In some surgical procedures, the stomach is left in place with intact blood supply. In some cases, it may shrink a bit and the lining (mucosa) may atrophy, but it generally remains unchanged.
The lower stomach still contributes to the function of the intestines even though it does not receive or process food — it makes intrinsic factor, necessary to absorb vitamin B12 and contributes to hormone balance and motility of the intestines. In BPD procedures, some portion of the stomach is completely removed.