Overview
The intestines are a long, continuous tube running from the stomach to the anus. Most absorption of nutrients and water happen in the intestines. The intestines include the small intestine, large intestine, and rectum.
The intestine is a muscular tube which extends from the lower end of your stomach to your anus, the lower opening of the digestive tract. It is also called the bowel or bowels. Food and the products of digestion pass through the intestine, which is divided into two sections called the small intestine and the large intestine.
What Is the Small Intestine?
The small intestine is made up of three segments, which form a passage from your stomach (the opening between your stomach and small intestine is called the pylorus) to your large intestine:
- Duodenum: This short section is the part of the small intestine that takes in semi-digested food from your stomach through the pylorus, and continues the digestion process. The duodenum also uses bile from your gallbladder, liver, and pancreas to help digest food.
- Jejunum: The middle section of the small intestine carries food through rapidly, with wave-like muscle contractions, towards the ileum.
- Ileum: This last section is the longest part of your small intestine. The ileum is where most of the nutrients from your food are absorbed before emptying into the large intestine.
By the time food reaches your small intestine, it has already been broken up and mashed into liquid by your stomach. Each day, your small intestine receives between one and three gallons (or six to twelve liters) of this liquid. The small intestine carries out most of the digestive process, absorbing almost all of the nutrients you get from foods into your bloodstream. The walls of the small intestine make digestive juices, or enzymes, that work together with enzymes from the liver and pancreas to do this.
How Can The Small Intestine Digest So Much?
Looking at the small intestine as a pipe, it seems hard to believe that an organ so narrow could do such a big job. However, looks can be deceiving. The absorptive surface area of the small intestine is actually about 250 square meters (almost 2,700 square feet) – the size of a tennis court! How is this possible? The small intestine has three features which allow it to have such a huge absorptive surface area packed into a relatively small space:
- Mucosal folds: The inner surface of the small intestine is not flat, but thrown into circular folds. This not only increases the surface area, but helps regulate the flow of digested food through your intestine.
- Villi: The folds form numerous tiny projections which stick out into the open space inside your small intestine (or lumen), and are covered with cells that help absorb nutrients from the food that passes through.
- Microvilli: The cells on the villi are packed full of tiny hairlike structures called microvilli. This helps increase the surface of each individual cell, meaning that each cell can absorb more nutrients.
What Is the Large Intestine?
Your large intestine is about five feet (or 1.5 meters) long. The large intestine is much broader than the small intestine and takes a much straighter path through your belly, or abdomen. The purpose of the large intestine is to absorb water and salts from the material that has not been digested as food, and get rid of any waste products left over. By the time food mixed with digestive juices reaches your large intestine, most digestion and absorption has already taken place.
What’s left is mainly fiber (plant matter which takes a long time to digest), dead cells shed from the lining of your intestines, salt, bile pigments (which give this digested matter its color), and water. In the large intestine, bacteria feed on this mixture. These helpful bacteria produce valuable vitamins that are absorbed into your blood, and they also help digest fiber. The large intestine is made up of the following parts:
- Cecum: This first section of your large intestine looks like a pouch, about two inches long. It takes in digested liquid from the ileum and passes it on to the colon.
- Colon: This is the major section of the large intestine; you may have heard people talk about the colon on its own. The colon is also the principal place for water reabsorption, and absorbs salts when needed.
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