Etiology/ Pathophysiology of Ascites

The accumulation of ascitic fluid is due to an excess of sodium and water in the body.  But there are a few different theories that propose the actual cause of ascites because that is still somewhat unclear.  The theories are the under-filling, overflow, and peripheral arterial vasodilation theories.

http://medical-dictionary.thefreedictionary.com/Peritoneal+cavity+fluid
1) Under-filling Theory: the kidneys' sodium and water retention is due to ineffective blood volume circulation and portal hypertension which causes the splanchnic vascular bed to fill up with fluid in the wrong manner.

2) Overflow Theory: proposes that the main problem in ascites is that the kidney does not retain sodium and water as well as it should even when the volume is not abnormally low.

3) Peripheral Arterial Vasodilation Theory: includes parts of both of the above theories.  Suggests that portal hypertension causes vasodilation and a less effective arterial blood volume but also that more sodium in the kidney is retained and the volume of plasma expands as the disease that is causing the ascites continues.  The excess of fluid from the renal retention of sodium ends up overflowing into the peritoneum.  Says that usually in the earlier stages under-filling is common and overflow is common later in the progression of the disease which is usually cirrhosis.

    

https://www.netterimages.com/pathophysiology-of-ascites-formation-labeled-runge-im-2e-internal-medicine-frank-h-netter-19946.html


References:
http://emedicine.medscape.com/article/170907-overview#a5