Bicarbonate secretion serves as protection for the epithelium, particularly in the sections closest to the pylorus in the duodenum, guarding against damage caused by acid and pepsin. The small intestine plays a vital role in absorbing water-soluble vitamins (e.g., vitamin C, cobalamin) and minerals such as calcium, magnesium, and iron. These substances are typically taken up from the contents within the intestine through specific transporters. The uptake of these substances is also regulated by feedback mechanisms that monitor their concentration in the bloodstream and adjust the expression of transporters accordingly.

The smooth muscle layers in the small intestine generate motility patterns that aid in mixing chyme with digestive secretions and propel fluids along the length of the intestine, facilitating the absorption of nutrients, water, and electrolytes. During the postprandial period, the motor patterns of the small intestine mainly focus on mixing and consist of segmenting and retropulsive contractions that slow down the passage of the meal while digestion is ongoing. Segmentation refers to rhythmic contractions that are regulated by the enteric nervous system and are displayed in a stereotypical pattern as depicted in Fig. 30.20. Although the hormonal mediators of this fed pattern of motility are not well-defined, CCK is known to contribute to this process. CCK also plays a significant role in slowing down gastric emptying when the meal is in the small intestine, as explained earlier in this chapter. This mechanism ensures that nutrient delivery matches the capacity to digest and absorb the meal components.

After the meal has been digested and absorbed, it is necessary to clear any undigested residues from the lumen to prepare the intestine for the next meal. Peristalsis, which is a coordinated sequence of contractions above the intestinal contents and relaxation below, facilitates the movement of contents over considerable distances. Peristalsis is triggered by acetylcholine and substance P released towards the site of intestinal distention, causing contraction of the circular muscle. Additionally, the inhibitory effects of VIP and nitric oxide on the caudal side play a role in peristalsis. Similar to segmentation, peristalsis occurs when action potentials generated by intrinsic innervation overlap with sites of cellular depolarization dictated by the basic electrical rhythm. During fasting, peristaltic motor patterns are organized into phases known as the migrating motor complex (Fig. 30.21).
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 (