One plan of wall, one nervous system
and a circulation that doubles as a reservoir.

1. One plan of wall, repeated. Outermost to innermost: serosa, longitudinal muscle, circular muscle, submucosa, mucosa. The mucosa itself is muscularis mucosae, lamina propria, epithelium. The two plexuses sit in fixed places inside that plan, and knowing where tells you what each controls.
2. The enteric nervous system is not a relay. It is a complete reflex arc in the wall, with sensory neurones, interneurones and motor neurones, and it runs peristalsis with every extrinsic nerve cut. The autonomic supply modulates it.
3. Myenteric for movement, submucosal for secretion. The myenteric (Auerbach) plexus lies between the two muscle layers and controls motility; the submucosal (Meissner) plexus lies in the submucosa and controls secretion, absorption and mucosal blood flow.
4. Parasympathetic excitatory, sympathetic inhibitory. Vagus and pelvic nerves increase motility and secretion through acetylcholine; sympathetic fibres from T5 to L2 reduce them, and at the same time contract the sphincters and constrict the splanchnic vessels.
5. Two hormone families, and the family predicts the behaviour. Gastrin and cholecystokinin share their five C-terminal amino acids; secretin, glucagon, vasoactive intestinal peptide and gastric inhibitory peptide are the other family. Gastrin prepares the stomach to receive food; secretin and cholecystokinin slow it down once the meal has moved on.
6. The splanchnic bed is two capillary beds in series. Three arteries supply three beds in parallel with each other, and every drop of that then passes through the liver before it reaches the heart. About 30% of the cardiac output.
7. It is the body’s largest blood reservoir. About 500 mL sits in the hepatic capacitance vessels, and about 1000 mL can be returned from the splanchnic bed as a whole under sympathetic stimulation.

The gastrointestinal tract is about 9 metres of tube, and it constitutes roughly 5% of body mass. What makes it learnable is that the wall has one plan. Regional specialisation is superimposed on that plan — an extra oblique muscle layer in the stomach, taeniae coli in place of a continuous longitudinal layer in the colon, stratified squamous rather than columnar epithelium in the oesophagus — but the sequence of layers does not change, and neither does where the nerves sit in it.

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A three-dimensional cutaway of the small-intestinal wall. At the top, finger-like villi project into the lumen; each contains a central lacteal and a capillary network within the lamina propria, bounded below by the thin muscularis mucosae. Beneath the mucosa lies the submucosa, containing a submucosal arteriole, a lymphatic vessel and the submucosal (Meissner) plexus drawn as purple ganglia on a yellow nerve network. Below that is the circular muscle layer, seen end-on as bundles of cut fibres, then the myenteric (Auerbach) plexus as a sheet of ganglia between the two muscle layers, then the longitudinal muscle layer with fibres running lengthwise, and finally the serosa. A vagal parasympathetic fibre enters from the left and a sympathetic postganglionic fibre from a ganglion on the right; both terminate on the plexuses rather than on the muscle.

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Two features of the mucosa are worth separating from the rest because they are commonly confused. The muscularis mucosae is a thin sheet of smooth muscle inside the mucosa; it agitates the villi and stops luminal contents settling against the epithelium, and it is not part of the muscularis externa that produces peristalsis. The lamina propria is connective tissue carrying blood vessels, nerve endings and a standing population of immune cells; in the ileum those are organised into Peyer’s patches, which is where section 09 picks the story up.

The enteric nervous system is the third division of the autonomic nervous system, and the only one that can complete a reflex without the central nervous system. It contains sensory neurones that respond to stretch, to luminal chemistry and to mucosal deformation; interneurones that integrate them; and motor neurones that act on smooth muscle, on secretory cells and on blood vessels. A length of intestine removed from the body and kept perfused will still generate a peristaltic wave when it is distended. That single observation is what the word “intrinsic” is doing.

Two consequences follow that recur throughout the module. First, an intervention that reduces sympathetic outflow to the gut — a thoracic epidural, for example — increases motility and shortens the duration of postoperative ileus. Second, the same sympathetic outflow that stops the bowel also empties the splanchnic veins, which is section 08.

