Food fuels the body but will not have any use for the body without proper processing. Digestion takes place in the gastrointestinal (GI) tract. It is also known as digestive tract or alimentary canal. It is a muscular tube which is about 33 feet (10m) long if it is fully spread.

A series of hollow organs make up the GI tract. They are joined in one long twisting tube from the mouth. The hollow organs that make up the GI tract are mouth, oesophagus, stomach, small and large intestines with the anus as the endpoint. The digestive system also has solid organs which are the liver, gallbladder and pancreas.

Digestion process

Complete digestion takes place when food goes through these parts of GI tract.

Mouth

This is the starting point of digestion. The mouth tests the food for taste and temperature using the tongue. The front teeth (incisors) bite the solid food, and the back teeth (molars) do the chewing. Saliva pours from the salivary glands near lower jaw into the mouth to moisten the food. It also has enzymes that begin the digestion process. By the end of chewing, the mouthful of food has changed to a softball known as the bolus. It is at an ideal temperature and ready for swallowing.

Pharynx

Pharynx is a muscle-lined cavity at on the back of the mouth. It the first point where the tongue pushes the bolus of food for controlled swallowing to prevent interference with breathing.

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Esophagus

The oesophagus also the gullet is a tube joining the mouth to the stomach. Muscle action known as peristalsis pushes down the food in the form of wave action but not through gravity. Cardiac sphincter muscles always keep the oesophagus shut except when swallowing to prevent highly acidic contents in the stomach from flowing upwards into the gullet. The sphincter relaxes and opens the pathway to the stomach when a bolus of food is passing down the oesophagus.

The stomach

The stomach is a collapsible muscular bag for storing food so that there is no need to eat incessantly. After food enters the stomach, their muscle mixes it with digestive juices then releases it gradually into the small intestines. Food processing in the stomach takes 2-6 hours. Various chemicals in the stomach convert the bolus into a liquid known as a chyme. Pylori sphincter muscle guards the stomach exit and controls the chyme that flows into the small amounts to a small amount.

Small intestine

The small intestine is longer than other parts of the alimentary canal. It is the point where most of the digestive action takes place. It is 6 meters (20 feet) long, but the width is just 4 centimetres (1.5 inches) hence its name.

The muscles in the small intestine mix the chime with digestive juices from the liver (bile) and pancreas (pancreatic secretions). After a thorough mix for further digestion, small intestine walls absorb water and digested nutrients from the food into the bloodstream. Peristalsis continues opening a valve at the end of small intestines for remaining waste products from the digestive process to move into larger intestines.

Large intestine

Large intestines are a tube that is 1.5 meters (3 feet) long and 7.5 centimetres (3 inches) wide that accommodates the large solid waste products from the digestive process. The waste includes undigested food, fluids and old cells from GI tract lining. The large intestine absorbs water from waster turning it from a liquid into the stool. Peristalsis again moves the stool to the rectum.

Rectum

The rectum is the lower end of the large intestine that stores stool until the time for bowel movement when it exits from the digestive system from the anus. In infancy, the sphincter opens automatically if the anal canal fills. As a child grows, the nervous system matures and learns to override this automatic signal. It takes 6-8 hours for food to pass through the stomach and small intestine before ending up in the colon (large intestine) for further digestion and absorption of water before elimination of the undigested food.

The full process up to the elimination of waste depends on an individual and gender. The average transit time is 33 hours for a man and 47 hours for a woman.

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