Ascites

Definition

Ascites is the buildup of fluid inside the peritoneal cavity. It occurs when fluid moves out of the vascular system faster than the peritoneal lymphatics can remove it. This fluid shift is most commonly seen in liver cirrhosis but can also appear in several other systemic disorders.

Etiology

Cirrhosis remains the leading cause of ascites. Other conditions that can contribute include:

• Heart failure
• Constrictive pericarditis
• Abdominal cancers
• Nephrotic syndrome
• Severe protein–calorie malnutrition

These disorders disrupt the normal balance of vascular and lymphatic systems, making fluid accumulation more likely.

Pathophysiology

The splanchnic vasodilation model is the main explanation for how ascites forms and progresses. In cirrhosis, excess nitric oxide and other vasodilators widen the splanchnic arteries. This dilation causes blood to pool in the splanchnic region, lowering the adequate circulating blood volume, even though total body fluid volume increases.

The body senses this low effective volume and activates the renin–angiotensin–aldosterone system and antidiuretic hormone. These responses promote sodium and water retention as an attempt to restore arterial filling. As fluid is retained, plasma volume expands, but the continued vasodilation prevents proper redistribution, creating an underfill state. Underfill enhances sodium and water retention, driving ongoing fluid accumulation.

Fluid Shift Into the Peritoneal Cavity

At the same time, pressure inside the liver rises, leading to increased lymph formation. When the liver can no longer clear this lymph load, fluid leaks into the peritoneal space and becomes ascitic fluid. Systemic inflammation, intestinal changes, malignancy, and cardiac or renal dysfunction can further worsen this process.