The small intestine exhibits a unique histological structure that significantly enhances its function in digestion and nutrient absorption. These structures include circular folds, villi, and various specialized cells that collectively facilitate the digestion of food.

The intestinal lining features transverse folds called circular folds, each housing fingerlike projections known as intestinal villi. These villi are covered by a layer of simple columnar epithelium, also referred to as absorptive cells or enterocytes. Equipped with microvilli and enzymes that break down food, these absorptive cells are integral to nutrient absorption from small intestinal chyme. Additionally, goblet cells present in the epithelium secrete mucus, contributing to a smooth digestion process.

Each villus contains an extensive network of capillaries in its lamina propria, which transports absorbed nutrients to the hepatic portal circulation for subsequent delivery to the liver. Besides capillaries, every villus houses a central lymphatic vessel, or lacteal, and nerve endings. At the base of the villi lie the entrances to the intestinal glands or crypts, which produce intestinal juice to aid digestion.

Absorptive cells and goblet cells aren't the only inhabitants of the intestinal glands. Paneth cells and enteroendocrine cells also reside here. Paneth cells secrete lysozyme, a bactericidal enzyme, and can perform phagocytosis, suggesting a potential role in regulating the microbial population in the small intestine. Enteroendocrine cells in the small intestine comprise S cells, CCK cells, and K cells, each secreting different hormones that regulate various physiological processes.

The duodenum has few circular folds and small villi, It primarily receives and neutralizes the acidic chyme from the stomach to protect the absorptive surfaces of the small intestine. Additionally, the duodenum is a vital site for digestion, where bile from the liver and digestive enzymes from the pancreas mix with chyme. This combination allows for the breakdown of fats, proteins, and carbohydrates, initiating the absorption process that continues through the jejunum and ileum.

In contrast, the proximal half of the jejunum features prominent circular folds and villi, facilitating most nutrient absorption before materials reach the ileum.

As one progresses towards the distal parts of the ileum, the size of the circular folds and villi gradually decreases. The distal ileum has few circular folds, and houses aggregated lymphoid nodules, or Peyer's patches, in the lamina propria. These nodules are an essential part of the immune system, helping protect the gastrointestinal tract from pathogens.