The small intestine is before the large intestine. It consists of the duodenum, followed by the jejunum, then ileum. The large intestine comes directly after the small intestine and constists of the cecum, followed by the colon, then rectum.

Microvilli surround villa which line the small intestine. These structures greatly increase the surface area (and therefore nutrient absorption capability) of the small intestine.

Microvilli increase the surface area in the small intestine, and thus allow a greater amount of absorption of nutrients once digestion occurs. They do not actually participate in digestion, nor are they linked to intestinal cancer.

The majority of digestion in the small intestine (and the body in general) occurs in the duodenum, the first section of the small intestine, after passing through the stomach. The digestive enzymes secreted by the pancreas enter the digestive tract in the duodenum.

The next two sites, the jejunum and ileum, are sites of absorption. The cecum is part of the large intestine, not the small intestine.  

Calcium is absorbed in the jejunum of the small intestine. Iron is absorbed in the duodenum, calcium in the jejunum, and vitamin B12 (complexed with intrinsic factor) is absorbed in the ileum.

The villi, characteristic of the small intestine, serve to increase the surface area over which nutrients can be absorbed and allow for the appropriate uptake of micro- and macronutrients. While other parts of the gastrointestinal system contains pits and folds that allow for the secretion of digestive enzymes, only the high surface area of the small intestine allows for the necessary uptake of nutrients.

The small intestine is composed of the duodenum, jejunum, and ileum. The duodenum receives chyme from the stomach and continues to breakdown food particles with additional enzymes from the pancreas. The jejunum is responsible for absorbing most micro- and macronutrients, and the ileum is responsible for absorption of bile salts, vitamin B12/IF, and ascorbic acid. The colon is a part of the large intestine and is primarily involved in water reabsorption.

The ileum is primarily responsible for absorbing ascorbic acid, vitamin B12 complexed to intrinsic factor (IF), and bile salts complexed with fats ingested as part of a meal. Iron is primarily absorbed in the jejunum of the small intestine.

The small intestine is a basic environment. Secretions from the pancreas into the duodenum help to neutralize chyme transferred from the stomach, resulting in a slightly basic environment in the small intestine.

The stomach is highly acidic, which helps kill microbes and assists with digestion. The vagina is also acidic to prevent the growth of pathogens. Lysosomes and phagosomes are acidic vacuoles that break down foreign pathogens.

Digestion of lipids is primary driven by the enzyme lipase and the introduction of bile salts to emulsify fats. Lipase and bile salts are both introduced to digestion in the small intestine.

The mouth contains salivary amylase, which initiates carbohydrate digestion, but the majority of food digestion will occur in the small intestine. The stomach helps mechanically break down food, while the large intestine helps to resorb water. The purpose of the esophagus is simply to transport food from the mouth to the stomach.