Portal hypertension: pathogenesis

Portal hypertension is a pathological increase in pressure in the portal vein system, leading to the formation of collaterals and complications of cirrhosis, including variceal bleeding, ascites, and hepatic encephalopathy. In modern hepatology, the key outcome of portal hypertension is not the pressure level itself, but rather the progression to clinically significant portal hypertension, as it is this that dramatically increases the risk of decompensation. [1]

Hemodynamically, portal pressure is described by simple logic: pressure increases if vascular resistance in the portal bed increases, if blood flow into the portal system increases, or if both mechanisms are present. In cirrhosis, both components are almost always present, so the process tends to be self-perpetuating and progressive. [2]

The hepatic venous pressure gradient (HVPG) is considered the gold standard for invasive assessment in cirrhosis. Threshold values are clinically important: HVPG greater than 5 mmHg corresponds to portal hypertension, HVPG of 10 mmHg and above corresponds to clinically significant portal hypertension, and the likelihood of variceal bleeding increases with HVPG of 12 mmHg and above. [3]

Critically, HVPG reflects primarily the sinusoidal form of portal hypertension typical of cirrhosis. In presinusoidal variants (e.g., some forms of noncirrhotic portal hypertension), portal pressure may be elevated while HVPG remains normal because the bottleneck is located before the sinusoids. [4]

Table 1. HVPG threshold values and clinical significance

Indicator	Meaning	What does it usually mean?
Portal hypertension	more than 5 mm Hg	Hemodynamic fact, but there may not be any complications yet
Clinically significant portal hypertension	10 mmHg and above	The risk of clinical decompensation becomes significantly higher
Threshold associated with variceal bleeding	12 mmHg and above	The risk of bleeding from varicose veins increases

[5]

The main driver in cirrhosis: increased intrahepatic vascular resistance

Cirrhosis has a "mechanical" side: fibrosis and regenerative nodes deform the liver's vascular architecture, compressing sinusoids and small branches of the portal tract, creating a persistent increase in blood flow resistance. This creates a baseline level of portal pressure, upon which functional mechanisms are then "built up." [6]

The structural component includes not only fibrosis. Significant contributions are made by sinusoidal remodeling, capillarization of the sinusoidal endothelium, changes in the extracellular matrix, and microvascular abnormalities, which impair intrahepatic patency even with the same degree of fibrosis. [7]

An important contemporary detail is the role of microthrombosis and endothelial dysfunction as factors in resistance progression. Damage to the sinusoidal endothelium and inflammatory cascades can support intrahepatic vascular dysfunction, so "vascular architecture" and "vascular tone" reinforce each other. [8]

Even in the compensated stage of the disease, the increase in intrahepatic resistance can be sufficient to cause clinically significant portal hypertension. This explains why some patients with relatively "moderate" biochemical abnormalities already have a high risk of varicose veins and decompensation. [9]

Table 2. Structural causes of increased intrahepatic resistance

Mechanism	What happens in liver tissue?	Hemodynamic result
Fibrosis	Thickening and "rigidity" of the stroma	Compression of sinusoids and venules
Regeneration nodes	Deformation of lobules and vessels	Distortion of blood flow pathways
Capillarization of sinusoids	Loss of endothelial fenestrations, matrix changes	Growing resistance at the micro level
Microvascular remodeling	Remodeling of small vessels and sinusoids	Chronic fixation of increased resistance

[10]

The "Dynamic" Component: Why Resistance Increases Even Without Additional Fibrosis

In cirrhosis, resistance increases not only due to the fibrotic framework but also due to functional spasm of the intrahepatic vessels. This is called the dynamic component: it is associated with an imbalance of vasodilators and vasoconstrictors within the sinusoids and with increased contractility of the hepatic stellate cells. [11]

The key mechanism is endothelial dysfunction: the bioavailability of nitric oxide (NO) in the sinusoids decreases, and the effect of vasoconstrictors, including endothelin 1, increases. In practice, this means that the intrahepatic blood flow loses its ability to "relax" in response to flow, and pressure rises more rapidly. [12]

Hepatic stellate cells, activated by chronic injury, acquire the properties of myofibroblasts: they contract, alter sinusoid tone, and simultaneously enhance fibrogenesis. Therefore, the dynamic component is closely intertwined with the structural component: inflammation and fibrosis create conditions for spasm, and spasm maintains hypoperfusion and inflammatory signals. [13]

It is this dynamic component that explains why portal pressure can decrease when vascular tone and neurohumoral mechanisms are affected, even when fibrosis has already formed. From a pathogenetic perspective, this means that portal hypertension is not just a "scar" but also an active vascular disease. [14]

Table 3. Dynamic component: main mediators and effects

Participant	What changes with cirrhosis?	Portal pressure result
Nitric oxide (NO) in sinusoids	Bioavailability decreases	Less vasodilation, more resistance
Endothelin 1	The vasoconstrictor effect is enhanced	Increased intrahepatic tone
Hepatic stellate cells	Contractility and fibrogenesis increase	Sinusoidal spasm plus remodeling
Oxidative stress and inflammation	Increase endothelial dysfunction	Self-maintenance of portal hypertension

[15]

