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.