Small Intestine: Functions, Diseases, and Resources
What is the Small Intestine and What Does It Do?
The small intestine is the part of the digestive tract that extends from the stomach to the large intestine. It is the site of chemical digestion and absorption of nutrients by the body, including vitamins, minerals, carbohydrates, proteins and fats. The small intestine is the longest section of the digestive system. How big is the small intestine? The small intestine is a tube-shaped and coiled, it measures about 16 feet long when uncoiled and is approximately 2.5 to 3 centimeters in diameter. Although the small intestine is about three time longer than the large intestine, it is called “small” because of the narrow size of its diameter.
What Parts Make up the Small Intestine?
The small intestine is divided into three structural parts: the duodenum, the C-shaped first part; the jejunum, the coiled midsection; and the ileum, the final section that leads to the large intestine.
What is the Primary Function of the Small Intestine?
By the time food reaches the small intestine, it has already been somewhat digested and reduced to a thick liquid, called chyme, by the teeth, saliva and the churning activity in the stomach. Once the food is digested to the right consistency, chyme from the stomach is allowed into the duodenum by a trap-door muscle called the pylorus, or pyloric sphincter, and then continues traveling through the small intestine by contractions called peristalsis. Once in the duodenum, the small intestine further digests the food by breaking it down into nutrient molecules small enough to pass into the bloodstream. The duodenum receives enzymes from the pancreas that help with the breakdown of three major classes of nutrients – carbohydrates, fats and proteins. It also receives bile from the liver, which helps with fat absorption. The muscles around the small intestine constrict about seven to 12 times per minute to stir up the chyme. The small intestine also absorbs enormous quantities of water, mainly from bodily fluids. Together, the intestines process two to three gallons of liquids, foods and bodily waste every day.
Digested food is now able to pass into the blood through the walls of the intestine by the process of diffusion, which is when matter moves from areas of higher concentration to areas of lower concentration. These absorbed substances are transported via the blood vessels to different organs of the body. The food that remains undigested and unabsorbed passes into the large intestine and is eventually excreted.
How are nutrients absorbed from your intestines?
Nutrients are absorbed through the lining of the intestines, called the mucosa, and transferred into the blood stream. Once in the blood stream, the different nutrients flow throughout the body where they are needed.
The transfer of nutrients through the intestines into the blood stream is the final step of a long digestive process. As soon as food is taken into the mouth, liquids and physical motion begin to breakdown the food into smaller components able to be absorbed. By the time the food reaches the intestines, it has passed through three organs; the mouth, esophagus and stomach.
At a general level, the food-liquid mix that moves from the stomach into the intestines is further mixed with additional liquids, mostly enzymes, while in the intestines. These enzymes get into the intestines through glands located in the walls of the intestines or through secretions from two other digestive organs: the pancreas and gallbladder.
Once the nutrients finish the last stage of digestion in the intestines, they can then move through the lining of the intestines into the bloodstream. The mucosa or lining has a complex surface of millions of tiny finger-like structures called villi. On these villi are even tinier protruding pieces, called microvilli. The combination of these millions of villi and microvilli create a vast surface area which capture the nutrients and push them through the lining into the bloodstream, where they can then travel through the entire body.
How are Nutrients Broken down in the Intestines?
More specifically, each of the major categories of nutrients gets broken down in the intestines and moved into the bloodstream in slightly different ways, as the nutrients themselves have different physical makeups. The macronutrient categories are carbohydrates, proteins and fats. There are also micronutrients, mostly vitamins and minerals.
How are Carbohydrates Broken down in the Intestines?
Carbohydrates have two digestible parts: starch and sugar. Once it reaches the intestines, starches are broken down by pancreatic juices into molecules called maltose. From there, the carbohydrates are broken down into glucose molecules in a second step by enzymes native to the intestines. Ultimately, the glucose is transferred through the bloodstream to the liver where it can be used for energy, or is stored if not immediately needed.
How is Protein Broken down in the Intestines?
Protein is made up of large molecules that get broken down by multiple enzymes throughout the entire digestive process. Once in the intestines, multiple enzymes from the pancreas and lining of the intestines itself further digest the protein molecules. While in the intestines, these large protein molecules are broken into much smaller molecules called amino acids. These smaller molecules are then able to move through the intestine lining into the bloodstream where they can strengthen all cells of the body.
How is Fat Broken down in the Intestines?
Fat goes through the most complex process of digestion and absorption. The fat is first dissolved in the juices of the intestines and bile acids. These liquids transform the fat molecules into little droplets which are then attacked by enzymes produced by the pancreas and the intestines. The results of this process are much smaller fat molecules, many of which are fatty acids and cholesterol. These fat molecules combine with bile acids to get absorbed into the cells of the mucosa. Once within the intestine lining, these small molecules rejoin to create larger molecules which enter lymph tissue, which is then transferred towards the heart. From there, the bloodstream carries these fat molecules where they deposit in various places of the body.
How are Vitamins Absorbed in the Small Intestines
Vitamins may be absorbed from the intestines depending on which type of the vitamin they are. Fat-soluble vitamins are dissolved in the intestines and then stored within the liver or in fat cells as they absorb through the intestines. Water-soluble vitamins are more difficult to store and thus most are washed out of the body through urination.
