Portal circulation

PORTAL CIRCULATION – AN INTEGRATED REVIEW

  1. Introduction
    The portal circulation (also called the portal venous system) conveys venous blood from the abdominal viscera to the liver before it enters the systemic venous return. This unique “second capillary bed” arrangement allows the liver to act as a metabolic gatekeeper, detoxifier, immune sentinel, and reservoir. Disruption of portal haemodynamics has far-reaching systemic consequences and underlies a spectrum of clinical entities, most notably portal hypertension and its complications.

  2. Developmental Anatomy
    During the fourth‒tenth embryonic weeks, paired vitelline veins that drain the yolk sac undergo selective involution and anastomosis to form a reticular plexus around the duodenum. The caudal segments coalesce into the superior mesenteric vein (SMV), while cranial portions form the splenic vein (SV); the communicating anastomosis becomes the trunk of the portal vein (PV). Failure of normal remodeling explains rare anomalies such as congenital portocaval shunts (Abernethy malformation) and pre-hepatic portal vein atresia – both predisposing to hyper-ammonemia and pulmonary vascular disorders in childhood.

  3. Gross Anatomy
    3.1 Extra-hepatic portion
    • The main PV (average length 6–8 cm; diameter 11–13 mm) is formed posterior to the neck of the pancreas by confluence of the SMV and SV.
    • It ascends within the hepatoduodenal ligament (HDL) posterior to the common bile duct (CBD) and hepatic artery proper (HAP) – the classic “Mickey-Mouse” triad on cross-sectional imaging (CBD right, HAP left, PV posterior).
    • Major tributaries: left gastric (coronary), right gastric, cystic, para-umbilical, superior pancreaticoduodenal and gastroepiploic veins.

3.2 Intra-hepatic branching
At the porta hepatis the PV divides into right and left branches that follow the corresponding portal pedicles. Successive dichotomous splitting produces segmental, sub-segmental, and interlobular portal venules, each accompanied by a hepatic artery branch and bile duct, forming the portal triad in Glisson’s sheath.

3.3 Portal–systemic (portocaval) anastomoses
These communications become prominent when portal pressures rise:
• Gastro-oesophageal (left gastric → azygos) → varices
• Rectal (superior rectal → middle/inferior rectals) → haemorrhoids
• Para-umbilical (recanalised ligamentum teres → epigastric) → caput medusae
• Retroperitoneal (colic/omental → lumbar)
While physiologically insignificant, they are haemodynamically critical in portal hypertension.

  1. Haemodynamics and Physiology
    4.1 Flow characteristics
    Normal portal venous inflow averages 1.0–1.2 L/min (~20–25 % of cardiac output). Hepatic arterial flow (0.4–0.6 L/min) complements portal inflow, giving a total hepatic flow of 1.5–1.8 L/min. Portal venous blood is nutrient-rich (post-prandial glucose, amino acids, chylomicron remnants) but oxygen-poor (O2 saturation 60–65 %); nevertheless, it supplies 50–70 % of hepatic oxygen requirement by virtue of its volume.

4.2 Regulation and the hepatic arterial buffer response
Because the PV lacks intrinsic sphincters, portal inflow is largely passive and mirrors splanchnic arterial inflow and mesenteric vascular tone (modulated by autonomic, hormonal, and paracrine factors). When portal flow decreases, adenosine accumulates in the space of Mall, causing hepatic arteriolar vasodilation – the “hepatic arterial buffer response” (HABR) – thereby maintaining total hepatic perfusion and oxygen delivery. Conversely, increased portal inflow induces hepatic arterial vasoconstriction.

4.3 Sinusoidal microcirculation
Terminal portal venules empty into hepatic sinusoids lined by fenestrated endothelial cells and patrolled by Kupffer cells. Pressure within sinusoids (portal pressure) normally ranges 5–10 mmHg; the gradient between portal vein and hepatic veins (hepatic venous pressure gradient, HVPG) is <5 mmHg. Sinusoidal blood exits via central veins to hepatic veins and then the inferior vena cava (IVC).

  1. Functions of the Portal Circulation
    • First-pass metabolism of nutrients (gluconeogenesis, glycogenesis, deamination), xenobiotics (CYP450 oxidation, conjugation), and hormones (insulin, glucagon).
    • Detoxification of gut-derived ammonia to urea; scavenging of bacteria/LPS by Kupffer cells – an innate immune firewall.
    • Delivery of trophic factors (GLP-1/2) regulating hepatic growth and metabolism.
    • Maintenance of systemic homeostasis (lipid, protein, and carbohydrate balance).

  2. Portal Hypertension – Pathophysiology and Clinical Significance
    Portal hypertension is defined as HVPG ≥10 mmHg (clinically significant ≥12 mmHg). It results from increased resistance to portal flow, increased portal inflow, or both.

6.1 Aetiologic classification
• Pre-hepatic: portal vein thrombosis, congenital atresia/stenosis, massive splenomegaly.
• Intra-hepatic
– Presinusoidal: schistosomiasis, nodular regenerative hyperplasia, sarcoidosis, primary biliary cholangitis.
– Sinusoidal: cirrhosis (most common; architectural distortion plus sinusoidal endothelial dysfunction).
– Postsinusoidal: veno-occlusive disease, sinusoidal obstruction due to chemotherapy (oxaliplatin, busulfan).
• Post-hepatic: Budd–Chiari syndrome, right-sided heart failure, constrictive pericarditis.

