Physiology of lower gastrointestinal tract.

Abstract

The lower gastrointestinal (GI) tract, formed from the midgut and hindgut, encompasses the colon, rectum and anal canal. The aim of this review is to provide an overview of the anatomy and physiology of the lower GI tract. Literature review on anatomy and physiology of the lower GI tract, including normal motility and phases of defecation. It derives its blood supply from the superior and inferior mesenteric arteries while it is innervated by the extrinsic autonomic (the thoracolumbar and sacral nerves) and the intrinsic enteric nervous system. The colon has four layers: mucosa, submucosa, muscularis externa and serosa. The anal canal ends in the internal and external anal sphincters (EASs) involved in continence and defecation. The lower GI tract is predominantly involved in digestion, absorption, defecation and protection. Defecation is a complex process that requires inter-neural (enteric and autonomic nervous systems), neurohormonal and neuromuscular coordination. It has four phases which include basal, pre-expulsive, expulsive and end phase. High-propagating contractions in the colon propel stool to the rectum leading to rectal distention and the recruitment of the recto-anal inhibitory reflex. Once able, the EAS, under full conscious control, is then relaxed allowing stool to be evacuated. Other defecation reflexes include the gastrocolic, gastroileal and coloanal reflexes. Recent advances provide novel techniques to investigate motility patterns including high-resolution manometry protocols with automated assessments, magnetic resonance imaging techniques for defecography, wireless motility capsules and fecobionics.