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| DIGESTIVE
SYSTEM
Anatomy of Stomach The Digestive System Movement of Food through the System Production of Digestive Juices Absorption and Transport of Nutrients How is Digestive Process Controlled Organs involved in Digestion of Food Macroscopic Anatomy of Stomach THE ANATOMY OF THE STOMACH The digestive system is a group of organs that work like wrecking equipment to break down the chemical components of food, through the use of digestive juices, into tiny nutrients which can be absorbed to generate energy for the body. This system also builds and replaces cells and tissues, which are constantly dying. Digestion begins in the mouth with the teeth, which grind the food into small particles; the tongue, a powerful muscle which detects "good" and "bad" flavors in food and manipulates the food between the teeth for chewing, and saliva, a watery fluid which lubricates chewing and swallowing and begins the process of digestion. The digestive system begins in the mouth, continues in the pharynx (throat) and esophagus and into the "gut" region: the stomach, small and large intestines, the rectum and the anus. Food is chewed, pulped and mixed with saliva to become a soft mass which will easily travel down the esophagus. The tongue traps the food and forces it into the throat, which is a mass of muscles and tissues which transports food into the gut system for final processing and distribution. The throat closes the top of the breathing pipes, but if the food accidentally enters these pipes, it goes down the "wrong way" and must be coughed up before the body chokes or asphyxiates. As food passes through the body, it is mixed with an unbelievable amount of chemicals, which break it down into small units that can be absorbed into the blood and lymph systems. Some is used for energy, some as building blocks for tissues and cells, and some is stored for future or emergency use. The liver and the pancreas also secrete digestive juices that break down food as it passes through the digestive ducts. Not all that we eat can be digested, so the waste must be disposed of in an efficient way. It may not be a savory ending for the food or drink we thought was so delicious in the mouth, but it is just as important for our health. The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus ( see the figure below ). Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. Digestion is the process in which ingested food is chemically and physically broken down in the alimentary canal into a form that can be absorbed and assimilated by the tissues of the body to provide nourishment for cells and energy for the body to carry out it's functions.
Primary/Major Organs 1. Mouth- Chewing physically breaks up the food into smaller portions (increasing surface area) while the enzymes found in saliva begins to chemically breakdown the food and provide lubrication for the esophagus 2. Esophagus- connects the mouth and the stomach. It has a valve-like epiglottis that prevents food/bolus from entering the trachea by a wave like movement called peristalsis 3.Stomach - Holding tank for food. About 60% of all protein digestion is done here by chemical and mechanical digestion. At it's total capacity the stomach can hold approximately about one liter of food and prepares food for treatment and absorption by small intestine. Proteins are broken down in the stomach by the enzyme pepsin , which is secreted by the chief cells in the form pepsinogen. The reason why the chief cells are secreted in the form of pepsinogen is because the inactive pepsinogen, which is mixed with HCL acid secreted by perietal cells, turns into the active form of pepsin. This is done so that the pepsin will not become active too soon and eat away at the lining of the stomach. To further insure that the stomach is protected from self-digestion the stomach is covered with a lining of mucus and the cells that line the stomach are reproduced every three days. 4.Small intestine - Is a long convoluted tube specialized for the completion of digestion and the absorption of vital nutrients. The small intestine is divided into three sections duodenum, jejunum and the ileum. The small intestine is protected by a layer of mucus and villi for absorption. Most digestion and absorption of nutrients is completed in the small intestine and is aided by enzymes made by the intestinal cells and the pancreas. Enzymes of the small intestine digest carbohydrates proteins and fats. 5. Large intestine - Is divided into three sections: ascending colon, transverse colon and descending colon. Its main function is the absorption of water and the production of several vitamins and minerals that leads to the fecal formation as well as the expelling of waste. Unlike the small intestine the large intestine is not protected by a layer of mucus and has no villi or folds and does not have the ability to produce digestive enzymes. Fecal Formation- the formation of feces Convoluted- a folding curve Pepsin- breaks down proteins Pepsinogen- inactive form of pepsin Chief cells- secrete gastric acid Perital cells- secrete hydrochloric acid (HCL) Secondary/Accessory Organs 1. Liver - Cells of the liver produce bile which is needed to emulsify or cut fat filtration of the blood. Cleansing it of excess vitamins as well as toxins. 2. Gall bladder- Holding container for bile, and while digestion is taking place in the stomach and in the small intestine, the gallbladder contracts and secretes the concentrated bile into the duodenum. 3. Pancreas- Produces enzymes that enter the duodenum and break down proteins, carbohydrates and fats. There are also two solid digestive organs, the liver and the pancreas, which produce juices that reach the intestine through small tubes. In addition, parts of other organ systems (for instance, nerves and blood) play a major role in the digestive system. Why Is Digestion Important? When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. How Is Food Digested? Digestion involves the mixing of food, its movement through the digestive tract, and chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine. The chemical process varies somewhat for different kinds of food.
