Fig. 30.19.
Bicarbonate secretion protects the epithelium, particularly in the most proximal portions of the duodenum immediately downstream from the pylorus, from damage caused by acid and pepsin.
Absorption of Minerals and Water-Soluble Vitamins
The small intestine is also an important site for absorption of water-soluble vitamins (e.g., vitamin C, cobalamin) and minerals such as calcium, magnesium, and iron. In general these substances are taken up from the luminal contents via the activity of specific transporters. Their uptake is •Fig. 30.18 MechanismofCl−secretioninthesmallandlargeintestines.CFTR,cysticfibrosistransmembraneconductanceregula-tor;NKCC1,sodium/potassium/2chloridecotransporter1.Notethatadditionalchloridechannels(notshown),suchasthoseregulatedbyintracellularcalciumconcentration,mayalsoexistintheapicalmembraneandcontributetooveralltransport.˜Ca˜˜Cl°2K˜˜˜cAMPNKCC1ATP˜ also controlled by feedback mechanisms that sense the concentration of the relevant substrate in the circulation and adjust the expression of transporters and accessory molecules accordingly.
Motor Patterns of the Small Intestine
The smooth muscle layers in the small intestine produce motility patterns that mix chyme with the various digestive secretions and propel fluid along the length of the intestine so that nutrients (along with water and electrolytes) can be absorbed. Motor patterns of the small intestine during the postprandial period are directed predominantly toward mixing and consist largely of segmenting and retropulsive contractions that retard the meal while digestion is still ongoing. Segmentation is a stereotypical pattern of rhythmic contractions that is displayed in
Fig. 30.20 and presumably reflects programmed activity of the enteric nervous system superimposed on the basic electrical rhythm. Hormonal mediators of this fed pattern of motility are poorly defined, although CCK contributes. CCK also plays important roles in slowing gastric emptying when the meal is in the small intestine, as described at the beginning of this chapter. This makes sense as a mechanism to match nutrient delivery to the available capacity to digest and absorb the components of the meal.
•Fig. 30.19 Mechanismsofbicarbonatesecretionintheduodenum.CA,carbonicanhydrase;CFTR,cysticfibrosistransmembranecon-ductanceregulator;DRA,downregulatedinadenoma;NBC-1,sodium/bicarbonatecotransporter1;NHE-1,sodium/hydrogenexchanger-1;PAT1,putativeaniontransporter1.3Na˜Na˜Na˜H˜Cl°Cl°K˜NBC-1 NHE-1 DRA? PAT1? CFTR ATP CO2 + H2O CA HCO3 °HCO3 °
After the meal has been digested and absorbed, it is desirable to clear any undigested residues from the lumen to prepare the intestine for the next meal. Such clearance is effected by peristalsis (
Fig. 30.21 ), a coordinated sequence of contraction occurring above the intestinal contents and relaxation below that permits the contents to be conveyed over considerable distances. Peristalsis reflects the action of acetylcholine and substance P released orad to a site of intestinal distention, which serves to contract the circular muscle, as well as the inhibitory effects of VIP and nitric oxide on the caudad side. Like segmentation, peristalsis originates when action potentials generated by intrinsic innervation are superimposed on sites of cellular depolarization dictated by the basic electrical rhythm. The peristaltic motor patterns that occur during fasting, moreover, are organized into a sequence of phases known as the migrating motor complex (