How Long Does it Take Food to Pass Through the Small Intestine?
Food may take as long as four hours to pass through the small intestine, with the average length of time hovering at about 2.5 to three hours. The exact length of time is dependent on the individual makeup of each person, the type of food being digested, gender, psychological stress and even reproductive status.
How do researchers determine how long it takes for food to pass through the small intestine? The time it takes for food to pass through the small intestine has been measured in a number of ways. One technique uses radiography following a barium-labeled meal, which allows researchers to follow the barium through the digestive tract. This technique is unpleasant for the patient and exposes him/her repeatedly to radiation. Another technique is a breath hydrogen analysis, which is based on the fact that when a specific type of food reaches the large intestine, hydrogen gas is released and may be detected in the breath. Using this technique, researchers can estimate transit time based on how long it takes for the hydrogen to be detected in the breath. Scintigraphic analysis has patients consume food laced with a radioactive label, which can then be monitored using a gamma camera.
Studies have shown that not all foods move uniformly through the digestive system, and foods do not necessarily pass through the digestive tract in the same order as they were consumed. This means that if one eats a meal of fish and chips and washes it down with a glass of milk, the calcium of the milk may be absorbed faster than the protein from the fish or the carbohydrates from the chips.
Resources about the Small Intestine
Introduction and index to the small intestine
Color images of the small intestine
List of diseases of the small intestine
How do Pathogens Affect the Human Small Intestine?
What are Pathogens
Pathogens are micro-organisms that cause disease. Bacteria and viruses are both types of pathogens and interfere with the normal functioning of the organ they attack. There are a wide variety of diseases that are caused by pathogens in the small intestine. Depending on the specific pathogen, whether it is foreign to the small intestine or merely present in abnormal levels that are too many or too few, different symptoms and diseases present. Because the small intestine is permeable, that is how nutrients transfer from the small intestine into the bloodstream to move throughout the rest of the body, pathogenic bacteria can also enter.
A healthy small intestine already houses trillions of micro-organisms. Many of these micro-organisms assist humans, including controlling the growth of harmful pathogens. Enzymes in particular are helpful in maintaining the right balance of bacteria in the intestines, which in turn helps control the volume of pathogenic bacteria. Indeed, most bacteria in the intestine act as a powerful and effective immune system, attacking pathogens that might enter and keeping the intestines healthy.
However, pathogens can produce toxins and carcinogens which cause an array of gastrointestinal (GI) diseases. They are particularly harmful when coupled with any condition that limits the normal, mechanical workings of the intestine. The intestines protect themselves not just through positive bacteria which fight pathogens, but also through their physical convulsions, called peristalsis, where the muscles in the intestinal lining move to push digested food, among other material, through the small intestine into the colon, ultimately to be expelled entirely from the body.
What are the Most Common Diseases in the Small Intestine?
The most common array of these diseases are known collectively as Irritable Bowel Disease (IBD), and include Celiac or Crohn’s disease. The exact cause of IBD is unknown, but is widely believed to be the result of excessive growth of some sort of bacterial pathogen since symptoms of IBD have been successfully treated with antibiotics. A consequence of IBD and many of the GI diseases is particular sensitivity to certain foods. For example, those suffering from Celiac Disease cannot process wheat in their intestines. If one with Celiac eats wheat or gluten products, the lining of their intestine will become even more inflamed and they are unable to process these foods through normal digestion. In the case of Crohn’s disease, it is believed that the active immune system within the intestine confuses normal bacteria and foods for pathogens, and begins to actively fight off pathogens that aren’t there. However the means of fighting the pathogens, the influx of white blood cells to the intestinal lining, when they are not there leads to swelling and intestinal damage.
What are Some Symptoms of Diseases of the Small Intestine?
There are also innumerable effects and symptoms of these diseases which can also present simply as the symptom, but which can be chronic themselves if the pathogens remain. Such symptoms include excess gas, diarrhea, abdominal bloating, abdominal distention, and abdominal pain. On occasion, the excess pathogens may cause constipation, rather than diarrhea. Because the intestine is used to transfer necessary nutrients into the bloodstream, a common symptom can also be fatigue as the pathogens can interfere with proper absorption and transfer of necessary nutrients from food digestion.
Resources about IBD
Questions and answers about IBD that includes; What is irritable bowel syndrome (IBS)? What causes IBS? What are the symptoms of IBS? How is IBS diagnosed? How is IBS treated? Does stress cause IBS?
Irritable Bowel Syndrome Self Help and Support Group
Resources about Crohn’s Disease
Questions and answers about Crohn’s disease that includes; What is Crohn’s disease? What causes Crohn’s disease? What are the symptoms? How is Crohn’s disease diagnosed? What are the complications of Crohn’s disease? What is the treatment for Crohn’s disease? Can diet control Crohn’s disease? Can stress make Crohn’s disease worse? Is pregnancy safe for women with Crohn’s disease?
Definition of Crohn’s disease
These diseases do not usually lead to serious medical conditions, although they are chronic and not curable. They can profoundly affect the quality of life for those who suffer from them if they are not managed well. IBD can be managed through a range of treatments, such as the traditional means of medication and nutrition, all the way to psychotherapy and alternative medicines.
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