Both hormonal and neural mechanisms control the exocrine secretions of the pancreas( figure 21.23) An acidic chyme in the duodenum stimulates the release of secretin. Secretin stimulates the secretion of the aqueous component of pancreatic juice, which contains a large amount of bicarbonate ions. The bicarbonate ions increase the pH of chyme in the duodenum so that the duodenum is not damaged by the low pH. In addition, pancreatic and microvilli enzymes do not function at a low pH.