Referrence:Digestive System(Chinese Version)
Digestive System (KidsHealth-Teens-Your Body-Body Basics Library http://kidshealth.org/index.html)
If you're like most people, you probably think digestion begins when you first put food in your mouth. But the digestive process actually starts even before the food hits your taste buds.
It begins when you smell something irresistible or when you see a favorite food you know will taste good. Just by smelling that homemade apple pie or thinking about how delicious that ice cream sundae is going to taste, you begin to salivate - and other parts of the digestive process begin to kick in.
If it's been a while since your last meal or if you see or smell something irresistible, you feel hungry. You eat until you're satisfied and then go on about your business. But for about the next 20 hours or so, your digestive system is doing its job as the food you ate travels through your body.
During the digestive process, food is changed into the substances your body uses for energy and survival. The smooth muscle in the walls of the stomach, intestines, and other digestive organs helps rhythmically and efficiently move the digesting food along the digestive tract.
Meanwhile, your blood picks up the nutrients (including carbohydrates, proteins, fats, vitamins, and minerals) derived from food passing through the digestive tract and takes them to the rest of the body.
How Is the Digestive System Important for Living? Almost all animals, even earthworms, have a tube-type digestive system - food enters the mouth, passes through a long tube, and exits through the anus. Each animal's digestive tract is specially designed to break down and handle the kinds of foods it eats.
Food is the body's fuel source. The nutrients in food give the body's cells the energy and other substances they need to operate.
Before food can do any of these things, it has to be changed into a form the body can absorb into the bloodstream and use. For example, protein in food is broken down into amino acids, which the body needs to make the substances necessary to function, grow, and repair itself. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.
The parts of food the body can't use are passed out of the body as waste.
The kinds and amounts of food we eat and how the digestive system processes that food play key roles in maintaining our health. How well our body functions, and ultimately, how good we feel depends on other factors, too, such as getting enough exercise and sleep, and lifestyle choices, like not smoking or taking drugs.
Basic Anatomy
Doctors often call the digestive tract the gastrointestinal tract. The adult digestive tract is about 30 feet long. Every morsel of food we eat has to be broken down into nutrients that can be absorbed while traveling through this tract, which is why it takes hours to fully digest food.
As it passes through the tube-shaped organs of the digestive tract, food is broken down into microscopic particles, or molecules, and separated into nutrients and waste matter. Nutrients pass through channels in the intestinal wall to the bloodstream, which distributes them to nourish every cell in the body.
The digestive system is made up of individual organs that perform different functions. The digestive tract begins with the alimentary canal, a long tube made up of smooth muscle, blood vessels, and other tissues that runs from the mouth down to the anus, where waste matter exits from our bodies.
Although just the sight or smell of food may trigger the digestive system to gear up for action, the actual breakdown of food begins when it enters the mouth. Food is torn and chopped by the teeth and mixed with saliva, which moistens it for easy swallowing. A digestive enzyme called amylase (pronounced: am-eh-lace), which is found in saliva, starts to break down some carbohydrates (starches and sugars) found in food even before it leaves the mouth.
After leaving the mouth, food passes down the throat and is moved down a muscular tube in the chest called the esophagus (pronounced: eh-saf-uh-gus).
Food then enters the stomach, a pouch-like organ located in the upper middle of the abdomen. The stomach stores food, while thoroughly mixing it with acids and enzymes, breaking it into much smaller, digestible pieces. When it's empty, an adult's stomach has a volume of one fifth of a cup, but it can expand to hold more than 8 cups of food after a large meal.
The stomach empties into the small intestine, which further breaks down food so that the body can absorb the nutrients from it into the bloodstream. The small intestine is made up of three parts:
the duodenum (pronounced: due-uh-dee-num), the C-shaped first part
the jejunum (pronounced: jih-ju-num), the coiled midsection
the ileum (pronounced: ill-ee-um), the final section that leads into the large intestine
The inner wall of the small intestine is covered with microscopic, finger-like projections called villi (pronounced: vill-lie). It is through the villi that nutrients and water are absorbed from the inside of the intestine so they can enter the bloodstream and travel to other parts of the body.