6.2 Haemodynamic alterations
In cirrhosis, intra-hepatic resistance rises because of fibrosis and dynamic vasoconstriction mediated by endothelin-1, reduced nitric oxide (NO), and activated stellate cells. Simultaneously, splanchnic vasodilation (excess NO, CO, endocannabinoids) increases portal inflow – a vicious circle. Hyperdynamic circulation (↑CO, ↓SVR) develops, contributing to ascites, hepatorenal syndrome, and cardiomyopathy.

6.3 Complications
• Gastro-oesophageal variceal bleeding (mortality 15–20 % per episode)
• Ascites ± spontaneous bacterial peritonitis
• Hepatic encephalopathy (due to porto-systemic shunting and impaired ammonia detoxification)
• Portopulmonary hypertension and hepatopulmonary syndrome
• Hypersplenism, musculoskeletal fragility (vitamin D malabsorption).

  1. Investigation of Portal Circulation Disorders
    • Doppler ultrasound: first-line; assesses calibre, direction, and velocity (normal 15–20 cm/s) of portal flow.
    • HVPG measurement (wedged minus free hepatic venous pressures): gold standard for portal pressure and prognostication.
    • Contrast-enhanced CT/MRI: delineate thrombosis, collaterals, cavernous transformation, and pre/post-hepatic obstruction.
    • Elastography: surrogate for portal pressure in cirrhosis (liver stiffness >20–25 kPa suggests CSPH).

  2. Therapeutic Manipulation of Portal Haemodynamics
    8.1 Pharmacologic
    • Non-selective β-blockers (propranolol, carvedilol) decrease portal inflow via β1-mediated ↓CO and β2 blockade–induced splanchnic vasoconstriction (unopposed α).
    • Somatostatin and octreotide inhibit splanchnic vasodilator hormones, used acutely for variceal bleeding.
    • Vasopressin analogues (terlipressin) cause splanchnic vasoconstriction; standard in variceal haemorrhage and hepatorenal syndrome.

8.2 Endoscopic and Radiologic
• Endoscopic variceal ligation and sclerotherapy control bleeding and eradicate varices.
• Transjugular intra-hepatic portosystemic shunt (TIPS) creates a low-resistance channel between portal and hepatic veins, reducing HVPG by ≥40 %. Used for refractory variceal bleeding, ascites, and Budd–Chiari syndrome.

8.3 Surgical
• Non-selective shunts (e.g., side-to-side portocaval, mesocaval) and selective shunts (distal splenorenal) now largely superseded by TIPS but remain options where expertise exists.
• Devascularisation procedures (Sugiura, Hassab) for bleeding varices in resource-limited settings or when shunt contraindicated.
• Orthotopic liver transplantation is definitive therapy for decompensated cirrhosis with portal hypertension.

  1. Other Disorders of the Portal Circulation
    9.1 Portal Vein Thrombosis (PVT)
    Etiology: cirrhosis, myeloproliferative neoplasms (JAK2 V617F), protein C/S deficiency, antiphospholipid syndrome, pancreatitis, abdominal sepsis. Acute PVT presents with abdominal pain and bowel congestion; chronic PVT leads to cavernous transformation and portal hypertension without cirrhosis (EHPVO). Anticoagulation within 6 weeks achieves recanalisation in >60 %.

9.2 Congenital Portosystemic Shunts
May be extra-hepatic (type 1) or intra-hepatic (type 2). Manifestations include neonatal cholestasis, encephalopathy, pulmonary hypertension, and hepatoblastoma. Closure (endovascular or surgical) is recommended in symptomatic cases.

9.3 Non-cirrhotic Portal Hypertension (NCPH)
Includes idiopathic portal hypertension, nodular regenerative hyperplasia, and schistosomiasis; important in Asia and Africa. Portal pressure is high despite preserved hepatic synthetic function. Management parallels cirrhotic portal hypertension but prognosis is better.

  1. Future Perspectives
    • Shear-wave and multiparametric MRI techniques aim to non-invasively estimate HVPG.
    • Modulators of intra-hepatic vasodilatory pathways (e.g., statins, simvastatin-rifaximin combination) show promise in reducing portal pressure and fibrosis.
    • Bioengineered vascular grafts and stem-cell–derived liver buds may eventually replace or supplement shunt surgery and transplantation.

  2. Conclusion
    The portal circulation is central to hepatic physiology and systemic homeostasis. Its distinctive two-capillary arrangement permits metabolic screening of gut-derived substrates but renders it susceptible to pressure and flow disturbances. Understanding the developmental anatomy, haemodynamic principles, and adaptive–maladaptive responses underpins rational diagnosis and therapy of portal hypertension and related disorders. Continued advances in imaging, molecular pharmacology, and interventional radiology are poised to refine our ability to monitor and manipulate this vital vascular system.

Suggested Core Reading – Harrison’s Principles of Internal Medicine, 21st ed.
• Chapter 368: Cirrhosis and Its Complications (portal hypertension)
• Chapter 361: Approach to the Patient with Liver Disease (includes portal circulation overview)
• Chapter 362: Evaluation of Abnormal Liver Chemistry Tests
• Chapter 419: Vascular Diseases of the Liver (portal vein thrombosis, Budd–Chiari)