http://www.britannica.com/EBchecked/topic/484656/pylorus?anchor=ref237588
pylorus, 
cone-shaped constriction in the gastrointestinal tract that demarcates the end of the stomach and the beginning of the small intestine. The main functions of the pylorus are to prevent intestinal contents from reentering the stomach when the small intestine contracts and to limit the passage of large food particles or undigested material into the intestine.

The internal surface of the pylorus is covered with a mucous-membrane lining that secretes gastric juices. Beneath the lining, circular muscletissue allows the pyloric sphincter to open or close, permitting food to pass or be retained. 


 

Causes

Stomach and pyloric valve

In dumping syndrome, food and gastric juices from your stomach move to your small intestine in an unregulated, abnormally fast manner. This accelerated process is most often related to changes in your stomach associated with surgery. For example, when the opening (pylorus) between your stomach and the first portion of the small intestine (duodenum) has been damaged or removed during an operation, dumping syndrome may develop.

Dumping syndrome may occur at least mildly in one-quarter to one-half of people who have had gastric bypass surgery. It develops most commonly within weeks after surgery, or as soon as you return to your normal diet. The more stomach removed or bypassed, the more likely that the condition will be severe. It sometimes becomes a chronic disorder.

Gastrointestinal hormones also are believed to play a role in this rapid dumping process.


http://emedicine.medscape.com/article/173594-overview

 

Background

The stomach serves as the receptive and storage site of ingested food. The primary functions of the stomach are to act as a reservoir, to initiate the digestive process, and to release its contents downstream into the duodenum in a controlled fashion. The capacity of the stomach in adults is approximately 1.5-2 liters, and its location in the abdomen allows for considerable distensibility. Gastric motility is regulated by the enteric nervous system, which is influenced by extrinsic innervation and by circulating hormones. Alterations in gastric anatomy after surgery or interference in its extrinsic innervation (vagotomy) may have profound effects on gastric emptying. These effects, for convenience, have been termed postgastrectomy syndromes.

Postgastrectomy syndromes include small capacity, dumping, bile gastritis, afferent loop syndrome, efferent loop syndrome, anemia, and metabolic bone disease. Postgastrectomy syndromes are iatrogenic conditions which may arise from partial gastrectomies, independent of whether the gastric surgery was initially done for peptic ulcer disease, cancer, or for weight loss (bariatric). The surgical procedures include Billroth-I, Billroth-II, and Roux-en-Y.¹

Pathophysiology

Dumping is the effect of altered gastric reservoir function, and abdominal postoperative gastric motor function.²The early dumping syndrome and reflux gastritis are less frequent when segmented gastrectomy rather than distal gastrectomy is performed for early gastric cancer.³ In persons with long segment Barrett esophagus treated with a truncal vagotomy, partial gastrectomy, plus Roux-en-Y gastrojejunostomy, 41% developed dumping within the first 6 months after surgery, but severe dumping is rare (5% of cases).⁴ 

The dumping syndrome occurs in 45% of persons who are malnourished and who have had a partial or complete gastrectomy.⁵ The late dumping syndrome is suspected in the person who has symptoms of hypoglycemia in the setting of previous gastric surgery, and this late dumping can be proven with an oral glucose tolerance test (hyperinsulinemic hypoglycemia), as well as gastric emptying scintigraphy that shows the abnormal pattern of initially delayed and then accelerated gastric emptying.⁶ 

Clinically significant dumping syndrome occurs in approximately 10% of patients after any type of gastric surgery. Dumping syndrome has characteristic alimentary and systemic manifestations. It is the most common and often disabling postprandial syndrome observed after a variety of gastric surgical procedures, such as vagotomy, pyloroplasty, gastrojejunostomy, and laparoscopic Nissan fundoplication. Dumping syndrome can be separated into early and late forms, depending on the occurrence of symptoms in relation to the time elapsed after a meal. Both forms occur because of rapid delivery of large amounts of osmotically active solids and liquids into the duodenum. Dumping syndrome is the direct result of alterations in the storage function of the stomach and/or the pyloric emptying mechanism.

Pathophysiology of dumping syndrome.

Gastric resection reduces the fundic reservoir, thereby reducing the stomach's receptiveness (accommodation) to a meal. Vagotomy increases gastric tone, similarly limiting accommodation. An operation in which the pylorus is removed, bypassed, or destroyed increases the rate of gastric emptying. Duodenal feedback inhibition of gastric emptying is lost after a bypass procedure, such as gastrojejunostomy. Accelerated gastric emptying of liquids is a characteristic feature and a critical step in the pathogenesis of dumping syndrome. Gastric mucosal function is altered by surgery, and acid and enzymatic secretions are decreased. Also, hormonal secretions that sustain the gastric phase of digestion are affected adversely. All these factors interplay in the pathophysiology of dumping syndrome.