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.

The small intestine is divided into three subsections in the following order: duodenum, jejunum, and ileum. The duodenum is responsible for neutralizing the acidity of the chyme entering the intestine from the stomach and introducing digestive enzymes. The jejunum and ileum are responsible for absorbing nutrients.

The small intestine is essential for the chemical digestion of nutrients and for their absorption. Pancreatic digestive enzymes are secreted into the duodenum, where they catabolize nutrients. These nutrients are then absorbed in the jejunum and ileum. Proteins and carbohydrates are absorbed directly into the blood, while fats are absorbed into the lymphatic lacteals.

Water, however, is not absorbed in large quantities in the small intestine. Instead, most water absorption occurs in the large intestine.

The lower esophageal sphincter separates the esophagus from the stomach. When stomach acid and contents leak through the lower esophageal sphincter, the acid irritates the esophagus and can lead to pain around the area of the heart. The heart is not actually affected.

The pyloric sphincter is found between the stomach and the duodenum of the small intestine. The other 3 sphincters are found in different parts of the body. 

Food enters the stomach from the esophagus through the cardiac sphincter.

The pyloric sphincter connects the stomach to the duodenum. The ileocecal sphincter connects the ileum to the large intestine, while the internal (anal) sphincter controls the exit of fecal matter from the body. There is also an external anal sphincter, which is under voluntary control, while the internal is involuntary.