Final answer:
Ascites formation in the initial phases of portal hypertension is due to increased hydrostatic pressure, resulting from the resistance to blood flow within the liver.
Step-by-step explanation:
The initial phases of portal hypertension lead to ascites primarily through increased hydrostatic pressure in the portal venous system. This increase in pressure is caused by the resistance to blood flow through the liver, as seen in conditions such as cirrhosis, resulting in a backlog of fluid that gets pushed out into the peritoneal cavity, leading to ascites.
Hydrostatic pressure is the force exerted by the blood against the blood vessel walls. In the case of portal hypertension, when this pressure rises in the portal vein system, it opposes the osmotic pressure that normally helps retain fluid within the vessels, causing fluid to leak into the peritoneal cavity. Ascites form when this imbalance between pressures favors the leaking of fluid rather than its retention.