The liver (located beneath the rib cage in the right upper part of the abdomen), gallbladder (hidden just underneath the liver), and the pancreas (next to the stomach) produce and store enzymes and other substances that travel through special channels or ducts directly into the small intestine to aid in digestion. The liver also plays a major role in the handling and processing of nutrients that are carried to the liver in the blood.
The large intestine receives food matter and water that wasn't absorbed by the small intestine. The large intestine is made up of three parts:
the cecum (pronounced: see-kum), a small, sac-like section of intestine, at the end of which hangs the appendix, a small, hollow, finger-like pouch
the colon, which extends from the cecum, up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum
the rectum, which ends at the anus
The cecum and colon are about 5 feet long, whereas the rectum is only 5 inches long.
As waste products move along the length of the large intestine, water is removed. By the time the final products of digestion reach the rectum, they are in the form of solid waste called feces (also known as stool). The rectum stores the feces until the body is ready to eliminate it out through the anus as a bowel movement.
Normal Physiology
When we see, smell, taste, or even imagine a tasty snack, our salivary glands, which are located under the tongue and near the lower jaw, begin producing a flow of saliva that enters the mouth. This flow is set in motion by a brain reflex that's triggered when we sense food. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.
Food enters the body through the mouth, where chewing begins the mechanical process of breaking down a meal. Our teeth are specially designed to transform food into a form that is easy to swallow. Sharp-edged teeth chop and tear food, while the flatter molars grind it up.
As we chew, saliva softens and moistens food, while enzymes in the saliva help break down starches and sugars.
Swallowing, which is accomplished by voluntary and coordinated muscle movements of the tongue and mouth, passes the food into the throat, or pharynx. The pharynx (pronounced: fair-inks), a passageway for food and air, is about 5 inches long. A flexible flap of tissue called the epiglottis (pronounced: epp-ih-glaht-us) automatically closes over the windpipe when we swallow to prevent choking from food entering the windpipe and lungs.
Food then travels into the esophagus, where waves of contractions called peristalsis (pronounced: per-uh-stall-sus) force it down through this muscular tube to the stomach. Peristalsis is an involuntary muscular action controlled by the nervous system.
As food passes through the digestive tract we normally aren't aware of the movements of the esophagus, stomach, and most of the intestine.
At the end of the esophagus, a muscular valve called a sphincter (pronounced: sfink-ter) allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. Some substances, such as water, salt, sugars, and alcohol can be absorbed directly through the stomach wall. Most other substances in the food we eat are digested further and travel down into the intestine before being absorbed.
Peristalsis in the stomach churns food back and forth, mixing it with mucus and gastric juices. These juices contain enzymes and hydrochloric acid because an acidic environment is needed for the digestion that takes place in the stomach. Gastric glands in the stomach lining produce about 3 quarts of these digestive juices each day.
By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme (pronounced: kime). A short muscular channel at the bottom of the stomach called the pylorus (pronounced: pie-lore-us) keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the duodenum of the small intestine by the action of peristalsis.
In the small intestine, where most nutrients are absorbed, enzymes and other digestive chemicals work on the partially digested food. The pancreas and liver connect with the small intestine and send secretions into it to aid digestion.
These secretions include:
pancreatic juice, which contains enzymes that help digest carbohydrates, proteins, and fats
bile, which breaks down and aids in the absorption of fat
Though bile is produced in the liver, it is stored in the gallbladder and sent into the small intestine through a narrow tube called the bile duct.
The pancreas secretions contain:
sodium bicarbonate (pronounced: buy-kar-buh-nate), a salt that neutralizes stomach acid
amylase, which breaks down carbohydrates (starches and sugars)
lipase, which, with bile, breaks down fats into fatty acids and glycerol
trypsin, which breaks down proteins into amino acids
Once broken down, these food products are dissolved and absorbed directly into the bloodstream through the walls of the small intestine, along with vitamins and minerals. This process is called absorption.
