Abdomen
The abdomen refers to the cavity in the mid-portion of the body bounded superiorly by the diaphragm that separates it from the thorax. The abdomen contains a number of organs including the pancreas, stomach, intestines, liver, spleen, bladder, and gallbladder.
The abdominal cavity is the largest cavity in the human body. It is somewhat oval in shape, with the long axis of the oval running upward and downward. There is a flaring out of the abdomen from bottom to top. The abdomen more accurately resembles an egg, with the narrower region of the oval pointing down. Variation in the size of the abdomen occurs with age and sex. The abdomen of an adult male tends to be oval but flattened in the front. In the adult female the oval shape tends to be narrower to accommodate the larger pelvic region. In children the bottom portion of the abdominal egg shape tends to be more pointed.
The abdomen sits between the lungs and the pelvis. At the top of the abdomen is the dome-like diaphragm. The bottom of the abdomen extends into the bony pelvic girdle. The muscles in this region are referred to as the diaphragm of the pelvis.
Anatomists tend to divide the abdomen into two imaginary regions for the purposes of description. The upper part of the oval is called the abdomen proper. The lower part of the oval is referred to as the pelvis. While there is no barrier between these regions, the abdomen does narrow, and this narrow region forms the boundary.
The abdomen proper is surrounded by muscle and a coating of muscle that is called the fascia. The action of the muscles allows this region of the abdomen to change in size and shape. This is particularly evident when someone is panting for breath.
The middle region of the abdomen houses the stomach. The lower region of the abdomen contains the bladder, colon, rectum, some of the small intestine, and some reproductive organs.
Another feature of the abdomen, which is especially evident when the region is surgically exposed, is a glistening membrane that covers many of the organs and the wall of the abdomen. This membrane, called the peritoneum, is the largest membrane in the body. The peritoneum is highly lubricated, which assists the packaging of the various organs into the abdominal space and the movement of these over one another during digestion and as the abdomen changes shape. Folds of the peritoneum, which are called the mesentery, extend from the abdominal walls to the various organs. The mesentery suspends the organs in place and houses blood vessels that supply the various organs with nutrients. Infection of the peritoneum can occur. This malady is called peritonitis.
Superficially the abdomen is divided by anatomists into nine regions. In the midline, the most superior region (closet to the thorax) is the epigastric region. The epigastric region is flanked by left and right hypochondriac regions. The navel lies in the center of the umbilical region. The umbilical region is flanked by left and right lumbar regions. The most inferior midline abdominal region is the hypogastric region. The hypogastric region is flanked by left and right iliac regions.
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