m2t03

=MODULE 2.3=

= = =I. Topic Title: Digestive system=

=**II. Topic Description:**=


 * Digestion** is the process by which food and liquid are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. The diet of any animal contains hundreds if not thousands of different molecules, but the bulk of the ingested nutrients are in the form of huge macromolecules that cannot be absorbed into blood without first being reduced to a much simpler and smaller forms. Even table sugar (sucrose) cannot be absorbed without first being enzymatically broken into glucose and fructose.

=**III. Terminal Objectives:**=

1. To identify the in-depth process of digestion. 2. To identify the essential as well as accessory digestive organs. 3. To explain the passage of food from the mouth down to the anus. 4. To identify the enzymes and other chemicals essential for the digestive process.

=**IV. Content Presentation:**=



http://training.seer.cancer.gov/module_anatomy/unit10_3_dige_region4_intestine.html

=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.

In many ways, the digestive system can be thought of as a well-run factory in which a large number of complex tasks are performed. The three fundamental processes that take place are:

· **Secretion:** Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood. · **Absorption:** Transport of water, ions and nutrients from the lumen, across the epithelium and into blood. · **Motility**: Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents.

=**DIGESTIVE ORGANS**=


 * 1. Mouth:**

· **Lips**- regulates the opening and the closing of the mouth. It keeps the food in place.

· **Teeth**- humans have the incisors to cut the food, the canines to keep the food in place, the pre-molars and the molars to grind the food. Adults do have 32 teeth. Children do gave approximately 20 teeth.

· **Tongue**- humans have tongues to taste the food and to aid them in speech. Lower forms of animals such as frogs use their tongues for catching the prey. Dogs use their tongues to aid them in breathing by panting. Snakes, on the other hand, use their tongues to smell the location of a potential prey or a mate. Some animals have taste receptors (chemosensors) on their feet (e.g., butterfly) or even over their entire bodies (e.g., catfish and earthworm). Birds have very few taste buds and therefore their sense of taste is probably not as good as other animals.The tongue is capable of recognizing, sweet taste at the tip, sour at the side, salty postero-lateral part and bitter at the middle of the tongue. And last basic taste is called "umami". Umami is a taste that occurs when food with glutamate (like MSG) are ingested.

· **Palatine tonsils-** located at the side of the oral cavity


 * 2. Pharynx**, is a passageway for food and air; it is about 5 inches (12.7 centimeters) long.


 * 3. Epiglottis**-a flexible flap of tissue which reflexively closes over the windpipe when we swallow to prevent choking.


 * Swallowing**

Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. From the throat, food travels down a muscular tube in the chest called the //esophagus.//

The Typical movement of the esophagus, stomach, and intestine is called //peristalsis//. 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.

In leaving the mouth, a bolus of food must cross the respiratory tract by a complicated mechanism known as //swallowing// which empties the mouth and ensures that food does not enter the windpipe. Swallowing involves the coordinated activity of the tongue, soft palate pharynx and esophagus. The first phase is voluntary, the food is 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 while swallowing food may enter the respiratory passages and a cough reflex expels the bolus.


 * 4. 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. Waves of muscle contractions called //peristalsis// force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract. At the end of the esophagus, a muscular ring called a //sphincter// allows food to enter the stomach.

The esophagus is like a stretchy pipe that's about 10 inches long. It moves food from the back of your throat to your stomach. But also at the back of your throat is your windpipe, which allows air to come in and out of your body. When you swallow a small ball of mashed-up food (or liquids), a special flap called the epiglottis flops down over the opening of your windpipe to make sure the food enters the esophagus and not the windpipe. If you've ever drunk something too fast, started to cough, and heard someone say that your juice "went down the wrong way," the person meant that it went down your windpipe by mistake. This happens when the epiglottis doesn't have enough time to flop down, and you cough involuntarily (without thinking about it) in order to clear your windpipe.


 * 5. Stomach:** The food now enters the stomach, and then squeezes shut to keep food or fluid from flowing back up into the esophagus. It has three mechanical tasks to do.

o 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. o The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The stomach muscles churn and mix the f ood with acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about 3 quarts (2.8 liters) of these digestive juices each day. o The third task of the stomach is to empty its contents slowly into the small intestine.

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 need further digestion and must travel into the intestine before being absorbed. When it's empty, an adult's stomach has a volume of one fifth of a cup (1.6 fluid ounces), but it can expand to hold more than 8 cups (64 fluid ounces) of food after a large meal.


 * 6. Small intestine**– is the site of final digestion of food.

The parts of the Small Intestine: · the duodenum, (2 feet) the C-shaped first part · the jejunum, (8 feet) the coiled midsection · the ileum (12 feet) the final section that leads into the large intestine.

The small intestine is a long tube that's around 1 1/2 inches to 2 inches in diameter. The small intestine is about 22 feet long. The small intestine has the important job of breaking down the food mixture so your body can absorb all the nutrients it needs from food-vitamins, minerals, proteins, carbohydrates, and fats. Only the small intestine can unleash its power for you it won't go directly into your body's tissues to give you energy. But the small intestine can't break down food for its nutrients by itself - it gets help from three accessory organs of the digestive system. These parts are the pancreas, the liver, and the gallbladder. These are accessory digestive organs because they are not part of the digestive tract itself, but they help to make up the whole digestive system. This is done by sending different juices to the first part of the small intestine. These juices help to digest food and allow the body to absorb its nutrients. The pancreas makes juices that help the body digest fats and protein. A juice from the liver called //bile// helps to absorb fats into the bloodstream. And the gallbladder is like a warehouse for this bile, holding onto extra amounts of it ready for release when the body needs it.

