Food and Digestion
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Digestion
Can you imagine having a 30-foot-long tube running from your mouth to your anus? That is exactly what you have in your digestive system. Food contains nutrients, proteins, carbohydrates, and fats that the cells in our body use as energy. The process of digestion involves breaking down the food so that cells can use its components to fuel our body and keep it functioning. This module explains how organisms digest, absorb, and utilize food to obtain the materials they need for their cells to sustain life, grow, and multiply.Usually in biological science, we learn most of what we know from studies of animals. But some of our first notions of what happens in digestion comes from the study of a live human patient who had an unhealed wound that led from the stomach to the outside. Read the interesting Story Time of Dr. Beaumont and his patient.
Why do we need a digestive tract?
If the body did not have a digestive tract, you could not enjoy your favorite pizza, hamburger, or other food. The human body must obtain its energy by eating food. Therefore, the main purpose of the digestive system is to provide the body with amino acids, carbohydrates, fats, and vitamins to keep our cells functioning. The digestive system provides these essential materials to the 75 trillion cells that live in our bodies. Wow! Do you know how many 75 trillion is?
The digestive tract takes both liquids and food and breaks them down into single molecules that can be absorbed by cells in the small intestine. These cells transport the molecules into the blood stream so that other cells in the body can use them. The digestive tract also serves to eliminate what your body doesn't absorb during the digestive process.
Why do I have a stomach?
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The stomach has two major functions in your body: one for digestion and one for defense. (See the illustration on the right.). Digestion breaks down proteins with acidic stomach juice, secreting enzymes that split up proteins into individual amino acids (see Cells Are Us unit on proteins). The defense component kills most foreign organisms that you ingest so that they can't get into your bloodstream or anywhere else in your body. These organisms are destroyed by the highly acidic stomach fluid that is secreted within the stomach.
Why do you have two intestines?
The intestines are where the body absorbs all of the food, vitamins, fluid, and minerals that you eat. The intestines are long giving your body a greater opportunity to absorb more of what you eat. The small intestine breaks down proteins, carbohydrates, and fats so that they can be absorbed into the body and the bloodstream. The large intestine is where moisture is absorbed from what is left of the food. Anything that is not absorbed by the intestines is then passed through as solid waste (or feces).
Why are the liver and pancreas important in digestion?
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Your liver secretes bile into your small intestine. This bile contains worn out red blood cells that must be removed from the body. If the old red blood cells are not removed, they can become toxic. The bile secreted by your liver also separates the fats that you eat so that they can be broken down by enzymes. (See the picture on the right.) Fat clumps together and when bile is not secreted into the small intestine, your enzymes are less efficient at breaking down fats into their fatty acid components. The enzymes that break down these separated fats come from the pancreas, as do those enzymes that break down carbohydrates and proteins. The pancreas also secretes a basic solution that neutralizes the acidic solution left over from the stomach. The pancreas also secretes insulin, which helps move glucose (blood sugar) from blood into cells. Diabetes is a common and very serious disease that results from lack of insulin.
How we find out?
Over the years scientists have worked to unlock the once mysterious process of how humans digest food. Some of the approaches can even be used by you to discover what happens in the digestive tract!
How do we learn about the different parts of the digestive system?
When we dissect a dead animal, we can see the different parts along the tract that begins in the mouth and ends in the anus. See pictures here. The cells can be seen by making microscope slides.
How could we see where digestion begins?
How would you find out if saliva has any digestion function? A quick way to find out is to compare a piece of steak (which has a lot of protein) and a cracker (which has a lot of starches). Place the meat in your mouth and do not chew it for about 2 minutes. What happens? You should discover that the steak keeps its shape. Next, place the cracker in your mouth without chewing it for about two minutes. You should notice that the cracker becomes soft in your mouth and you may even taste something sweet. This is because the cracker is composed of carbohydrates, which saliva is able to digest at least partly. The carbohydrates in the cracker begin to be broken down into individual glucose molecules by an enzyme in your saliva. This enzyme only breaks down carbohydrates, not proteins. Since the steak was mostly composed of proteins, it did not start breaking down like the cracker.
How do we know what goes on in our stomach?
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The stomach contains acids that turn most of your food into a liquid. Have you ever been sick and felt a burn in the back of your throat after vomiting? If you have, then you have experienced the power of the acids in your stomach. Stomach secretions also contain strong enzymes. The digestive functions of the stomach were discovered during an experiment performed by Doctor Beaumont. In his experiment, Doctor Beaumont tested the digestive power of stomach fluids through a "fistula" (see the graphic on the right) that connected the stomach to the patient's skin. Doctor Beaumont tied food to a string, poked it through the fistula, and pulled it back out at varying times to see what had happened to the food. He observed that the food had changed, varying with the amount of time spent in the stomach and the nature of the food itself. One of the major things that was noticed by Doctor Beaumont is that proteins were especially degraded by the acids of the stomach.
