Why cirrhosis causes ascites: pressure, protein and sodium together

Step one: portal pressure pushes fluid out

Fibrosis and regenerating nodules increase resistance to blood flowing through the liver's sinusoids. Pressure rises in the portal vein and in the vessels that drain into it. Higher hydrostatic pressure in the sinusoids and the gut's capillaries forces fluid out into the tissues, and when lymph drainage cannot keep up, fluid collects in the peritoneal cavity.

Portal hypertension is the main driver of ascites in cirrhosis. It is also why ascitic fluid from cirrhosis typically has a high serum-ascites albumin gradient, a laboratory clue clinicians use to separate portal hypertension from causes such as infection or cancer. The nurse may see this result after a diagnostic paracentesis and should recognise what it points to.

Step two: low albumin weakens the pull back in

Albumin is made by the liver and creates oncotic pressure, which draws fluid back into blood vessels. As liver function declines, albumin production falls. With less oncotic pull opposing the higher hydrostatic push, the balance of Starling forces tips further toward fluid leaving the circulation, both into the abdomen and into the legs as peripheral oedema.

This explains why the client can be fluid overloaded in the tissues yet effectively short of volume in the arteries. It also explains why intravenous albumin is often given alongside large-volume paracentesis: removing several litres of fluid can shift volume out of the circulation again, and albumin helps support circulating volume and kidney perfusion.

Step three: the kidneys keep refilling the leak

Nitric oxide and other vasodilators widen the splanchnic arteries, so blood pools in the gut circulation. The kidneys sense an underfilled arterial system and activate the renin-angiotensin-aldosterone system and sympathetic nerves. They retain sodium avidly, and water follows. That retained fluid enters the high-pressure portal system and leaks out again, so ascites keeps reaccumulating.

This step is the target of standard treatment. Dietary sodium restriction limits what the kidneys can hold onto, and spironolactone blocks aldosterone, often combined with a loop diuretic. Fluid restriction is usually reserved for low serum sodium rather than for ascites itself. Each intervention makes sense once the client's sodium retention is understood as the engine of reaccumulation.

Signs that the mechanism is going wrong

The same processes that cause ascites can tip into complications. Tense ascites can push the diaphragm upward and cause breathlessness, especially when lying flat; positioning with the head of the bed raised can ease breathing. Diuretics that remove fluid too quickly can lower blood volume further, so rising creatinine, dizziness, falling sodium or new confusion during treatment need prompt reporting.

Ascitic fluid is low in protein and has weak defences against bacteria, which is part of why spontaneous bacterial peritonitis develops. It may present subtly, with mild fever, vague abdominal discomfort or worsening encephalopathy rather than dramatic pain. Kidney function can also decline as the circulation becomes more underfilled, so urine output is a useful bedside indicator of how the client is coping.

Nursing assessment and a worked scenario

Daily weight at the same time and on the same scale tracks fluid better than girth alone, though girth measured at a marked level adds information. Monitor intake, output, serum sodium, potassium and creatinine during diuretic therapy. Fever, abdominal tenderness or new confusion in a client with ascites raises concern for spontaneous bacterial peritonitis and needs prompt reporting.

In a hypothetical case, a client with cirrhosis has gained weight, has a tense abdomen and asks why salt matters when the problem is water. Options include advising extra fluids, explaining that the kidneys hold sodium and water follows it, or suggesting a high-protein salty snack. The middle option is correct because it names the mechanism the restriction targets.

Sources and further reading

Merck Manual Professional: Ascites. Portal hypertension, splanchnic vasodilation, low albumin and renal sodium retention; SAAG; sodium restriction, diuretics and albumin with paracentesis.

Merck Manual Professional: Portal hypertension. Increased sinusoidal resistance in cirrhosis as the cause of raised portal pressure.

NIDDK: Symptoms and causes of cirrhosis. Ascites and lower-leg oedema as features of advancing cirrhosis.