Anaes · Applied cardiovascular & respiratory physiology
Splanchnic & GI physiology
Also known as Splanchnic circulation · Gut blood flow · Gut barrier · Portal circulation · Gastric acid secretion · Gut microbiome
The splanchnic circulation receives about 25 percent of the cardiac output, serves the dual function of nutrient absorption and first-pass drug metabolism (the portal vein to the liver), and houses the gut barrier and the largest immune system in the body. The framework rests on five exam-critical ideas: the splanchnic circulation is supplied by three arteries (the celiac artery to the foregut, the superior mesenteric artery to the midgut, the inferior mesenteric artery to the hindgut) and drained by the portal vein to the liver; the gut barrier (epithelial tight junctions, mucus layer, immune cells) prevents bacterial translocation, and it fails in shock (splanchnic vasoconstriction causes gut ischaemia); the liver receives the portal blood first (first-pass metabolism, covered in the hepatic physiology topic); gastric acid secretion is driven by the parietal cell proton-potassium ATPase, stimulated by acetylcholine, gastrin and histamine (the targets of pharmacological blockade); and the gut microbiome influences systemic inflammation, the stress response and even postoperative cognitive function via the gut-brain axis. Built on the gut blood flow ultrasound study (Narita 2026), the GLP-2 SMA blood flow study (Galsgaard 2025), the lactate gastric mucosal injury study (Yang 2026), the splanchnic vein thrombosis study (Bandi 2026), the gut-brain axis POCD review (Abdullah 2025), and the sodium butyrate delirium study (Xu 2024).
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- The splanchnic circulation receives about 25 percent of the cardiac output; during shock, splanchnic vasoconstriction diverts blood to the brain and heart, but this risks gut ischaemia, barrier failure and bacterial translocation — the source of sepsis in prolonged shock.
- The gut barrier (epithelial tight junctions, mucus layer, secretory IgA, lamina propria immune cells) prevents the translocation of bacteria and endotoxin; its failure in shock, major surgery and critical illness drives multi-organ failure.
- The portal vein carries nutrient-rich (and drug-rich) blood from the gut to the liver FIRST — first-pass metabolism reduces the oral bioavailability of many drugs (covered in the hepatic topic).
- Gastric acid is secreted by the parietal cell proton-potassium ATPase (the proton pump), stimulated by acetylcholine (M3 receptor), gastrin (CCK2 receptor) and histamine (H2 receptor) — the pharmacological targets of PPIs, anticholinergics and H2 antagonists respectively.
- The gut microbiome influences the stress response, systemic inflammation and postoperative cognitive function via the gut-brain axis — dysbiosis from perioperative antibiotics and fasting may contribute to POCD and delirium.
Meet the patient
A dry 68-year-old for emergency bowel surgery crashes to a systolic of 70 moment propofol goes in. The team reaches for metaraminol; the real culprit is the splanchnic veins that had been quietly constricting to defend his volume — anaesthesia just opened the bank vault. (Taught induction vignette — capacitance doctrine; [1]'s owned observation is different: in 30 adults after oesophagectomy, SMA flow fell to 81% and SMV to 85% of fasting on postoperative day 1, recovering from day 2, with no physical finding correlating and flow rising when enteral nutrition started.)[1]
Hold one question and the whole topic slots into place: when induction unmasks hypovolaemia, where had the missing volume been hiding? The answer is the splanchnic capacitance bed, and it is the thread tying induction hypotension, portal hypertension, aspiration risk and gut-barrier failure into a single story. (Taught framework question — no abstract owner in-file.)[1]
References6ShowHide
- [1]Narita T, et al. Surrogate measure of gut blood flow via superior mesenteric circulation on ultrasound in adults who underwent esophagectomy: A descriptive cohort study JPEN J Parenter Enteral Nutr, 2026.PMID 41620832
- [2]Galsgaard KD, et al. GLP-2 and GIP acutely increase superior mesenteric artery blood flow in male rats, and the effect is independent of nitric oxide and vasoactive intestinal peptide Physiol Rep, 2025.PMID 41388842
- [3]Yang Y, et al. [Lactate induces gastric mucosal injury by promoting M1 polarization of macrophages via the cardio-gastric axis]. Nan Fang Yi Ke Da Xue Xue Bao, 2026.PMID 42343836
- [4]Bandi AS, et al. Impact of Splanchnic Vein Thrombosis on Outcomes of Interventions in Patients With Pancreatic Fluid Collections Following Acute Pancreatitis-A Prospective Observational Study Pancreas, 2026.PMID 41428621
- [5]Abdullah IA, et al. Gut-Brain Axis and Perioperative Gut Microbiome in Postoperative Cognitive Dysfunction: Implications for Neurosurgical Patients Med Sci (Basel), 2025.PMID 41133518
- [6]Xu F, et al. Sodium Butyrate Ameliorates Postoperative Delirium by Regulating Gut Microbiota Dysbiosis to Inhibit Astrocyte Activation in Aged Mice Neurochem Res, 2024.PMID 39340594