Aspects of the physiology of the colon

physiology of the colonThe primary functions of the colon are absorption, secretion, motility, and intraluminal digestion. These interrelated phenomena process ileal effluent and convert it into semisolid feces that are stored until defecation is convenient. Regional variations in function are significant. The proximal colon absorbs electrolytes and water more efficiently than do the descending colon and rectum, and motility and intraluminal digestion differ by region also. Loss of colonic function through disease or surgery results in a continuous discharge of food wastes and increases daily intestinal losses of water and electrolytes, chiefly sodium and chloride.

The small intestine digests and absorbs most nutrients from ingested foods. The role of the colon in human nutrition is not well defined. Metabolism of carbohydrate to absorbable volatile fatty acids is probably important. Ureolysis—conversion of circulating urea to ammonia, which is reabsorbed and reused—may be significant. The colon also absorbs amino acids, bile acids, and vitamin K, but the contribution of the colon to homeostasis by these mechanisms has not been quantified.

Intestinal Gas

The volume and composition of intestinal gas vary greatly among normal individuals. The small intestine contains approximately 100 mL of gas and the colon somewhat more. Some gas is absorbed through the mucosa and excreted through the lungs, and the remaining 400–1200 mL/d are discharged as flatus.

Nitrogen (N2) comprises 30–90% of intestinal gas. Swallowed air is the principal source of intestinal N2, but N2 also can diffuse across the mucosa from blood to lumen when other gases are produced in sufficient volume to lower the partial pressure of N2 and establish a gradient for diffusion. Other intestinal gases include oxygen (O2), carbon dioxide, hydrogen, methane and odoriferous trace substances such as methyl sulfide, hydrogen sulfide, indole, and skatole. H2 and CO2 are generated by fermentation of ingested non-absorbed carbohydrate, especially carbohydrate present in polysaccharides (eg, fiber) and some starches. Lactose in milk provides the substrate in lactase-deficient persons. Mucus is the main endogenous source of carbohydrate in the colon; intestinal glycoproteins are 80% carbohydrate. Only about one-third of the population produces CH4, which is a product of colonic bacteria that use hydrogen to reduce CO2. Stools of CH4 producers nearly always float, even in the absence of fecal fat. CH4, like H2, can be measured in the breath. H2 and CH4 are explosive gases, and caution must be exercised when using electrocautery in the bowel lumen.

Patients with “excessive gas” may complain of abdominal pain and distention, increased flatus, and watery stools. Some of these patients have irritable bowel syndrome. “Increased” flatus may reflect extreme sensitivity of the rectum to small volumes, resulting in frequent passage of gas. Alternatively, gas may be produced in excessive quantities in symptomatic patients. Almost invariably, hydrogen is the culprit. Measurement of breath hydrogen is a potentially useful test for malabsorption states. Treatment of overproduction of gas at present is directed toward elimination of lactose, legumes, and wheat from the diet.

Absorption

The colon participates in maintaining the body economy by absorption of water and electrolytes, but the absorptive function of the colon is not essential to life. Although amino acids, fatty acids, and some vitamins can be absorbed slowly from the large bowel, only a small amount of these nutrients reaches the colon normally. Perhaps 10–20% of ingested starch, however, passes unabsorbed into the colon, where bacterial fermentation converts starch to short-chain volatile fatty acids (eg, acetate). Absorption of fatty acids contributes importantly to assimilation of calories. Dietary celluloses and hemicelluloses are degraded by colonic bacteria.

Approximately 1000–2000 mL of ileal effluent consisting of 90% water enters the cecum each day. This material is desiccated during transit through the colon, so that only 100–200 mL of water is excreted in the feces. Table 30–1 gives average values for the electrolyte and water composition of ileal effluent and feces; the differences provide a rough estimate of colonic absorption and secretion. Data are listed also for the estimated maximal absorptive capacity, which is greater in the right colon than in the left. This capacity depends on the rate at which fluid enters the cecum. Normally, formed feces are composed of 70% water and 30% solids. Almost half of these solids are bacteria; the remainder is food waste and desquamated epithelium.

Sodium is absorbed by an active transport mechanism that is enhanced by mineralocorticoids, glucocorticoids, and volatile fatty acids produced by bacteria. Volatile fatty acids may be essential mucosal nutrients for normal colonic absorption of electrolytes and water. There are segmental differences in the mode of absorption of sodium and water. Normally, sodium absorption is so efficient that a person can remain in balance on as little as 5 meq in the daily diet, but colectomy increases the minimum daily requirements to 80–100 meq to offset losses from the ileostomy. Potassium enters feces by passive diffusion and by secretion in mucus. Excessive mucus production may occur in colitis or with certain tumors such as villous adenomas and may lead to substantial potassium losses in the stool. Chloride is absorbed in exchange for bicarbonate.

Classification of Constipation and Obstructed Defecation

Constipation

  • Normal colon
  • Normal transit
  • Slow transit
  • Megacolon/megarectum
  • Congenital
  • Acquired

Obstructed defecation

  • Descending perineum syndrome
  • Rectal intussusception
  • Complete rectal prolapse
  • Anismus (inappropriate sphincter contraction)

A thorough history and physical examination may elucidate the origin of the symptoms, eg, depression, psychotropic or other drugs, or anatomic abnormalities. Further investigation of chronic idiopathic constipation requires assessment of colonic transit and study of pelvic floor function. Colonic transit is evaluated by obtaining serial plain abdominal x-rays after ingestion of tiny radiopaque markers or by scintigraphy after ingestion of radiolabeled solid pellets. Tests of pelvic floor function include defecography, anorectal manometry, electromyography, nerve conduction studies and magnetic resonance imaging.

Obstructed defecation related to rectal prolapse responds to operative repair of the prolapse. Rectal intussusception is treated with fiber, water, and stimulating bowel movements with suppositories for mild to moderate situations. Patients are instructed to stimulate a bowel movement in the morning with a suppository and to ignore the urge to defecate during the day. The sense of rectal fullness that the patient experiences is a result of the proximal bowel prolapsing into the distal rectum. With this behavioral modification, the symptoms usually resolve. Biofeedback therapy may be a helpful adjunct. Surgical repair is reserved for severe cases of rectal intussusception.

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