Which substance is secreted by parietal cells that is necessary for vitamin B12 absorption?

Parietal cells in the stomach are essential for the absorption of vitamin B12 due to their secretion of intrinsic factor. Intrinsic factor is a glycoprotein that binds to vitamin B12 in the stomach. This complex then travels to the ileum, the final part of the small intestine, where specific receptors on the intestinal cells recognize the intrinsic factor-vitamin B12 complex, allowing for the absorption of the vitamin into the bloodstream.

Vitamin B12 is crucial for several bodily functions, including red blood cell formation and neurological function. Without intrinsic factor, vitamin B12 cannot be effectively absorbed, leading to potential deficiencies, which can cause anemia and other health issues.

The other substances mentioned have different roles: hydrochloric acid, also secreted by parietal cells, helps digest food and creates an acidic environment for enzymes to function but does not directly facilitate the absorption of vitamin B12. Mucins are secreted by mucus cells to protect the stomach lining, and secretin is a hormone that regulates water homeostasis and the secretion of digestive enzymes in the pancreas, but it is not involved in vitamin B12 absorption. Thus, the only substance critical for vitamin B12 absorption provided by parietal cells is intrinsic factor.