In the upper part of the small intestine the liquid food mix combines with the juices from the pancreas and the liver. After all this mixing of juices the liquid food mix has now become very thin and watery. It moves into the lower part of the small intestine, getting broken down even further. When the mix is almost at the end of its journey through the small intestine the nutrients from the food can finally pass through the wall of the small intestine and into the blood. Once the nutrients have entered your blood, the blood takes them to a major checkpoint: the liver.

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called //villi//. The villi are the vehicles through which nutrients can be absorbed into the body.


 * 7. Large Intestine** is shorter but wider than the small intestine (around 3 inches to 4 inches around), and it's almost the last stop on the digestive tract. Like the small intestine, it is packed into the body, and if it were stretched out would be about 5 feet long. The large intestine has a tiny tube with a closed end coming off it called the //appendix.// Although the appendix is part of the digestive tract, it doesn't do anything at all.

The //cecum// is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area expands in diameter, allowing food to travel from the small intestine to the large. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Doctors believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.

The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then extends down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the //ascending colon// and //transverse// //colon,// which absorb fluids and salts, and the //descending colon//, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.

Any useful substances in the leftovers such as spare water and body minerals, are absorbed through the walls of the large intestine back into the blood. The remains are formed into brown, semi-solid feces ready to be removed from the body.

//Appendicitis// an inflammation of the appendix, the finger-like pouch extending from the cecum located in the lower right part of the abdomen. The classic symptoms of appendicitis are abdominal pain, fever, loss of appetite, and vomiting. Kids and teens between the ages of 11 and 20 are most often affected by appendicitis, and it requires surgery to correct.


 * 8. Rectum**- is the part where feces are stored until they leave the digestive system through the anus as stool. These are finally squeezed through a ring of muscle, the anus and out of the body.


 * ACCESSORY DIGESTIVE ORGANS**


 * 1. Salivary Glands**

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 saliva. The flow of saliva is set in motion by a brain reflex that's triggered when we sense food or think about eating. 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. As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called //amylase,// which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth. The saliva is secreted by three kinds of salivary glands according to location.


 * Parotid glands**. Mumps begins as infective parotitis in the parotid glands in the cheek. The others open into the floor of the mouth.


 * Submandibular or Submaxillary glands** are located below the mandible or lower jaw.


 * Sublingual glands-** are located below the tongue.


 * 2. Pancreas** - the only gland that is both exocrine and endocrine. As an exocrine gland part the pancreas secretes many enzymes which enter the duodenum via the pancreatic duct. As and endocrine gland it secretes //insulin//, a blood sugar regulator. The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also makes a substance that neutralizes stomach acid.


 * 3. Liver**- the largest gland of the body. The liver produces yet another digestive juice-the //bile.// The bile is stored between meals in the gallbladder. At meal time, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in 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.

The liver even helps figure out how many nutrients will go to the rest of the body, and how many will stay behind in storage. The liver stores certain vitamins and a type of sugar that your body uses for energy. Once everything has passed through the liver for inspection, the liver then gives the signal and the nutrients can be carried in the blood to the rest of the body. It is also referred as the //graveyard// of red blood cells because it collects dead red blood cells.


 * 4. Gallbladder** – is a small baglike part is tucked under the liver. It stores a fluid called bile, which is made in the liver. As food from a meal arrives in the small intestine, bile flows from the gall bladder along the bile duct into the intestine. It helps to digest fatty foods and also contains wastes for removal.

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 hence on demand via the common bile duct to an opening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments (mainly bilirubin, essentially the non-iron part of hemoglobin) cholesterol and phospholipids. Bile salts and phospholipids emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, that can block the hepatic or common bile ducts causing pain and jaundice.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called //chyme//. A muscular tube at the outlet of the stomach called the //pylorus// keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine where digestion of food continues so the body can absorb the nutrients into the bloodstream. Attached to the end of the esophagus is the stomach, a stretchy "sack" that's shaped like the letter "j." The stomach is like a mixer, churning and mashing together all the small balls of food that came down the esophagus into smaller and smaller pieces. This is accomplished with the help of the strong muscles in the walls of the stomach and the gastric juices that also come from the stomach's walls. These gastric juices help to break down food and make a sort of liquid mixture in the stomach. They also help to kill bacteria that might have come along with the food. It takes about 4 hours for the stomach to do this job completely and get the mixture ready for the next part of the digestive tract.


 * V. Activities:**

http://school.discovery.com/quizzes35/jorimi/DigestiveSystem.html


 * VI. References:**

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http://www.talkorigins.org/faqs/vestiges/appendix.html http://training.seer.cancer.gov/module_anatomy/unit10_3_dige_region4_intestine.html

=VIII. Disclaimer and Copyright Notice= This work is licensed under a [|Creative Commons Attribution-NonCommercial-NoDerivs 2.5 License]

=IX. Topic Writer:= Ms. Joanne R. Miranda jorimi@gmail.com User name: jorimi

X. Document History:
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