exit. Everything more evolved than flatworms has an internalised digestive system to increase the surface area for absorption and allow a specialised environment for digestion. One advantage of being a flatworm is that you can swim (or wriggle) up to you food, digest it and then swim away from your waste products. If you have an internalised digestive system you have to move the food to the correct part of the gut for digestion and absorption and then move the waste products away for excretion. This important digestive function is achieved by co-ordinated waves of contraction or peristaltic movements of the gut which forces food to move along.

Given that the outermost (that in contact with the food) layer of the gut is composed of epithelial cells we can deduce that underlying layers must include muscle (because the gut can contract) and nerves (because the contractions are co-ordinated: enteric nervous system). In fact the gut has a uniform structure throughout (except that the oesophagus and rectum lack serosa & mesentery.).

As food passes through the gut, activity in one part is communicated to the next by a combination of nerves and hormones. Hormonal activity can be either endocrine (via bloodstream) or paracrine (in local area). Hormones can have simultaneous endocrine and paracrine activity.

gut layers

Components of the digestive system.

GI Tract

Oral cavity

The salivary glands produce saliva (1500 ml/day) containing digestive enzymes (primarily salivary amylase) and mucous. Mastication breaks the food into small particles for swallowing, coats the particles with mucous and mixes amylase inside the bolus where it can continue to act even when the bolus passes to the stomach.

Stomach.

stomachAcid secretion (parietal cells). Pepsinogen secretion (Chief cells) Secretion of hormones, e.g. gastrin (G cells). Stomach pH 1.0 low enough to cause tissue damage except mucous is also secreted (Aspirin inhibits prostaglandin synthesis which inhibits mucous production). Acid kills bacteria helps protein digestion provides environment for pepsin to digest protein. Helps stimulate bile and pancreatic juice. (2500 ml gastric juice/day)

gastric pits

Small intestine

regulation of gut hormone releaseMassive increas in surface area by infolding
Absorption of sugars, amino acids, water etc. Hormone secretion e.g. CCK (pancreozymin). Input from pancreas (1500 ml/day), bicarbonate (neutralise stomach acid), trypsin (as trypsinogen, autocatalysis in intestine to active form). Input from liver (500 ml/day), bile salts, important in emulsification of fat and subsequent digestion. Total of 7-9 l/day of fluid enter the small intestine (2 l with food rest from saliva pancreas etc.), all but 1-2 l/day absorbed. Small intestine also contains lymph nodes (Peyer's patch) involved in immune response.

Large intestine, Water reabsorption. 1-2 l/day enter colon <100 ml lost in faeces. The colon has massive absorptive powers & can reabsorb water against a large hydrostatic gradient. Colon contains bacteria, mainly E.coli. Route of choice for administration of drugs etc. because they can be absorbed very rapidly.

The passage of food through the gut activates each part of the gut in turn. Negative feedback (dotted lines) helps to inactivate higher gut sections as the food passes ever onwards.

Enteric nervous system.

Controls motility & involved in regulation of secretion. Composed of myenteric (Auerbach's) and submucous (Meissner's) plexi. Contains motor neurones, secretory neurones and sensory neurones (stretch, tonicity, glucose or amino acids). > 1 million neurones. Sometimes called 3rd division of autonomic nervous system (along with sympathetic and parasympathetic). Neurones contain ACh, noradrenaline, serotonin (5-HT), enkephalins, VIP, substance P, somatostatin, gastrin-releasing peptide (GRP), neurotensin and maybe angiotensin II. Some substances released as neurotransmitters (some putative neurotransmitters), some as neuromodulators some have paracrine actions.

Extrinsic innervation from both parasympathetic and sympathetic systems. Preganglionic Parasympathetic efferents (mainly from vagus) release ACh (onto cholinergic enteric) nerves and increase activity of the gut. Postganglionic Sympathetic efferents release noradrenalin which decrease activity of the gut, Sypathetic efferents terminate on postganglionic ACh neurones and act by inhibiting ACh release.

Motility

Peristaltic waves

Gastric slow wave in stomach from fundus to pyloris, 3/min, regulated by vagus. Helps co-ordinate gastric emptying.

Small bowel slow wave 12/min duodenum, 9/min ileum. No need for extrinsic innervation but myenteric plexus must be intact.

Large bowel slow wave 2/min ileocaecal valve up to 6/min at sigmoid. Colon also also has ‘mass action contraction’, simultanous contraction of large area of smooth muscle. Involved in moving material to rectum. Rectal distension initiates defecation reflex.

Blood supply

Splanchnic circulation. Arterial supply to intestine pancreas and spleen from mesenteric arteries. Drainage unusual via hepatic portal vein which goes straight to liver. Blood flow in intestine regulated by metabolic activity i.e. doubles after a meal. therefore throughput to liver also doubles.

Transit time

4 hours after meal first shows up in caecum. Everything, including undigested parts by 8-9 hours. Up to 70% through and excreted in 72 hours, may take up to a week for everything. Brown colour of stools due to bile pigments.