Surgicaltreatmentofobesity,so-calledbariatricsurgery,canachievesubstantialandlastingweightlossandalsohelpameliorateassociatedhealthproblemssuchasinsulinresistance,hyperlipidemia,andelevatedbloodpressure.Initially,surgeryinvolvedjejunoileal bypass, theremovalofasubstantialpartoftheabsorptivesmallintestine,butthisprocedureisassociatedwithmalabsorptionandsubsequentundesirablesequelaesuchasdiarrhea.Avarietyofrevisedsurgicalapproachestoobesityhavebeendevised,includingRoux-en-Ygastricbypassandverticalsleevegastrectomy.Themechanismsbywhichtheseproceduresarethoughttobesuccessfullieinthesmallsizeoftheresidualgastricpouch,wherebymealsizeisdecreasedbecauseofearlysatiety,andabeneficialeffectofthebypassontheprofilesofGIhormones.Recentdataimplythateffectsofsurgeryonbileacidsandthemicrobiomemayalsocontributetobothweightlossandmetabolicbenefits.
Most of the nutrients ingested by humans are in the chemical form of macromolecules. However, such molecules are too large to be assimilated across the epithelial cells that line the intestinal tract and must therefore be broken down into their smaller constituents by processes of chemical and enzymatic digestion. Secretions arising from the pancreas are quantitatively the largest contributors to enzymatic digestion of the meal. The pancreas also provides additional important secretory products that are vital for normal digestive function. Such products include substances that regulate the function or secretion (or both) of other pancreatic products, as well as water and bicarbonate ions. The latter are involved in neutralizing gastric acid so that the small intestinal lumen has a pH approaching 7.0. This is important because pancreatic enzymes are inactivated by high levels of acidity and also because neutralization of gastric acid reduces the likelihood that the small intestinal mucosa will be injured by such acid acting in combination with pepsin. Quantitatively the pancreas is the largest contributor to the supply of bicarbonate ions needed to neutralize the gastric acid load, although the biliary ductules and duodenal epithelial cells also contribute.
As in the salivary glands, the pancreas has a structure that consists of ducts and acini. The pancreatic acinar cells line the blind ends of a branching ductular system that eventually empties into the main pancreatic duct and from there into the small intestine under control of the sphincter of Oddi. Also in common with salivary glands, a primary secretion arises in the acini, which is subsequently modified as it passes through the pancreatic ducts. In general the acinar cells supply the organic constituents of the pancreatic juice in a primary secretion whose ionic composition is comparable to that of plasma, whereas the ducts dilute and alkalinize the pancreatic juice while reabsorbing chloride ions (Fig. 30.2 ). The major constituents of pancreatic juice, which amounts to approximately 1.5 L/day in adult humans, are listed in
Box 30.1 . This list also outlines the functions of pancreatic secretory products. Many of the digestive enzymes produced by the pancreas, particularly the proteolytic enzymes, are produced as inactive precursor forms. Storage in these inactive forms is critically important in preventing the pancreas from digesting itself.