By the time food reaches the large intestine, digestion is almost finished. The large intestine's main function is to remove water from the undigested matter and form these waste products into feces that will be excreted (removed) from the body in a bowel movement.
The contents of the small intestine enter the cecum (the first part of large intestine) through another sphincter muscle, which prevents their return to the small intestine. The cecum passes these contents to the colon (which makes up most of the large intestine).
Fluids and salts are absorbed in the ascending colon, more water is removed from waste materials in the transverse colon, and the descending colon holds the resulting waste.
Bacteria in the colon help to digest the remaining food products. The final waste product, in the form of feces, is then stored in the rectum until it exits the body through the voluntary opening of another sphincter at the anus.
Diseases, Conditions, and Disorders
Nearly everyone has a digestive problem at one time or another. Some conditions, such as indigestion, are common; they result in mild discomfort and get better on their own or are easy to treat. Others, such as inflammatory bowel disease, can be long lasting or troublesome.
Esophageal Disorders
Conditions affecting the esophagus may be congenital (which means a person is born with them) or noncongenital (which means a person develops them after birth):
Congenital malformations (birth defects) of the esophagus include tracheoesophageal fistula (pronounced: tray-kee-oh-eh-saf-uh-jee-ul fish-chuh-luh), an abnormal connection between the esophagus and the trachea (windpipe), and esophageal atresia (pronounced: eh-saf-uh-jee-ul uh-tree-zhuh), a condition in which the esophagus comes to a dead end instead of connecting to the stomach. Both of these conditions are usually detected soon after birth or sometimes even before birth, and they require surgical repair.
Noncongenital disorders include esophagitis (pronounced: eh-saf-uh-jeye-tus), an inflammation of the esophagus, which can be caused by infection or as a side effect of certain medications. Esophagitis is also commonly caused by gastroesophageal reflux disease (GERD), a condition in which the esophageal sphincter (the tube of muscle that connects the esophagus with the stomach) becomes weakened and allows the acidic contents of the stomach to move backward, or reflux, up into the esophagus. This causes inflammation and sometimes damage to the tissues of the lower esophagus. Irritation of the esophagus due to reflux may cause a burning pain in the lower chest called "heartburn."
Disorders of the Stomach and Intestines
The stomach and the intestines are subject to conditions that may be present from birth or that develop later on. Some examples:
Gastrointestinal infections are common in teens and can be caused by viruses, by bacteria (such as Salmonella, Shigella, Campylobacter, or E. coli); or by parasites or protozoan organisms. Abdominal pain or cramps, diarrhea, and sometimes vomiting are the common symptoms of gastrointestinal infections.
Diarrhea is a condition in which bowel movements are abnormally frequent and stools are loose and watery. Diarrhea results when muscle contractions move the contents of the intestines along too quickly and there isn't enough time for water to be absorbed before it is pushed out of the body. If diarrhea lasts for more than a couple of days, the body may lose too much salt and water and become dehydrated. In addition to bacterial and viral infections, diarrhea can also be caused by food poisoning (when food or drinking water is contaminated with harmful bacteria or their diarrhea-causing by-products), lactose intolerance, and stress.
Appendicitis is an inflammation of the appendix, a finger-like pouch extending from the colon that is located on the lower right side of the abdomen. The classic symptoms of appendicitis are abdominal pain, fever, and vomiting. Appendicitis affects 7% of the U.S. population and is the most common reason for a child to need emergency abdominal surgery. Kids and teens between the ages of 11 and 20 are most often affected.
Gastritis and peptic ulcers. Under normal conditions, the stomach and duodenum are extremely resistant to irritation by the strong acids produced in the stomach. Sometimes, however, the growth in the gastrointestinal tract of a bacterium (most commonly Helicobacter pylori) or the chronic use of certain drugs or medications weakens the protective mucous coating of the stomach and duodenum, allowing acid to get through to the sensitive lining beneath. Both the acid and bacteria can irritate and inflame the lining of the stomach (a condition known as gastritis) or cause peptic ulcers, which are sores or holes that form in the lining of the stomach or the duodenum and cause pain or bleeding. Medications are usually successful in treating these conditions, including a course of antibiotics if Helicobacter pylori is involved.
