How is required pH maintained in the stomach and small intestine?

Gastric glands present on the walls of the stomach release HCl. HCl create an acidic medium in stomach to facilitate the action of enzyme pepsin.

Bile juice from liver makes the food alkaline in small intestine for pancreatic enzymes to act.

Enzymes work because they have very specific shapes - active sites - which will only allow certain chemicals to bond with them (and therefore be affected). At temperatures or pH's that are above optimum, the enzyme becomes less efficient until it cannot work at all. We say that the enzyme is denatured.

In the small intestine, the enzymes that "work" there need an alkaline pH in order to have optimum working conditions.

A second reason is that strong acids (like hydrochloric) can end up damaging the lining of your intestine. The stomach is specially adapted to cope with these conditions (e.g. thick lining, regularly replaced cells) but the small intestine is not. Therefore, it needs the pH to be reduced to avoid damage.

Gastric acid or the digestive juice, formed in the stomach is composed of hydrochloric acid (HCl) and large quantities of potassium chloride (KCl) and sodium chloride (NaCl). The stomach contains parietal cells that secrete HCL. The HCL creates acidic pH in the stomach necessary for the action of enzyle pepsin. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring that it does not become too acidic. The pH of gastric acid is 1.35 to 3.5 in the human stomach, the acidity being maintained by the proton pump H+/K+ ATPase. The small intestine has a pH of around 7 to 8, that is, it is either neutral or slightly alkaline in nature. The pancreas secretes the bicarbonate ions through bile juice in the small intestines. These bicarbonates neutralize the acidic pH of the food from stomach and create an alkaline pH, thus maintain the required pH in small intestines.