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.
Pachypodium habitats
"Rapanarivo et al." measured the pH levels for the soil in half the sites of "Pachypodium" endemic to Madagascar. The pH ranges approximately from 3.5 to 7. Strictly acid soils that had a pH level of 3.5 to 5 were preferred by "Pachypodium brevicaule", "P. cactiples", "P. densiflorum", "P.eburneum", and "P. rosulatum". The next level, acid to basic soils, with a pH between 4.5 and 7 were suitable for "P. lamerei" and "P. rutenbergianum". On soils with a neutral pH level of 7, "P. meridionale" inhabit. "P. sofiense" grows on calcareous soils but also in acid soils with a pH level of 4. "P. lealii" grows on outcrops of granite in fairly fertile, acid soil.