Digestion is the process by which food and drink are broken down into small components so that the body can use them to build and nourish cells and to provide energy.
The digestive system comprises a series of hollow organs that are joined in a long, twisting tube from the mouth to the anus, called the digestive tract, as well as other organs that help the body break down and absorb food.
It includes the mouth, oesophagus, stomach, small intestine, large intestine (colon), rectum, and anus. These hollow organs are lined by a layer of tissue called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. Along the digestive tract too, is a layer of smooth muscle that helps to break down food and to move it along.
There are two organs; the pancreas and the liver, that are also critical for digestion. They produce digestive juices that reach the intestine through small tubes called ducts. The gallbladder stores the liver's digestive juices until they are needed in the intestine.
For digestion to occur, there are three important sub-processes that have to take place:
A defect in any of these will result in inefficient digestion.
Precise contractions of the layer of muscle around the hollow organs of the tract enable movement and propel food mass in one direction; from mouth to anus. These contractions are called are called peristalsis. The first movement occurs after swallowing. Upon swallowing, food is pushed from the mouth into the oesophagus that connects the throat with the stomach below. Once in the oesophagus, the movement is involuntary and under the control of the nervous system. Where the oesophagus meets stomach, there is a ringlike muscle, called the lower oesophageal sphincter, that closes the passage between the two organs. As food approaches the closed sphincter, the sphincter relaxes and allows the food to pass through to the stomach. Once in the stomach, this sphincter closes once again to prevent food from going back into the oesophagus. The muscles of the upper part of the stomach then relax to accommodate the food that has entered.
The food is temporarily stored in the stomach so that it can be slowly mixed with the digestive juices produced by the stomach itself. This mixing (also called churning) is facilitated by the muscles of the lower part of the stomach.
The length of time food spends in the stomach depends on several factors that include the type of food ingested. Carbohydrates, for example, spend the least amount of time in the stomach, while protein stays longer, and fats the longest. When food leaves the stomach, it is met by and dissolves in juices from the pancreas, the liver, and the intestine. The whole content is then mixed and moved forward. As it moves, the digested nutrients are absorbed through the intestinal walls before being transported throughout the body. The waste products of this process include the undigested parts of the food and older cells that have been shed from the mucosa. These materials are pushed into the colon and remain there until the faeces are expelled by a bowel movement.
Saliva produced by these glands contains an enzyme that begins to break down the starch from food into smaller molecules.
An enzyme is a substance that speeds up chemical reactions in the body.
The lining produces stomach acid and enzymes that break down proteins. A thick mucus layer coats the mucosa of the stomach and helps keep the acidic digestive juice from dissolving the tissue of the stomach itself. In most people, the mucosa is able to resist the juice but where the mucosa is compromised, the lining becomes irritated and ulcers may form.
a) The Pancreas produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food.
b) The Liver, produces yet another digestive juice—bile, that is stored between meals in the gallbladder. Upon eating, it is squeezed out of the gallbladder, through the bile ducts, and into the intestine to mix with the fat in food. The bile acids emulsify and so dissolve fat into the watery contents of the intestine. After fat is dissolved, it is digested by enzymes present.
c) Other enzymes that are active in the process come from glands in the wall of the intestine.
Most digested food molecules, water and minerals, are absorbed through the small intestine. The mucosa of the small intestine contains many folds that are covered with tiny finger-like projections called villi and the villi, in turn, are covered with microscopic projections called microvilli. This setup creates a vast surface area through which nutrients can be absorbed. Through specialized cells, absorbed materials cross the mucosa into the blood, where they are transported to other parts of the body for storage or further chemical change, depending on the type of nutrients.
Many carbohydrates contain both starch and sugars. Carbohydrates include bread, potatoes, rice, pasta, fruits, vegetables to name a few. Starch and sugars are broken into simpler molecules by enzymes in the saliva, the pancreatic juice and in the lining of the small intestine. Fiber is indigestible and moves through the digestive tract without being broken down by enzymes.
Proteins include meat, eggs, and beans among others. Proteins digestion starts with the acid and the enzymes in the juice of the stomach. In the small intestine, several enzymes of the pancreatic juice and the lining of the intestine complete the breakdown, ultimately to amino acids that will be used by the body to build and repair cells.
Fats include butter and margarine. Fat molecules are a rich source of energy for the body. Fat is insoluble with water and so to begin the process of breaking down fat, it is first emulsified by the bile acids into tiny droplets that are then broken down to smaller molecules by pancreatic and intestinal enzymes. Some of the resulting small molecules are fatty acids and cholesterol. Bile acids combine with these fatty acids and cholesterol and so help them to move into the cells of the mucosa. In these cells the small molecules reform into large ones and most then pass into vessels called lymphatics near the intestine. These vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.
Digestion is regulated by messenger chemicals (Hormones) as well as by electrical impulses (Nerves)
The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. They include:
The release of gastrin causes the stomach to produce acid for dissolving and digesting some foods. Gastrin is also necessary for normal cell growth in the lining of the stomach, small intestine, and colon.
Secretin causes the pancreas to release digestive juice that is rich in bicarbonate. The bicarbonate helps to neutralize the acidic stomach contents so that it doesn't damage the small intestine. Secretin also stimulates the stomach to produce pepsin, an enzyme that digests protein. Further, Secretin stimulates the liver to produce bile.
CCK stimulates the pancreas to produce the enzymes of the pancreatic juice. It causes the gallbladder to empty and also promotes normal cell growth of the pancreas.
Both of these latter hormones work on the brain to help regulate the intake of food for energy.
Two types of nerves help to control the action of the digestive system.
Common disorders of the Digestive System include Heartburn, GastroEsophageal Reflux Disease (GERD), Peptic and Duodenal ulcers, Chrohn's Disease, Inflammatory Bowel Syndrome (IBS), Appendicitis, Bloating, Flatulence, Diarrhoea, Oesophageal Cancer, Intestinal and Colon Cancers, Diverticulitis, Constipation, Haemorrhoids, among others.
For further reading and research into the causes, nature and risk factors of GERD and Peptic Ulcers, you may visit the links below, among others.