The second driver: increased blood flow into the portal system and hyperdynamic circulation

Against the backdrop of increased intrahepatic resistance, the body "redistributes" blood flow: splanchnic vasodilation develops in the abdominal vessels. As a result, more blood enters the portal system, which further increases portal pressure even without increased fibrosis. [16]

Nitric oxide (NO) is considered one of the leading mediators of splanchnic vasodilation, along with a number of inflammatory signals that enhance vasodilator production. This results in hyperdynamic circulation: increased cardiac output, decreased systemic vascular resistance, and decreased effective arterial blood volume. [17]

A decrease in effective arterial volume activates neurohumoral systems that retain sodium and water: the renin-angiotensin-aldosterone system (RAAS), the sympathetic nervous system, and antidiuretic hormone. These cascades are important for the pathogenesis of complications because they link portal hypertension with ascites and renal dysfunction. [18]

An additional mechanism of progression is angiogenesis and vascular remodeling in the splanchnic basin, which perpetuates the increased blood flow into the portal system. This creates a vicious cycle: increased pressure stimulates vascular adaptations, and these adaptations maintain pressure. [19]

Table 4. Why the inflow into the portal system increases

Mechanism	What's happening	How portal pressure increases
Splanchnic vasodilation	Dilation of the intestinal and mesenteric arteries	Increased portal vein flow
Hyperdynamic circulation	Increased cardiac output, decreased systemic resistance	Maintains high portal flow
Neurohumoral activation	RAAS, sympathetic system, antidiuretic hormone	Sodium and water retention associated with ascites
Angiogenesis and remodeling	Formation of stable vascular changes	Fixation of increased portal inflow

[20]

Collaterals and complications: how pressure "finds workarounds"

When pressure in the portal system becomes persistently elevated, portosystemic collaterals form—vascular pathways that divert part of the blood into the systemic circulation, bypassing the liver. While this serves as a compensatory relief for the portal system, it can also lead to dangerous complications, as collaterals have thin walls and are prone to rupture. [21]

The most clinically significant collaterals form in the esophagus and stomach, where varicose veins develop. The clinical significance of portal hypertension is closely linked to the risk of variceal formation and bleeding, so current consensus uses the concept of clinically significant portal hypertension as the "transition point" to complications. [22]

Splenomegaly and hypersplenism arise from chronic venous congestion in the splenic vein system and splenic remodeling. This is not simply an "enlarged spleen," but a component of the systemic response: blood cell storage changes and the risk of thrombocytopenia increases, which is important for procedures and bleeding. [23]

Pathogenetically, ascites reflects a combination of portal hypertension and neurohumoral activation with sodium and water retention, as well as changes in microcirculation and lymph formation in the liver. Therefore, ascites cannot be explained solely by "high blood pressure" or "low albumin"; it arises from the intersection of several mechanisms. [24]

Table 5. Complications of portal hypertension and pathogenetic relationship

Complication	What plays a key role in pathogenesis?	Why is this dangerous?
Varicose veins of the esophagus and stomach	Collaterals plus high pressure gradient	Risk of massive bleeding
Ascites	Portal hypertension plus RAAS and sodium retention	Infections, respiratory limitations, renal dysfunction
Splenomegaly, hypersplenism	Venous congestion and splenic remodeling	Thrombocytopenia, risk of bleeding
Hepatic encephalopathy	Porto systemic shunting and metabolic factors	Impaired consciousness, decreased survival

[25]

Where is the "block" located: prehepatic, intrahepatic and posthepatic portal hypertension

From a pathogenesis perspective, portal hypertension is not a single disease, but rather the general outcome of various "points of resistance." The prehepatic form is associated with an obstruction to the liver, such as portal vein thrombosis, when pressure increases due to a mechanical barrier at the entrance to the hepatic bloodstream. [26]

Intrahepatic variants are divided into presinusoidal, sinusoidal, and postsinusoidal. The sinusoidal form is most typical of cirrhosis and is best reflected by HVPG. Presinusoidal forms can cause severe portal hypertension with a relatively preserved sinusoidal zone, so their hemodynamic profiles differ. [27]

Posthepatic portal hypertension occurs when blood flow from the liver is impaired, such as in Budd-Chiari syndrome or severe right ventricular conditions. Here, the primary problem is a blockage at the outlet, and hepatic and portal pressures increase secondarily. [28]

This anatomical classification is important because identical complications can develop through different pathways, meaning the "weak links" in pathogenesis differ. In cirrhosis, a combination of structural and dynamic resistance plus hyperdynamic circulation often predominates, whereas in prehepatic forms, a local barrier to flow may be the dominant factor, with relatively intact hepatocyte function. [29]

Table 6. Localization of resistance and pathogenetic profile

Level	Typical examples	Leading mechanism
Prehepatic	Portal vein thrombosis	Mechanical barrier to the liver
Intrahepatic presinusoidal	Not all cirrhotic portal venopathy, some parasitic lesions	Increase in resistance to sinusoids
Intrahepatic sinusoidal	Cirrhosis	Structural plus dynamic growth of resistance
Posthepatic	Budd-Chiari syndrome, constrictive and right ventricular causes	Violation of venous outflow from the liver

[30]