Gastric acid
The main constituent of gastric acid is hydrochloric acid which is produced by parietal cells (also called oxyntic cells) in the gastric glands in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the hydrochloric acid is secreted into the lumen of the stomach. The pH of gastric acid is 1.5 to 3.5 in the human stomach lumen, the acidity being maintained by the proton pump H/K ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
Polycarbophil calcium
The human stomach presents a mild acidic environment due to the presence of HCl. Polycarbophil absorbs about ten times its own weight of water under acidic conditions, but the swelling ratio markedly increases at above pH 4.0 and reaches 70 times the initial weight under pH-neutral conditions. The swelling of polycarbophil is not affected by non-ionic osmolarity, but by ionic strength, showing a decrease with increase of ionic strength. Monovalent metal ions such as sodium and potassium ions in gastrointestinal fluid do not reduce the equilibrium swelling of polycarbophil, but divalent ions such as calcium and magnesium ions do. However, calcium ions only slightly reduce the equilibrium swelling under sodium-rich conditions.