The haustrating contractions are brought about by local contractions of the circular muscle layer. Many of the haustrating contractions are stationary or almost stationary. They give the colon its typical haustrated appearance. The movements of these contractions are so slow that during short observations one gets the impression that the haustrated appearance of the colon is based on preformed anatomical structures. Prolonged observations of colonic appearance on X-ray have made clear however that the characteristic haustration pattern changes continuously.

Using intraluminal manometry the haustrating contractions can be recognized as pressure waves of variable duration and amplitude, without a clearly recognizable rhythm (Fig. 7.4). Electromyographic studies of the colon have shown that electrical slow waves are present, but, unlike the situation in the small bowel, these lack regularity. Often multiple slow wave frequencies coexist in the colon.

The haustrating contractions of the colon do not lead to significant displacements of colonic contents. Their primary function is to mix and triturate the contents, enabling optimal contact with the mucosa. This ensures that the resorption of water and salts is optimal. The frequency and amplitude of haustrating contractions increase after a meal, and haustrating contractions almost cease during the night.

Mass contractions are powerful, prolonged circular contractions that travel in the direction of the anus. When studied manometrically, the pressure in the lumen of the colon is found to reach values of several hundreds of mmHg during these contractions. Hence, the alternative name high-amplitude peristaltic contractions (HAPCs). The propagation speed of the contractions is about 1 cm per second. HAPCs do not occur frequently; in the human colon about 6 HAPCs contractions can be seen per day. They occur preferentially in the morning after awakening, after breakfast, and after meals. HAPCs effectively propel colonic contents in the direction of the anus. When feces arrives in the rectum, a sensation of urgency can arise and defecation may follow.

It is difficult to define constipation. In fact, constipation comprises a variable assortment of symptoms. In the past often a low defecation frequency (fewer than three defecations per week) was used to define constipation. We now accept that constipation can also be present when defecation frequency is in the normal range. It has also been proposed to use fecal weight (in grams per 24 h) as an objective criterion for constipation. A production of less than 100 g would be abnormal. However, this criterion is difficult to use in daily practice. Many patients with constipation do not complain of a low frequency of defecation, and hardly any patient would report a low fecal weight. Rather, the patients’ presenting symptoms are hard stools, the feeling of incomplete evacuation, or the necessity to use straining (Table 7.1). In the Rome III classification all of these symptoms are taken into account for the diagnosis of functional constipation.

It is also difficult to define diarrhea. In addition to a higher frequency of defecation and an increased fecal output (more than 200 g per day), the Bristol stool form scale is used. In this scale consistency and shape of the stools are used to categorize the stools into one of seven types (Fig. 7.5).

Whereas constipation most often is caused by disordered motor function (of colon or pelvic floor), diarrhea is often the consequence of diminished resorption of water by the colonic mucosa. This can be caused by a variety of disorders such as inflammatory bowel disease (Crohn’s disease and ulcerative colitis). Diarrhea can also be the consequence of malabsorption at the level of the small intestine, as for instance in celiac disease. Both in cases with acute and chronic diarrhea motility disorders should only be considered when all other causes of diarrhea have been excluded (Table 7.2).

Not infrequently the gastroenterologist sees patients whose predominant complaint is that they have a distended abdomen. Some of these patients do not really have abdominal distension but they perceive a feeling of bloating. In others the abdomen is visibly distended. This condition can be caused by serious conditions such as ascites, hepatosplenomegaly, and large tumors or cysts in the abdomen, but often the distension is brought about by accumulation of gas, fluid, or feces in the gastrointestinal tract, in particular in the colon. Such accumulation of liquid, gaseous, or solid material in the colon can take place when the colonic transit is slow.

It is also possible however to have a visibly distended abdomen in the absence of an increase of the contents of the abdomen. This is brought about by a combination of a low position of the diaphragm, which is abnormally contracted, and relaxation of the abdominal wall muscles. Especially in patients with functional bowel disorders (such as irritable bowel syndrome) who complain of intermittent abdominal distension, this mechanism is frequently present.