The peritoneum

The peritoneal membrane is conveniently divided into two parts the visceral surrounding the viscera and the parietal lining the other surfaces of the cavity. The peritoneum has a number of functions (Table 56.1). The parietal portion is richly supplied with nerves and, when irritated, causes severe pain accurately localised to the affected area. The visceral peritoneum, in contrast, is poorly supplied with nerves, and its irritation causes vague pain which is usually located to the midline.

The peritoneal cavity is the largest cavity in the body, the surface area of its lining membrane (2 m2 in an adult) being nearly equal to that of the skin. The peritoneal membrane is composed of flattened polyhedral cells (mesothelium), one layer thick, resting upon a thin layer of flbroelastic tissue. Beneath the peritoneum, supported by a small amount of areolar tissue, lies a network of lymphatic vessels and rich plexuses of capillary blood vessels from which all absorption and exudation must occur. In health, only a few millilitres of peritoneal fluid is found in the peritoneal cavity The fluid is pale yellow, somewhat viscid and contains lymphocytes and other leucocytes; it lubricates the viscera allowing easy movement and peristalsis.

In the peritoneal space mobile gas-filled structures float upwards under the influence of posture, as does free air (‘gas’). In the erect position, when free fluid is present in the peritoneal cavity, pressure is reduced in the upper abdomen compared with the lower abdomen. When air is introduced it rises, allowing all of the abdominal contents to sink.

During expiration intra-abdominal pressure is reduced and peritoneal fluid, aided by capillary attraction, travels in an upward direction towards the diaphragm. Experimental evidence shows that particulate matter and bacteria are absorbed within a few minutes into the lymphatic network through a number of ‘pores’ within the diaphragmatic peritoneum. This upward movement of peritoneal fluids is responsible for the occurrence of many subphrenic abscesses.

The peritoneum has the capacity to absorb large volumes of fluid: this ability is used during peritoneal dialysis in the treatment of renal failure. But the peritoneum can also produce an inflammatory exudate when injured (Table 56.2). When a visceral perforation occurs the free fluid which spills into the peritoneal cavity runs downwards, largely directed by the normal peritoneal attachments. For example, spillage from a perforated duodenal ulcer may run down the right paracolic gutter.

When parietal peritoneal defects are created healing occurs, not from the edges, hut by the development of new mesothelial cells throughout the surface of the defect. In this way large defects heal as rapidly as small defects.