The major distinguishing feature of this C-shaped first pa of the small intestine duodenum is the presence of duodenal (Brunner's) glands in the submucosa. The mucous cells of these glands produce an alkaline secretion (pH 8.1-9.3) that enters the lumen through the crypts. It protects the duodenal lining from the acidity of the chyme and raises the luminal pH to optimize pancreatic enzyme activity.