The clear straw-colored serous fluid secreted by the cells of the peritoneum. The few milliliters present in the peritoneal cavity moisten the surfaces of the two peritoneal layers and allow them to glide over each other as the intestinal tract changes shape during the process of digestion and absorption. In certain disease states (such as right-sided heart failure, cirrhosis, or ovarian malignancy) the amount of peritoneal fluid is increased.