Gastrointestinal secretions and vomiting

Abstract

The two main gastrointestinal secretions are mucus and digestive enzymes. There are numerous pits or crypts throughout the gastrointestinal tract containing specialized secretory cells. Tubular glands are present in the stomach and proximal duodenum. Specialized glands include the salivary glands, pancreas and liver. The presence of food in the gastrointestinal tract causes direct stimulation of mucous (goblet) cells and nearby glands, which release digestive enzymes. The mucus protects the epithelium and acts as a lubricant. Mucous cells and glands are also stimulated by local enteric reflexes in response to distension or irritation. Parasympathetic stimulation increases secretion from salivary, oesophageal, gastric, pancreatic, duodenal Brunner's and distal large intestinal glands. Secretion from the rest of the small intestine and proximal large intestine is controlled predominantly by local neural and hormonal stimuli. Sympathetic stimulation alone slightly increases gastrointestinal secretion, but in the presence of parasympathetic or hormonal stimulation can reduce secretion because it induces constriction of splanchnic blood vessels. Nausea is a conscious awareness of arousal of the vomiting centre or related areas of the brain. It often precedes vomiting and may be initiated by higher CNS centres or local irritation in the gastrointestinal tract. Vomiting can be initiated by overdistension or irritation of the upper gastrointestinal tract, resulting in afferent impulses transmitted via the parasympathetic (vagus) and sympathetic nervous systems to the vomiting centre in the medulla. It can also be initiated by stimulation of other areas of the brain, especially the chemoreceptor trigger zone.