WHAT WE KNOW?
Digestive Structures and Functions
Over the years scientists have learned a lot about the digestive tract that has given doctors and researchers in the health science field the necessary information to cure or treat most of the digestive disorders that face us today.
Is pizza good for me?
Pizza contains the three major types of food your body needs to function and some essential vitamins, but it also contains cholesterol, which can cause blockage in your blood vessels. Pizza is very high in calories and fat.
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What organs are involved in digestion?
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Many organs are involved in the digestive process (see the diagram on the left). First, food enters the body via the mouth, where saliva from the salivary gland begins to break down carbohydrates. Next, the food goes down the food tube, the esophagus, pushed by muscular contractions. The food ends up in the stomach, where stomach acids breakdown much of the protein into amino acids and kill most bacteria in the food. The liver processes and regulates the substances entering the blood stream from the food that is ingested. The liver also produces bile, which contains worn-out red blood cells, that is stored in the gall bladder. The pancreas assists in digestion of carbohydrates, proteins, and fats, and it regulates blood sugar levels by directing cells to take up or to secrete sugar into the blood. Once leaving the stomach, the food travels into the small intestine where carbohydrates, amino acids and fats are absorbed. After entering the large intestine, moisture is absorbed from what is left of the food and bacteria breaks down some of the material that has not been digested. This creates waste (feces). The rectum is the final storage space for wastes, which is what becomes of the food once all of the nutrients and moisture have been removed. The wastes remain in the rectum until excreted.
The time food stays in the various parts of the digestive tract in a human under normal conditions is approximately as follows:
• Stomach: 4-6 hours
• Small intestine: 2-4 hours
• Large intestine: 24-48 hours
Knowing this, you should be able to answer the following:
1. Why do you get hungry at the times for breakfast, lunch, and dinner?
2. Why should you have a bowel movement every day or two?
STORY TIME:
William Beaumont (1785-1853)
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In 1822, an Army surgeon stationed at Fort Mackinac on the upper Michigan peninsula would change medical science. That surgeon, William Beaumont, was introduced to a 19-year-old gunshot victim, an employee of the American Fur Company. The patient, Alexis St. Martin, had multiple wounds, as you might expect from a shotgun. One of his wounds penetrated the stomach. Alexis agreed to let Beaumont experiment with him, allowing the surgeon to insert a six-inch tube into his stomach by way of an opening to the outside.
Beaumont was able to directly observe the release of stomach secretions in response to Alexis' eating different foods. He was also able to collect stomach contents, place them in glass tubes, and observe how much food was digested and the length of time it took for the food to break down. For centuries, many people believed that the stomach was hot and somehow "cooked" the food, acting as a fermenting vat or grinding mill. Beaumont, with the help of Alexis, proved that those superstitions were false.
Medicine in the early nineteenth century was primitive. Doctors did not realize the importance of cleanliness. For the pain of rheumatism, Beaumont and fellow doctors prescribed opium, wood resin, and turpentine. Despite such uninformed and primitive medicine, Beaumont proudly claimed that none of his 200+ patients died.
COMMON HAZARDS:
Ulcers
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An ulcer is a lesion in the mucus lining of the stomach. The mucus protects the underlying cells in the wall of the stomach from the toxic juices in the stomach. These juices contain a high concentration of HCl, which is toxic to the cells due to its highly acidic nature. An ulcer can become serious if not treated because prolonged exposure to HCl will eventually cause bleeding at the exposed portion of the stomach wall..png)
Causes
The leading cause of ulcers is a recently discovered bacterium. This bacterium, Helicobacter pylori (photo on left), was discovered in the early 1990's by a veterinarian who was studying ulcers in pigs. This bacterium resides in about 30% of the population; these individuals have a much greater risk of developing ulcers.
The chances of any person getting an ulcer are increased if they are exposed to certain chemicals or emotions. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, or stronger medications used in the treatment of arthritis or chronic inflammatory disease and ethyl alcohol (alcohol in alcoholic beverages) can weaken or break the mucus barrier that normally protects stomach cells from the acid. An ulcer can also be caused by stress and anxiety because these emotions cause an increase in the amount of HCl secreted by the cells of the stomach. Patients that suffer from severe infections or injuries are at a much greater risk of developing ulcers due to a breakdown in the mucus lining of the stomach.
Treatment
Treatment of ulcers includes the use of drugs to treat bacteria, reduce stomach acid, and protect the stomach lining. Antibiotics are used to treat bacteria, antacids are used to reduce stomach acid, and bismuth subsalicylate, a component of Pepto-Bismol®, is used to protect the mucus lining of the stomach. The most effective way to combat ulcers is a two-week treatment of these three types of drugs.
To learn more about ulcers visit the following websites:
