Final answer:
The deep folds that increase surface area in the small intestine are known as circular folds, or plicae circulares.
Step-by-step explanation:
The deep folds of both the mucosa and submucosa in the small intestine that increase surface area are known as circular folds (plicae circulares). These folds, which are also called a plica circulare, begin near the proximal part of the duodenum and end near the middle of the ileum. Their distinct shape causes the chyme to spiral, as opposed to moving in a straight line, which slows its movement and facilitates more thorough absorption of nutrients.
The small intestine's surface area is further enlarged thanks to features like villi and microvilli. The villi are folds on the small intestine lining, while the microvilli are cytoplasmic extensions on the absorptive cells, both of which aid in maximizing the surface area for nutrient absorption.