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ANATOMIC CONSIDERATIONS

The gastrointestinal (GI) tract extends from the mouth to the anus and is composed of organs with distinct functions. Sphincters that assist in gut compartmentalization separate the organs. The gut wall is organized into distinct layers that contribute to regional activities. The mucosa is a barrier to luminal contents or a site for fluid and nutrient transfer. Smooth muscle in association with the enteric nervous system mediates propulsion between regions. Many GI organs possess a serosal layer that provides a supportive foundation and permits external input.

Interactions with other systems serve the needs of the gut and the body. Pancreaticobiliary conduits deliver bile and enzymes into the duodenum. The vascular supply is modulated by GI activity. Lymphatic channels assist in gut immune activities. Intrinsic nerves provide the controls for propulsion and fluid regulation. Extrinsic neural input provides volitional or involuntary control that is specific for each gut region.

FUNCTIONS OF THE GI TRACT

The GI tract serves two main functions—assimilating nutrients and eliminating waste. In the mouth, food is processed, mixed with salivary amylase, and delivered to the gut lumen. The esophagus propels the bolus into the stomach; the lower esophageal sphincter prevents oral reflux of gastric contents. The squamous esophageal mucosa protects against significant diffusion or absorption. Aboral esophageal contractions coordinate with relaxation of the upper and lower esophageal sphincters on swallowing.

The stomach triturates and mixes the food bolus with pepsin and acid. Gastric acid also sterilizes the upper gut. The proximal stomach serves a storage function by relaxing to accommodate the meal. Phasic contractions in the distal stomach propel food residue against the pylorus, where it is ground and thrust proximally for further mixing before it is emptied into the duodenum. The stomach secretes intrinsic factor for vitamin B12 absorption.

Most nutrient absorption occurs in the small intestine. The mucosal villus architecture provides maximal surface area for absorption and is endowed with specialized enzymes and transporters. Triturated food from the stomach mixes with pancreatic juice and bile in the duodenum. Pancreatic juice contains enzymes for nutrient digestion and bicarbonate to optimize the pH for enzyme activation. Bile secreted by the liver and stored in the gallbladder is essential for lipid digestion. The proximal intestine is optimized for rapid absorption of most nutrients and minerals, whereas the ileum is better suited for absorbing vitamin B12 and bile acids. Bile contains by-products of erythrocyte degradation, toxins, medications, and cholesterol for fecal evacuation. Intestinal motor function delivers indigestible residue into the colon for processing. The ileocecal junction is a sphincter that prevents coloileal reflux, reducing microbial density.

The colon prepares waste for evacuation. The mucosa dehydrates the stool, reducing daily ileal volumes of 1000–1500 mL to 100–200 mL expelled from the rectum. The colon possesses a dense bacterial colonization that ferments undigested carbohydrates and short-chain fatty acids. The ...

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