Final answer:
The correct sections of the digestive tract critical for vitamin B12 absorption are the stomach's parietal cells, which produce intrinsic factor, and the small intestine's ileum, where the vitamin B12-intrinsic factor complex is absorbed through endocytosis.
Step-by-step explanation:
Vitamin B12 Absorption in the Digestive Tract
The proper absorption of vitamin B12 is a critical function that involves two key sections of the digestive tract: the stomach and the small intestine, specifically the ileum. In the stomach, parietal cells are responsible for secreting intrinsic factor, which is essential for B12 absorption. Without intrinsic factor, vitamin B12 cannot be absorbed, and this can lead to nutritional deficiencies impacting red blood cell production and neurological functions. In individuals who have undergone a total gastrectomy, intramuscular vitamin B12 injections are necessary to maintain health due to the absence of intrinsic factor production. Once intrinsic factor is bound to vitamin B12 in the stomach, it travels to the ileum, the final section of the small intestine. Here, within the ileum's mucosal lining, the complex of vitamin B12 and intrinsic factor is absorbed through endocytosis.
Therefore, the correct answer to the question of which two sections are critical for vitamin B12 absorption is: 4) Stomach parietal cells and small intestine.