The mammalian digestive tract ileum is the final section of the small intestine. The first and second section of the small intestine respectively are the duodenum and the jejunum.
The duodenum is where proteins and lipids get broken down with the aid of liver bile and stomach chyme, while the ileum is where bile acids and vitamin B12 get absorbed.
The microstructure of the ileum reveals many adaptations for its function. Firstly, it has multiple layers. The outermost layer is the serosa which serves to lubricate the outside of organs by secreting serous fluid. This prevents friction and inflammation, in addition to supporting the organ.
Further in is a muscle layers i.e. muscularis externa, followed by the submucosa and the muscularis mucosae. Everything else further in consists of the mucosa, which is the site of absorption and the main focus of this topic.
The muscularis externa is the muscle that keeps the intestine moving to maintain digestive function. The submucosa supports the mucosa by providing the blood and lymph vessels, while the muscularis mucosae is a thin muscle layer that specifically keeps the mucosa in a state of slight movement to maximise the exchange of products with the contents of the ileum.
For the purpose of absorption, the mucosa contains villi that increase the surface area for absorption, columnar epithelial cells that themselves have microvilli in their plasma membrane, as well as goblet cells that secrete mucus.
Additionally, the mucosa has access to blood vessels to supply the cells with nutrients as well as lacteals which are lymph vessels that carry away digested fat products.
This is the site of absorption via diffusion, active transport and pinocytosis.