The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ. The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves. The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ring like valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass. The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine. Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the stomach contents (the small intestine). As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion. Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa. These materials are propelled into the colon, where they remain, usually for a day or two, until the feces are expelled by a bowel movement.
The glands that act first are in the mouth--the salivary glands. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules. The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein. One of the unsolved puzzles of the digestive system is why the acid juice of the stomach does not dissolve the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot. After the stomach empties the food and its juice into the small intestine, the juices of two other digestive organs mix with the food to continue the process of digestion. One of these organs is the pancreas. It produces a juice that contains a wide array of enzymes to break down the carbohydrates, fat, and protein in our food. Other enzymes that are active in the process come from glands in the wall of the intestine or even a part of that wall. The liver produces yet another digestive juice--bile. The bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in our food. The bile acids dissolve the fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.
Digested molecules of food, as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. The absorbed materials cross the mucosa into the blood, mainly, and are carried off in the bloodstream to other parts of the body for storage or further chemical change. As noted above, this part of the process varies with different types of nutrients. Carbohydrates: An average American adult eats about half a pound of carbohydrate each day. Some of our most common foods contain mostly carbohydrates. Examples are bread, potatoes, pastries, candy, rice, spaghetti, fruits, and vegetables. Many of these foods contain both starch, which can be digested, and fiber, which the body cannot digest. The digestible carbohydrates are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps: First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose; then an enzyme in the lining of the small intestine (maltase) splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body. Table sugar is another carbohydrate that must be digested to be useful. An enzyme in the lining of the small intestine digests table sugar into glucose and fructose, each of which can be absorbed from the intestinal cavity into the blood. Milk contains yet another type of sugar, lactose, which is changed into absorbable molecules by an enzyme called lactase, also found in the intestinal lining. Protein: Foods such as meat, eggs, and beans consist of giant molecules of protein that must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the juice of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells. Fats: Fat molecules are a rich source of energy for the body. The first step in digestion of a fat such as butter is to dissolve it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. These small 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. Vitamins: Another vital part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins. There are two different types of vitamins, classified by the fluid in which they can be dissolved: water-soluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, and K). Water and Salt: Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. In a healthy adult, more than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours.
Hormone Regulators A fascinating feature of the digestive system is that it contains its own regulators. 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. These hormones are released into the blood of the digestive tract, travel back to the heart and through the arteries, and return to the digestive system, where they stimulate digestive juices and cause organ movement. The hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK): Gastrin causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon. Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile. CCK causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty. Nerve Regulators Two types of nerves help to control the action of the digestive system. Extrinsic (outside) nerves come to the digestive organs from the unconscious part of the brain or from the spinal cord. They release a chemical called acetylcholine and another called adrenaline. Acetylcholine causes the muscle of the digestive organs to squeeze with more force and increase the "push" of food and juice through the digestive tract. Acetylcholine also causes the stomach and pancreas to produce more digestive juice. Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs. Even more important, though, are the intrinsic (inside) nerves, which make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs. Purpose The digestive system prepares food for use by hundreds of millions of body cells. Food when eaten cannot reach cells (because it cannot pass through the intestinal walls to the bloodstream and, if it could, would not be in a useful chemical state. The gut modifies food physically and chemically and disposes of unusable waste. Physical and chemical modification (digestion) depends on exocrine and endocrine secretions and controlled movement of food through the digestive tract.
Mouth Mouth Food enters the digestive system via the mouth or oral cavity, mucous membrane lined. The lips (labia) protect its outer opening; cheeks form lateral walls, hard palate and soft palate form anterior/posterior roof. Communication with nasal cavity behind soft palate. Floor is muscular tongue. Tongue has bony attachments (styloid process, hyoid bone) attached to floor of mouth by frenulum. Posterior exit from mouth guarded by a ring of palatine/lingual tonsils. Food is first processed (bitten off) by teeth, especially the anterior incisors. Suitably sized portions then retained in closed mouth and chewed or masticated (especially by cheek teeth, premolars, molars) aided by saliva Ducted salivary glands open at various points into mouth. This process involves teeth (muscles of mastication move jaws) and tongue (extrinsic and intrinsic muscles). Mechanical breakdown, plus some chemical (ptyalin, enzyme in saliva). Taste buds allow appreciation, also sample potential hazards (chemicals, toxins) Swallowing In leaving the mouth a bolus of food must cross the respiratory tract (trachea is anterior to esophagus) by a complicated mechanism known as swallowing or deglutination that empties the mouth and ensures that food does not enter the windpipe. Swallowing involves co-coordinated activity of tongue, soft palate pharynx and esophagus. The first (buccal) phase is voluntary, food being forced into the pharynx by the tongue. After this the process is reflex. The tongue blocks the mouth, soft palate closes off the nose and the larynx rises so that the epiglottis closes off the trachea. Food thus moves into the pharynx and onwards by peristalsis aided by gravity. If we try to talk whilst swallowing food may enter the respiratory passages and a cough reflex expels the bolus. Esophagus The esophagus (about 10") is the first part of the digestive tract proper and shares its distinctive structure. Basic tissue layers of the gut are 1. Mucosa. Innermost, moist lining membrane. Epithelium (friction resistant stratified squamous in esophagus, simple beyond) plus a little connective tissue and smooth muscle. 2. Submucosa. Soft connective tissue layer, blood vessels, nerves, lymphatic 3. Muscularis externa. Typically circular inner layer, longitudinal outer layer of smooth muscle 4. Serosal fluid producing single layer. Stomach C shaped, left side abdominal cavity (because liver is on right). Cardio esophageal sphincter guarding entrance from esophagus is of doubtful anatomical integrity (though functionally the diaphragmatic pinch cock serves). Pyloric sphincter guarding the outlet is much better defined. Fundus, body and pylorus recognised as distinct regions. Stomach secretes both acid and mucus (for self protection). Surface area increased by rugae. Serves as a temporary store for food that is also churned by muscular layers (three here) to form chyme, creamy substance voided via pyloric sphincter to duodenum Duodenum First part of small intestine. C shaped 10" long and curves around head of pancreas and entry of common bile duct (accessory organs of digestion, pancreas, liver see below). Chemical degradation of small controlled amounts of food controlled by pyloric sphincter begins here, enzymes secreted by pancreas and duodenum itself aided by emulsifying bile (which also lowers pH). Duodenal ulcers caused by squirting of acid stomach contents into duodenal wall opposite sphincter. Small Intestine Jejunum (8 feet) and ileum (12 feet) continue degenerative process. Surface area increased by plica circulares (circular folds) carrying villi: cells of villi carry microvilli. Each villus has a capillary and a lacteal (lymphatic capillary) Absorption of digested foodstuffs is via these to the rich venous and capillary drainage of the gut. Towards the end of the small intestine accumulations of lymphoid tissue (Peyer's patches) more common. Undigested residue of food is rich in bacteria. Large Intestine Jejunum terminates at caecum. Caecum is small saclike evagination, important in some animals as a repository for bacteria/other organisms able to digest cellulose. A blind ending appendix may give trouble (appendicitis) if infected. The large intestine has three longitudinal muscle bands (taenia coli) with bulges in the wall (haustra) between them. These may evaginate in the elderly to become diverticuli and infected in