Stomach function

The stomach has a reputation as the place where food gets put to use, and that is not what it does. Its jobs are actually different: it forms chyme, the semisolid mixture it turns food into; it makes proteins needed to absorb vitamins; it takes part in microbial defense, meaning holding back whatever comes in alive with the food; and it propagates the peristaltic reflex, the wave of contraction that pushes contents along the digestive tract. What it does not do is absorb nutrients. Not one.

The mechanical side has a curious precision to it. Mixing waves, a specific type of peristalsis, run along its wall, churning and softening food with the gastric juices until it becomes chyme. And at the exit, the pylorus, the gateway to the intestine, works like a strainer: it lets only liquids and small particles through.

One detail changes the ranking of everything above. Of all the stomach's digestive functions, only one is necessary for life: producing intrinsic factor, a protein the small intestine needs in order to absorb vitamin B12 in its final stretch, the terminal ileum.