The intestinal tract did not evolve to compensate for mechanical bowel obstruction caused by an immobile, fixed lesion such as cancer. The physiologic changes that arise with obstruction would be adaptive to reversible forms of bowel obstruction that may have occurred for our ancient ancestors, but they are maladaptive for patients with cancer. Similarly, kidneys demonstrate a maladaptive response to heart failure. Decreased renal perfusion is sensed as dehydration. Fluid is retained to compensate when, in fact, the patient is drowning. We adjust for this by "overruling" the kidneys, telling them to get rid of salt and water and not hold on to them. The kidneys can be mistaken. What "misunderstanding" arises in bowel obstruction?
Imagine that you have been very hungry. Your tribe finally hunts down a mastodon, and it is time for a feast. You gorge yourself, eating great chunks of meat and causing a temporary obstruction. Your body would respond in the following way.
1. Mechanoreceptors and chemoreceptors would be stimulated by the distention caused by the large build-up of food proximal to the blockage. These receptors would tell your brain to stop eating.
- 2. The intestine proximal to the blockage would begin hypersecreting fluid, trying to flood the system and wash the intestinal contents downstream.
- 3. Intestinal motility would increase, further trying to push contents downstream and causing cramping.
- With luck, you would live to hunt another day. While this approach works well for ingested mastodons, it works poorly for malignant bowel obstruction.