diverticulitis. The large intestine resorbs water then eliminates drier residues as faeces. Regions recognised are the ascending colon, from appendix in right groin up to a flexure at the liver, transverse colon, liver to spleen, descending colon, spleen to left groin, then sigmoid (S-shaped) colon back to midline and anus. Anus has voluntary and involuntary sphincter and ability to distinguish whether contents are gas or solid. No villi in large intestine, but many goblet cells secreting lubricative mucus. Accessory digestive organs Salivary glands Three pairs, parotid, submandibular, and sublingual. Mumps begins as infective parotitis in the parotid glands in the cheek. The others open into the floor of the mouth. Saliva is a mixture of mucus and serous fluids, each produced to various extents in various glands. Also contains salivary amylase, (starts to break down starch) lysozyme (antibacterial) and IgA antibodies. In some mammals (and snakes!) saliva may be poisonous, quietening down living prey. Pancreas Endocrine and exocrine gland. Exocrine part produces many enzymes that enter the duodenum via the pancreatic duct. Endocrine part produces insulin, blood sugar regulator. Liver and gallbladder Bile, a watery greenish fluid is produced by the liver and secreted via the hepatic duct and cystic duct to the gall bladder for storage, and thence on demand via the common bile duct to an opening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments (mainly bile Rubin, essentially the non-iron part of hemoglobin) cholesterol and phospholipids. Bile salts and phospholipds emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, may block the hepatic or common bile ducts causing pain, jaundice. Liver Multifunctional: important in this context since the capillaries of the small intestine drain fat and other nutrient rich lymph into it via the hepatic portal system.
The main function of the stomach is to process and transport food. After feeding, the contractile activity of the stomach helps to mix, grind and eventually evacuate small portions of chyme into the small bowel , while the rest of the chyme is mixed and ground. Anatomically, the stomach can be divided into three major regions: fundus (the most proximal), corpus and antrum. Histologically, the fundus and corpus are hardly separable. In the antral area, the density of the smooth muscle cells increases. Macroscopic anatomy of the stomach. (View Image) The stomach wall, like the wall of most other parts of the digestive canal, consists of three layers: the mucosal (the innermost), the muscularis and the serosal (the outermost). The mucosal layer itself can be divided into three layers: the mucosa (the epithelial lining of the gastric cavity), the muscularis mucosae (low density smooth muscle cells) and the submucosal layer (consisting of connective tissue interlaced with plexi of the enteric nervous system). The second gastric layer, the muscularis, can also be divided into three layers: the longitudinal (the most superficial), the circular and the oblique. The longitudinal layer of the muscularis can be separated into two different categories: a longitudinal layer that is common with the esophagus and ends in the corpus, and a longitudinal layer that originates in the corpus and spreads into the duodenum. Structure of Gastric Muscularis. (View Image) A -- the longitudinal layer (the
area where the longitudinal fibers split is marked with a black
circle)
B -- the circular layer
C -- the oblique layer.
The area in the corpus around the
greater curvature, where the split of the longitudinal layers
takes place, is considered to be anatomically correlated with
the origin of gastric electrical activity . The circular layer
of the muscularis is continuous with the circular layer of the
esophagus, but is absent in the fundus . The thickness of the
circular layer increases in the antrum and especially in the pyloric
sphincter . It does not continue into the duodenum. The oblique
layer of the muscularis is clearly seen in the fundus and near
the lesser curvature of the corpus, but the oblique fibers disappear
distally (towards the antrum). The outermost main layer is the
serosa.
Cross-section of gastric wall. (View Image) Nerve plexi provide the interface between the mucosa and the muscularis, as well as between the longitudinal and circular layers of the muscularis. |