Inflammatory bowel disease is a chronic inflammation of the intestines that affects older kids, teens, and adults. There are two major types: ulcerative colitis, which usually affects just the rectum and the large intestine, and Crohn's disease, which can affect the whole gastrointestinal tract from the mouth to the anus. The most common symptoms of inflammatory bowel disease are chronic abdominal pain; intermittent, severe diarrhea, which may be bloody; and weight loss or poor growth in children. Flare-ups of inflammatory bowel disease can occur at any time, but are especially common during times of stress. They are treated with medications and, if necessary, intravenous (IV) feedings to provide nutrition. In some cases, surgery may be necessary to remove inflamed or damaged areas of the intestine.
Constipation. In this condition, the contents of the intestines do not move along fast enough. Waste materials stay in the large intestine so long that too much water is removed and the feces become hard. This results in infrequent bowel movements, which may be difficult or painful to pass. Occasional bouts of constipation are common and usually resolve without treatment. However, for some kids and teens, constipation can be more than an occasional annoyance: it sometimes requires a long course of daily medicines, increasing the amount of fiber (especially from fresh fruits, vegetables, and whole grains) in the diet, and changing toilet habits.
Disorders of the Pancreas, Liver, and Gallbladder
Conditions affecting the pancreas, liver, and gallbladder often affect the ability of these organs to produce enzymes and other substances that aid in digestion.
Pancreatic disorders. Cystic fibrosis is a chronic, inherited illness that typically affects the lungs and pancreas. The production of abnormal mucus blocks the passageways of the pancreas and prevents its digestive juices from entering the person's intestines, making it particularly difficult for the intestines to digest proteins and fats. This causes important nutrients to pass out of the body unused, and malnutrition (inadequate nutrition) results over time.
Liver conditions. Hepatitis, or inflammation of the liver, is most commonly caused by viral infection. Viruses such as hepatitis A, B, or C, cytomegalovirus (CMV), and Epstein-Barr virus (the virus that causes mononucleosis) can all cause hepatitis. Other conditions such as liver injury caused by a poison (chemical hepatitis) or liver damage caused by interruption of the organ's normal blood supply or by trauma can also cause hepatitis.
Gallbladder conditions. Cholecystitis (pronounced: koh-lee-sis-teye-tus) is an inflammation of the gallbladder, often the result of having gallstones. Although the condition is uncommon in kids and teens, it can occur in those who have a high risk for developing gallstones, such as children with sickle cell anemia or those taking certain long-term medications.
Glossary
absorption: a process by which substances such as nutrients and water pass from inside the intestine into the bloodstream
alimentary canal: the tube-like channel in the body through which food passes and is digested
amino acids: the building-block molecules that make up proteins
anus: opening at the end of the large intestine through which feces are excreted
ascending colon: the part of the colon, located in the right side of the abdomen, that moves digested food matter upward from the cecum to the tranverse colon
bile: a substance, produced in the liver and stored in the gallbladder, that aids in the digestion of fats
cecum: the first part of the large intestine; the appendix extends from it
chyme: food mixed with stomach chemicals ready to be sent to the small intestine
colon: longest part of the digestive system, part of the large intestine
descending colon: the part of the colon, on the left side of the abdomen, that moves digested food matter being formed into feces downward toward the rectum
duodenum: the C-shaped first portion of the small intestine
epiglottis: a flexible flap of tissue that automatically closes over the opening of the windpipe while food is being swallowed
esophagus: muscular tube that carries food from the mouth to the stomach
excretion: the process of discharging waste matter from the body
feces: waste matter eliminated from the bowels
gallbladder: small bag-like organ located beneath the liver used to store bile before it is secreted into the small intestine
ileum: the last part of the small intestine that empties into the large intestine
jejunum: the coiling midsection of the small intestine
large intestine: the portion of the intestine that extends from the ileum to the anus and includes the cecum, colon, rectum, and anus
liver: a large, reddish-brown organ that forms bile before it is stored in the gallbladder and is active in the formation of certain blood proteins; it filters waste products and toxins in the blood and is involved in the metabolism of carbohydrates, fats, and proteins
pancreas: a gland that produces enzymes and substances that help with the digestion of carbohydrates, fats, and proteins in the small intestine
