Phase I of the MMC is characterized by relative quiescence, while phase II sees the onset of small disorganized contractions. During phase III, which lasts approximately 10 minutes, large contractions propagate along the length of the intestine, stimulated by the hormone motilin, sweeping any remaining gastric and intestinal contents into the colon. The pylorus and ileocecal valve fully open during this phase, allowing even large undigested items to pass from the body. Subsequently, the motility of the intestine reverts to phase I of the MMC, completing the entire cycle in approximately 90 minutes in adults unless a meal is consumed. In the presence of a meal, the MMC is suspended until motilin levels rise again.

The small intestine, consisting of the duodenum, jejunum, and ileum, receives the meal after it leaves the stomach. Its primary function is to digest and absorb the nutrients present in the meal. Chyme in the duodenum slows down further gastric emptying, ensuring that nutrient delivery matches the small intestine's ability to digest and absorb substances.

The digestion and absorption process in the small intestine are facilitated by two digestive juices: pancreatic juice derived from the pancreas and bile from the liver. The presence of the meal in the small intestine triggers hormonal and neural signals, leading to the secretion of these digestive juices.

Pancreatic secretions, originating from the acini, contain various proteins that aid in meal digestion or act as essential cofactors. The secretion becomes diluted and alkalinized as it travels through the pancreatic ducts.

Bile, produced by the liver and stored in the gallbladder until required, contains bile acids, which are crucial components with detergent-like properties that solubilize the products of lipid digestion.

Migrating motor complexes (MMCs) in the duodenum and jejunum, as recorded through manometry in fasting humans, display intense contractions (phase III) propagating aborally. Sequential recording points along the length of the duodenum and jejunum are labeled as D1, D2, J1, J2, and J3.

Carbohydrates and proteins, water-soluble macromolecules, are digested and absorbed through analogous mechanisms. Lipids, the third macronutrient, require specialized mechanisms to transfer the products of lipolysis to the epithelial surface for absorption. Additionally, the small intestine absorbs fat and water-soluble vitamins, as well as minerals such as calcium, magnesium, and iron.
Fig. 30.22 ). Phase I of the MMC is characterized by relative quiescence, whereas small disorganized contractions begin to occur during phase II. During phase III, which lasts about 10 minutes, large contractions that •Fig. 30.20 A,Radiographicviewshowingthestomachandsmallintestinefilledwithbariumcontrastmediuminanormalindividual.Notethesegmentationoftheintestine.B,Sequenceofsegmentalcontractionsinthesmallintestine.Lines1to4representsequentialtimepoints.Thedottedlinesindicatewherecontractionswilloccurnext;thearrowsdepictthedirectionofmovementoftheintestinalcontents.(A,FromGardenerEMetal.Anatomy: A Regional Study of Human Structure. 4thed.Philadelphia:Saunders;1975;B,redrawnfromCannonWB. Am J Physiol 1902;6:251.) •Fig. 30.21 Peristalticmotilityintheintestinepropelsintestinalcontentsalongthelengthofthesmallintestine.
propagate along the length of the intestine are stimulated by the hormone motilin and sweep any remaining gastric and intestinal contents out into the colon. The pylorus and ileocecal valve open fully during this phase, so even large undigested items can eventually pass from the body. Motility of the intestine then reverts to phase I of the MMC, with the entire cycle taking about 90 minutes in adults unless a meal is ingested, in which case the MMC is suspended. After the meal, motilin levels fall, and the MMC cannot be resumed until they rise again.
1.
On leaving the stomach, the meal enters the small intestine, which consists (sequentially) of the duodenum, jejunum, and ileum. The principal function of the small intestine is to digest and absorb the nutrients contained in the meal.
2.
The presence of chyme in the duodenum retards additional gastric emptying, thus helping match nutrient delivery to the ability of the small intestine to digest and absorb such substances.
3.
Digestion and absorption in the small intestine are aided by two digestive juices derived from the pancreas (pancreatic juice) and liver (bile). These secretions are triggered by hormonal and neural signals activated by the presence of the meal in the small intestine.
4.
Pancreatic secretions arise from the acini and contain various proteins capable of digesting the meal or acting as important cofactors. The secretion is diluted and alkalinized as it passes through the pancreatic ducts.
5.
Bile is produced by the liver and stored in the gallbladder until needed in the postprandial period. Bile acids, important components of bile, are biological detergents that solubilize the products of lipid digestion.
•Fig. 30.22 Migratingmotorcomplexesintheduodenumandjejunumasrecordedfromafastinghumansubjectbymanometry.D1,D2,J1,J2,andJ3indicatesequentialrecordingpointsalongthelengthoftheduodenumandjejunum.Theintensecontractions(phaseIII)propagateaborally.(RedrawnfromSofferEEetal. Am J Gastroenterol 1998;93:1318.) 6.
Carbohydrates and proteins, water-soluble macromolecules, are digested and absorbed by broadly analogous mechanisms. Lipids, the third macronutrient, require special mechanisms to transfer the products of lipolysis to the epithelial surface where they can be absorbed. The small intestine also absorbs fatand water-soluble vitamins, as well as minerals such as calcium, magnesium, and iron.
7.