The Human Digestive System

What Is Digestion?

The human body obtains the energy and nutrients it needs from food. However, our cells cannot absorb these nutritional benefits until the food has been “digested” – meaning, “processed and converted into a useable form”. Thus digestion is the complex process of breaking down food molecules into energy and other useful components, which can then be absorbed into the bloodstream and distributed throughout the body to maintain good health. Food remnants which are not absorbed during the digestion process are expelled as waste in the form of feces.

The digestion of food in humans takes place in the gastrointestinal tract – a series of hollow organs (mouth, esophagus, stomach, large and small intestines) connected to form a long tube of about 24 feet in length which extends from the mouth to the anus. It is also referred to as the GI Tract, the alimentary canal, the digestive tract, or the gut. Above the large intestine, the digestive system is sometimes called the upper gastrointestinal tract, while everything below is the lower gastrointestinal tract. The tract has muscular walls that propel food along the tube (a process called peristalsis) breaking it down and mixing it with digestive juices for optimum absorption.Where Do We Digest Our Food?

The Functions Of The Digestive Tract

The gastrointestinal tract has four main functions. It ingests the food we eat; it breaks it down into simple chemical components for energy and nutritional purposes; it extracts nutrients from it (eg. macronutrients such as carbs, fats, proteins; as well as micronutrients like vitamins and minerals); and finally it expels the remaining food waste.

Step-By-Step Guide To The Digestive Process

The Mouth

Digestion starts in the mouth – the beginning of the digestive tract. Food smells cause the salivary glands in the mouth to secrete saliva (“mouth-watering”), so even before we start eating our digestive system is primed and ready for action! Saliva contains antibacterial compounds and various enzymes to aid the breakdown of food molecules. It also softens the food – enabling the tongue to mould it into a bolus or ball for swallowing. The tongue, teeth and saliva work together to start digestion and aid swallowing. Teeth chop and grind food, breaking the food down into pieces small enough to be digested and increasing the surface area over which the digestive enzymes in saliva can act. For more, see Guide To Digestion In The Mouth.

The Pharynx and Esophagus

Food is then swallowed and passes into the pharynx, or throat. When we swallow, passages to the lungs (windpipe) and the nasal cavity are automatically closed, and the food goes into the esophagus – a muscular tube extending from the pharynx to the stomach. Food is propelled through the esophagus and into the stomach by means of muscular contractions called peristalsis. At the bottom of the esophagus, just before the opening to the stomach is a ring-shaped muscle known as the lower esophageal sphincter (LES). This muscle relaxes (opens) to let food into the stomach and then tightens (closes) to prevent regurgitation. If the LES malfunctions and allows food in the stomach to re-enter the esophagus, it may cause a condition called gastroesophageal reflux disease (GERD), characterised by heartburn and regurgitation. For more, see Guide To Digestion In The Esophagus.

The Stomach

A large pouch with strong muscular walls, the stomach serves as a temporary holding station and food-processor for the chewed and swallowed food. It has the ability to expand or contract depending upon the amount of food it contains. The stomach aids digestion in two ways. Its strong muscular walls churn the food into chyme – a semi-fluid mixture resembling porridge – while glands within the walls secrete gastric juice – a blend of hydrochloric acid and various digestive enzymes – that helps to digest foods like protein, fats, a few carbohydrates and alcohol. To prevent the stomach from digesting itself(!) its walls are lined with a membrane called mucosa which secretes a protective slimy substance called mucus. Liquids pass through the stomach in a matter of minutes, while solid food can remain in the stomach for up to 5 hours. Chyme slowly exits the stomach and passes into the small intestine. For more, see Guide To Digestion In The Stomach.

The Small Intestine

Approximately 17 feet in length, the small intestine is a coiled tube made up of three sections – the duodenum, jejunum and ileum. As the semi-digested food (chyme) enters the duodenum from the stomach, the duodenal lining releases intestinal hormones that stimulate the gallbladder and pancreas to release special digestive juices (bile and pancreatic juice) which help to further break down food molecules in the chyme. It is in the small intestine that most nutrients are digested and absorbed, although different nutrients are absorbed at different speeds. Typically carbs are digested most rapidly, followed by proteins and finally fats. Micronutrients (vitamins and minerals) consist of molecules tiny enough for the body to absorb without breaking them down first, but water soluble vitamins are absorbed faster than fat soluble ones. The duodenum and jejunum is where the chyme is broken down, while the ileum is responsible for absorbing nutrients into the bloodstream. The absorbed nutrients pass through the bloodstream to the liver where they are processed and either stored or distributed to other parts of the body. After every useful, digestible ingredient other than water has been wrung out of the chyme, the remaining “waste” passes into the large intestine. For more, see Guide To Digestion In The Small Intestine.

Surplus Energy Converts To Stored Body Fat

Aside from breaking down and absorbing nutrients, the digestive system also converts food into energy to help power the muscles and fuel the millions of chemical reactions needed to sustain good health. After immediate energy requirements have been satisfied, any remaining surplus is stored as glycogen (a small reserve of liquid energy stored in the liver and muscles), or body fat. For more, see Guide To Body Fat And Adipose Tissue/Body Fat – How We Gain Fat.

The Large Intestine

Also known as the large bowel, the large intestine – consisting of 3 sections, the cecum, colon and rectum – is approximately 5 feet in length and has two main functions: to absorb all remaining water from the food waste and to compress the remaining matter into a compact bundle (feces or stool) so that defecation (excretion of waste) is easy and convenient. The cecum is a short pouch containing a valve which opens to receive chyme from the ileum. The colon absorbs water and through bacterial action reduces the bulk of fiber in the feces. The rectum is the terminal segment of the digestive tract, in which feces accumulate just prior to discharge. They are discharged through the anus which contains two important muscles – the internal sphincter and the external sphincter. The internal sphincter is always tight, except when feces enter the rectum, in order to keep us continent (eg) when we are asleep. When we get an urge to defecate, we depend upon the external sphincter to keep the stool in until we go to the toilet. In total, it takes about 36-48 hours or longer for waste matter to pass through the large intestine. As in the esophagus and the small intestine, the contents of the large intestine are pushed forward by a sequence of muscular contractions called peristalsis (a type of motility or muscular movement). Peristalsis is regulated by a large network of nerves, hormones and muscles. Malfunction of any of these components may lead to a range of intestinal problems, including indigestion and constipation. For more, see Guide To Digestion In The Large Intestine.