The Colonic Phase of the Integrated Response to a Meal

LEARNING OBJECTIVES

Upon completion of this chapter, you should be able to answer the following questions:

• 1

  What are the segments of the large intestine?

• 2

  How is motility of the large intestine controlled, and how does this subserve its physiological function?

• 3

  How is water and electrolyte transport in the large intestine controlled, and how does it differ from the process in the small intestine?

• 4

  What biological functions are contributed by the gut microbiota?

• 5

  How is defecation delayed until it is socially convenient, and what motility patterns underpin the evacuation of stool?

Overview of the Large Intestine

The most distal segment of the gastrointestinal tract is called the large intestine, which is composed of the cecum; ascending, transverse, and descending portions of the colon ; the rectum ; and the anus ( Fig. 31.1 ). The primary functions of the large intestine are to digest and absorb components of the meal that cannot be digested or absorbed more proximally, reabsorb the remaining fluid that was used during movement of the meal along the gastrointestinal tract, and store the waste products of the meal until they can conveniently be eliminated from the body. In fulfilling these functions, the large intestine uses characteristic motility patterns and expresses transport mechanisms that drive the absorption of fluid, electrolytes, and other solutes from the stool. The large intestine also contains the vast majority of a unique biological ecosystem, known as the gut microbiota, consisting of many trillions of commensal bacteria and other microorganisms that engage in a life-long symbiotic relationship with their human host. These microorganisms can metabolize components of the meal that are not digested by host enzymes and make their products available to the body via a process known as fermentation. Colonic bacteria also metabolize other endogenous substances such as bile acids and bilirubin, thereby influencing their disposition. There is emerging evidence that the colonic microbiota is critically involved in promoting development of the normal colonic epithelium and in stimulating its differentiated functions, as well as regulating development of the enteric nervous system. In addition, the microbiota can detoxify xenobiotics (substances originating outside the body, such as drugs) and protect the colonic epithelium from infection by invasive pathogens. Finally, the colon is both the recipient and the source of signals that allow it to communicate with other gastrointestinal segments to optimally integrate function. For example, when the stomach is filled with freshly masticated food, this triggers a long reflex arc that results in increased colonic motility (the gastrocolic reflex ) and eventually evacuation of the colonic contents. Similarly, the presence of luminal contents in the colon causes the release of both endocrine and neurocrine mediators that slow propulsive motility and decrease electrolyte secretion in the small intestine. This negative-feedback mechanism matches the rate of delivery of colonic contents to the segment’s capacity to process and absorb the useful components. Details of the signals that mediate this crosstalk between the colon and other components of the gastrointestinal system are reviewed in the next section.