When digestion is working normally, a person is unaware of the movements of the gullet, stomach, and most of the intestine. Swallowing is a voluntary muscular action. .
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In the stomach, a mixture of hydrochloric acid and Pepsin, which is secreted by the stomach wall, liquefies the food. At the same time, a substance called intrinsic factor binds Vitamin B in food and various gastric enzymes, such as Pepsin which begins protein digestion, which are secreted by the stomach wall. Secretions of mucus protect the stomach from its digestive enzymes. The food is then mixed in the stomach, turning it into chyme(digested food). Then is passed on by peristalsis into the small intestine. .
The small intestine is the longest section of the digestive tract, with an average length of about 6 metres. Although only 2.5 cm in diameter - which is why it is called the small intestine - its surface area for absorption covers the size of a tennis court. This is due to the numerous folds on its surface, covered in tiny projections known as villi, which in turn are covered in even tinier projections known as microvilli. .
The main parts of the small intestine are the duodenum, jejunum, and ileum. Two large glands, the liver and the pancreas, connect with the small intestine by ducts, or tubes. Through these ducts the liver and pancreas pour secretions which further aid digestion and absorption. .
Fluid from the pancreas is called pancreatic juice. Fluid from the liver is called bile. The pancreas is one of the most important glands in the body. It secretes up to a pint of pancreatic juice a day. This digestive fluid contains enzymes, which help digest carbohydrates, proteins, and fats. .
One of these enzymes is trypsin, which helps digest protein foods. Other enzymes are amylase and maltase, which help digest carbohydrates. The pancreatic enzyme lipase, along with bile from the liver, helps digest fat. The liver and gallbladder are responsible for a number of functions.