COLON , RECTUM AND ANUS


COLON , RECTUM AND ANUS

Colorectal Physiology

I. Normal Colon Function

A. Water absorption

Normal ileal effluent totals 900 to 1,500 mL/day, with stool water loss typically less than 200 mL/day. The right colon maximally can absorb 6 L of fluid/day, and only when large-bowel absorption is less than 2 L/day does an increase in fecal water content result in diarrhea.

B. Electrolyte transport

Sodium and chloride absorption occur by active processes in exchange for potassium and bicarbonate in the right colon.

C. Nutrition

Although absorption of nutrients is minimal in the colon, mucosal utilization of short-chain fatty acids (SCFAs) produced by colonic bacteria can account for up to 540 kcal/day. Chronic absence of SCFAs such as butyrate and proprionate results in “diversion colitis,” a condition characterized by rectal bleeding and rare stricture formation. Subclinical diversion colitis occurs in almost all diverted patients and uniformly resolves following stomal closure.

D. Motility patterns of the colon

Motility patterns of the colon allow for mixing and elimination of intestinal contents. Factors influencing motility include emotional state, amount of exercise and sleep, amount of colonic distention, and hormonal variations.

  • Retrograde movements occur mainly in the right colon. These contractions prolong the exposure of luminal contents to the mucosa and thereby increase the absorption of fluids and electrolytes.
  • Segmental contractions, the most commonly observed motility pattern, represent localized simultaneous contractions of the longitudinal and circular colonic musculature in short colonic segments.
  • Mass movements occur three to four times a day and are characterized by an antegrade, propulsive contractile wave involving a long segment of colon.

E. Microflora

One third of the dry weight of feces is normally composed of bacteria. Anaerobic Bacteroides species are most prevalent (1011/mL), whereas Escherichia coli has a titer of 109/mL. Bacteria produce much of the body’s vitamin K. Endogenous colonic bacteria also suppress the emergence of pathogenic micro-organisms. Antibiotic therapy can alter the endogenous microflora, resulting in changes in drug sensitivity (warfarin) or infectious colitides due to pathogenic microbial overgrowth (Clostridium difficile colitis).

F. Colonic gas

Colonic gas (200 to 2,000 mL/day) is composed of (1) swallowed oxygen and nitrogen and (2) hydrogen, carbon dioxide, and methane produced during fermentation by colonic bacteria. Because hydrogen and methane are combustible gases that may explode when electrocautery is used for biopsy, adequate bowel cleansing is mandatory before using hot-snare techniques during colonoscopy.