Final answer:
Circular folds in the wall of certain regions of the GI tract, such as the small intestine, increase the absorptive surface area.
Step-by-step explanation:
The folds in the wall of certain regions of the GI tract, such as the small intestine, that increase the absorptive surface area are called circular folds.
Also known as plica circulares, these deep ridges in the mucosa and submucosa cause the chyme to spiral through the small intestine, slowing its movement and providing more time for nutrients to be absorbed.
Circular folds are one of the three modifications of the small intestine wall that increase the absorptive surface area, along with villi (projections of the mucosa) and microvilli (projections of the plasma membrane of absorptive cells).