The intestines are an integral part of the digestive system, without which, we wouldn't be able to absorb the vital nutrients from our food. Together with organs including the liver and pancreas, the intestines perform an amazingly complex series of functions which work seamlessly to break down and process everything we eat. What's more, we don't have to consciously do anything other than enjoy the food we're putting into our mouth.
What are the intestines?
The small intestine is where the majority of all nutrient absorption takes place. Measuring over six meters (20 feet) in length, this long, narrow tube, is recognizable by its coiled and snake like appearance. The internal walls of the small intestine are covered with tiny hair-like projections called villi which vastly increase the overall surface area and allow for greater nutrient absorption. Inside each microvillus is a blood capillary which absorbs nutrients which then enter the blood stream.
The small intestine is made up from three main parts: the duodenum, jejunum and ileum, each contributing to the various stages of the absorption process. The large intestine or colon, is much wider than the small intestine, measuring around 1.5 meters (5 feet) and plays a minor part in the absorption of nutrients.
The gall bladder, pancreas and liver all work in conjunction with the small intestine to further breakdown nutrients. The liver for example, is responsible for producing bile which contains vital bile salts. These are essential for breaking down the fats in our food. The gall bladder stores bile and releases it into the small intestine when it's needed. The pancreas secretes digestive juices into the duodenum which break down proteins, carbohydrates and fats as well as maintaining a safe PH balance within the small intestine.
How the intestines work
Once we've chewed and swallowed our food, it's pushed through the esophagus by a process called peristalsis, whereby it arrives in the stomach and is further broken down. Food then moves from the stomach through a valve called the pyloric sphincter which controls the flow of partially digested food into the small intestine.
The first section of the small intestine, the duodenum, subjects food to an intense breaking down process. It is here that bile and pancreatic juices are combined with water and mucous from the duodenum to break down the remaining carbohydrates, fats and proteins from the food.
The second part of the small intestine is the jejunum which serves to absorb more water and nutrients after food has left the duodenum. The final part of the small intestine, the ileum, functions to absorb whatever nutrients are still left after passing through the jejunum, including reusable products such as bile salts. Another sphincter passes any undigested matter onto the large intestine (colon).
Whatever arrives in the large intestine basically consists of material which the body cannot digest or has no use for, plus any minerals which haven’t yet been absorbed. During a period of around three to ten hours, bacteria within the colon digests whatever it can, after which time any water or nutrients released will be absorbed into the colon. Finally, whatever is left over is compressed to form feces which then passes to the rectum.
Lisa Hark PhD RD & Dr Darwin Deen, 2007. Nutrition For Life, p.36. 3rd ed. United Kingdom: Dorling Kindersley Limited