More than fifteen types of hormone-secreting enteroendocrine cell have been identified in the mucosa of the stomach, small intestine and colon. Five of the products matter for this examination, and they fall into two structurally defined families. The gastrin family — gastrin and cholecystokinin — share the same five C-terminal amino acids, which is why the biological activity of both resides in that tail and why cholecystokinin has gastrin-like activity when it is given in large doses. The secretin family — secretin, glucagon, vasoactive intestinal peptide and gastric inhibitory peptide — share a different structure altogether.

Gastrointestinal regulation runs on three mechanisms that operate together, and the same molecule can be used in more than one of them. Endocrine signalling releases a mediator into the bloodstream to act on a distant segment or on an organ draining into it; gastrin is the standard example. Paracrine signalling releases a mediator that is too unstable to survive the circulation, so it acts only on cells in the immediate area; somatostatin from the D cell, diffusing onto the neighbouring G cell and parietal cell, is the standard example, and so is histamine from the enterochromaffin-like cell. Neurocrine signalling releases a transmitter from a nerve terminal; vasoactive intestinal peptide and nitric oxide from the inhibitory enteric neurones are the standard examples, and they are the pair that relaxes every sphincter in the tract.

The splanchnic circulation is unlike any other regional bed in one specific respect: its venous outflow does not return to the heart. It returns to another organ. Blood from the stomach, spleen, pancreas, small intestine and colon drains into the portal vein and perfuses the hepatic sinusoids before entering the hepatic veins, the inferior vena cava and the right atrium. That single arrangement explains first-pass metabolism, portal hypertension, the site of a gastrointestinal haemorrhage’s effect on ammonia, and why the liver is the first organ to see anything absorbed from the gut.

Those three sum to 1800 mL/min, and the viscera together with the liver receive about 30% of the cardiac output at rest. Feeding raises it substantially; vigorous exercise, haemorrhage and any other state of high sympathetic tone reduce it sharply, and the splanchnic bed is one of the first to be sacrificed. That vulnerability is the physiological basis of non-occlusive mesenteric ischaemia in a patient on high-dose vasoconstrictors, and of the mucosal injury that follows a prolonged period of low cardiac output.

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Two side-by-side panels. In the left panel, labelled "At rest", the abdominal aorta gives off the coeliac, superior mesenteric and inferior mesenteric arteries, which supply capillary networks in the stomach and spleen, the small intestine and the colon. All three networks drain into a wide portal vein, which enters the liver; hepatic veins leave the liver for the inferior vena cava. The right panel, labelled "Sympathetic stimulation", shows the same anatomy with narrowed arteries and a narrowed portal vein, and a broad arrow marked "autotransfusion to the central circulation" running from the hepatic veins into the inferior vena cava. Along the foot of the figure, four vessel cross-sections compare a dilated arteriole and a wide splanchnic vein packed with red cells at rest against a constricted arteriole and a collapsed, nearly empty vein under stimulation. A panel between them states that about 500 mL is held in the hepatic capacitance vessels, that sympathetic stimulation returns about 250 mL, and about 1000 mL with the splanchnic vessels together.

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The splanchnic veins, the portal venules and the hepatic veins are thin-walled, compliant and densely innervated by sympathetic fibres. At rest they hold a large volume at low pressure. When sympathetic tone rises — on standing, on exercise, on haemorrhage — those vessels constrict, and the volume they were holding is returned to the central circulation. The effect is an autotransfusion, and it happens before any of the slower compensations for hypovolaemia have begun.

The gastrointestinal epithelium is a single cell thick over an absorptive area of hundreds of square metres, and on its luminal side sits a bacterial population outnumbering the body’s own cells. The barrier that separates them has four components, and each fails in a way that shows up in the perioperative period.

Secretory immunoglobulin A is the specific immunological point worth carrying. It is the most abundantly produced immunoglobulin in the body, it is secreted as a dimer with a secretory component that protects it from luminal proteases, and it works by immune exclusion — binding organisms and preventing their adherence rather than opsonising them for destruction. That is why it does not fix complement and why it does not provoke an inflammatory response every time it does its job.