Parietal and chief cells are very impermeable to the acid of the gastric lumen, as well as to CO 2 and NH 3 . Loss of HCl from vomiting can lead to a metabolic alkalosis and a hypochloremia. This intestinal smooth muscle activity is established first by pacemaker smooth muscle cells that spontaneously depolarize to form slow waves of pacesetter potentials. Cell membranes contain digestive brush border enzymes, with active sites facing out. Proteins are initially hydrolyzed by pepsin, but pancreatic juice enzymes and peptidase brush border enzymes are specialized for different parts of the amino acid sequences to produce amino acids, di- and tripeptides, which are then carried across the epithelium.
Cytosolic peptidases can finish the digestion to single amino acids.
4 Lipids are emulsified with bile and then digested with pancreatic lipase into fatty acids and monoglycerides. Epithelial cells take these up, resynthesize triglycerides to form chylomicrons that then travel in the lacteals of the villi. 4 Diarrhea can therefore lead to a metabolic acidosis. The waste's that's left goes to the rectum, where there is an increase in rectal pressure and an urge to defecate. The internal anal sphincter keeps the feces from entering the anal canal, with a voluntary external anal sphincter as the final arbiter.