The small intestine

Once food has been stored in the stomach long enough to be partially digested, contractions of the stomach push partially digested food into the small intestine, where digestion is completed and nutrients are absorbed by the blood. This narrow, muscular, hoselike portion of the gastrointestinal tract is about 6-7 meters long, completely folded up within the abdominal cavity. The small intestine is divided into 3 sections: the duodenum, the jejunum and the ileum. The duodenum is the beginning portion of the small intestine, an 18 cm long receiving area for partially digested food from the stomach. The stomach is connected to the duodenum by the pyloric sphincter, where the food enters the small intestine. From here it is pushed through the intestines by peristalsis. The environment in the duodenum is slightly acidic, with the pH being 6.0 to 6.5.
The jejunum is the 2-8 meters long central division of the small intestine, where most of the nutrients are absorbed into the bloodstream. The pH in the jejunum is usually between 7 and 8, which is neutral or slightly alkaline.
The ileum is the final section of the small intestine. It is about 4 meters long. This is where the remaining nutrients are absorbed before moving into the large intestine. The pH in the ileum, similarly to the the jejunum is 7-8 (neutral or slightly alkaline).
For best absorption of nutrients the small intestinal walls are covered with fingerlike vascular projections, called villi. Each villus has additional cell membrane extensions, called microvilli. The villi and microvilli together increase intestinal absorptive surface area providing exceptionally efficient absorption of nutrients.
The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine. The lining of the small intestine secretes a hormone called secretin, which stimulates the pancreas to produce digestive enzymes. The duodenum receives secretions from the pancreas and liver via the pancreatic and common bile ducts. Bile is produced in the liver and stored in the gallbladder. When food enters the duodenum, the gallbladder contracts and discharges the bile into the common bile duct, that carries it to the small intestine, where it helps with digestion. The gallbladder and the ducts that carry bile and other digestive enzymes from the liver, gallbladder, and pancreas to the small intestine are called the biliary system.