Why does HCI acid not effect the stomach lining?

This is a misconception. Hydrochloric acid (HCl) does affect the stomach lining, but the stomach has several protective mechanisms to prevent damage.

The stomach lining is covered in a layer of mucus that acts as a barrier against HCl and other digestive juices. The mucus is produced by specialized cells in the stomach called mucous cells.

The stomach also produces bicarbonate ions (HCO3-) that help to neutralize HCl. Bicarbonate ions are produced by the parietal cells in the stomach.

In addition, the stomach lining is constantly being replaced by new cells. This process, called cell turnover, helps to repair any damage that is caused by HCl.

As a result of these protective mechanisms, the stomach is able to withstand the corrosive effects of HCl and other digestive juices.