pancreatic juice: secretions containing pancreatic enzymes and other substances that travel through the pancreatic duct into the small intestine that help to digest carbohydrates, proteins, and fats
peristalsis: synchronized involuntary (not under a person's control) muscular movements that push food through the alimentary canal
pharynx: the throat
pylorus: a muscle-walled channel at the bottom of the stomach that controls the flow of stomach contents from the stomach into the small intestine
rectum: lower end of the large intestine where feces are stored before being excreted out of the body through the anus
saliva: the watery mixture of secretions and enzymes produced by the salivary glands that lubricates and begins the breakdown of of food as it is chewed
salivary glands: glands in the mouth and near the lower jaw that secrete saliva
small intestine: the upper part of the intestine where most of digestion takes place and most of the nutrients are absorbed into the blood
sphincter: a muscular valve that acts to control flow through or prevent backflow through a tubular organ or opening in the body
stomach: a hollow, pouch-like organ that stores and thoroughly mixes food with acid and enzymes as part of the digestive process
transverse colon: the part of the colon that runs across the upper abdomen connecting the ascending and descending colon
villi: microscopic finger-like projections that line the small intestines and play a role in the absorption of nutrients
Functions of the Digestive System http://training.seer.cancer.gov/module_anatomy/unit10_1_dige_functions.html
The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body:
Digestion
Absorption
Elimination
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism.
The digestive system prepares nutrients for utilization by body cells through six activities, or functions.
Ingestion The first activity of the digestive system is to take in food through the mouth. This process, called ingestion, has to take place before anything else can happen.
Mechanical Digestion The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes. This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.
Chemical Digestion The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical digestion, through a process called hydrolysis, uses water and digestive enzymes to break down the complex molecules. Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.
Movements After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction. These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids. The movements that propel the food particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.
Absorption The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption.
Elimination The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.
General Structure of the Digestive System
The long continuous tube that is the digestive tract is about 9 meters in length. It opens to the outside at both ends, through the mouth at one end and through the anus at the other. Although there are variations in each region, the basic structure of the wall is the same throughout the entire length of the tube. The wall of the digestive tract has four layers or tunics:
Mucosa
Submucosa
Muscular layer
Serous layer or serosa
The mucosa, or mucous membrane layer, is the innermost tunic of the wall. It lines the lumen of the digestive tract. The mucosa consists of epithelium, an underlying loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosa. In certain regions, the mucosa develops folds that increase the surface area. Certain cells in the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen. In the mouth and anus, where thickness for protection against abrasion is needed, the epithelium is stratified squamous tissue. The stomach and intestines have a thin simple columnar epithelial layer for secretion and absorption.
The submucosa is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood vessels, lymphatic vessels, and nerves. Glands may be embedded in this layer.
The smooth muscle responsible for movements of the digestive tract is arranged in two layers, an inner circular layer and an outer longitudinal layer. The myenteric plexus is between the two muscle layers.
Above the diaphragm, the outermost layer of the digestive tract is a connective tissue called adventitia. Below the diaphragm, it is called serosa.
Regions of the Digestive System
At its simplest, the digestive system is a tube running from mouth to anus. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination throughout the body.
Regions of the digestive system can be divided into two main parts: the alimentary tract and accessory organs. The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus. Associated with the alimentary tract are the following accessory organs: salivary glands, liver, gallbladder, and pancreas.
To learn more about the regions of the digestive system, use the hyperlinks listed below to branch into a specific topic.
1. Alimentary Tract of the Digestive System
Pharynx and Esophagus
Stomach
Small and Large Intestine
2. Accessory Organs of the Digestive System
The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.