And now we reach the heart of the intestinal tract. Everything so far has been preparation for this discussion. Digestion, or breaking food down into smaller bits, is certainly important -- crucial even -- but to what purpose? The purpose, quite simply, is to get the nutrition inherent in the food you ate ready so that it can be absorbed into your body where it can be used by each and every single cell to survive and carry on its individual function.
When it comes to the intestinal tract, the key is absorption. It's not what you eat or digest that matters; it's what you absorb.And when it comes to absorption, the small intestine is the portal for virtually all nutrients that enter into the bloodstream. Note: much of this discussion is easy to understand, but the core of it, the actual act of absorption is quite technical and involves some chemistry. As always, I will only deal with as much chemistry as is absolutely necessary -- and will present it in such a way as to make it comprehensible. Digestion -- setting up absorption Before we can get to absorption, we have to cover the final stages of digestion that take place in the small intestine.
In fact, you get a combination of mechanical and chemical digestion and some absorption in the small intestine. Early in the intestine it is mostly digestion, very little absorption. However, the further on you move down the digestive tract, the more the ratio swings in favor of absorption. Effectively, the entire small bowel (duodenum, jejunum, and ileum) is devoted to these two processes: digestion and absorption. Digestion itself is divided into mechanical and chemical phases.
Mechanical digestionMechanical digestion, as we alluded to in our exploration of the anatomy of the small intestine, is the result of two very different, but complementary actions: * Segmentation contractions chop, mix, and roll the chyme (the mixture of food and digestive juices). * Peristalsis slowly propels the chyme forward toward the large intestine. Segmentation represents localized activity in the small intestine, whereas peristalsis represents the more global movement that takes place throughout the entire intestinal tract. In segmentation, circular muscles constrict and divide the small bowel into segments -- each about 3-4 inches long.A muscle then contracts between the two other muscles and subdivides the segment.
This is repeated many times per minute so that the chyme is moved back and forth in the same area of the segment. Localized contractions crush and mix food within that segment alone. This action mixes the chyme with intestinal juices and prolongs its contact with the absorptive surface of the small intestine. Relaxation allows the segments to coalesce, thus allowing chyme to move on down the intestinal tract -- pushed by peristalsis.
Peristaltic contractions represent a global movement that is designed to move chyme through the entire length of the small intestine and ultimately complement the mechanical process of segmentation that holds chyme in individual segments of the intestinal tract. Peristalsis is completely under the control of the autonomic nervous system and is coordinated by the myenteric plexi (plexuses). The myenteric plexus, also known as Auerbach's plexus, is a network of nerves between the circular and longitudinal layers of the muscles surrounding the intestinal tract.Segmentation in a sheep's small intestine It should be noted that peristaltic activity is weak (as opposed to segmentation), which means that food stays in the small bowel for a relatively long time (4-6 hours).
And it should also be noted that peristalsis can be fairly easily slowed or even stopped by outside factors. Culprits include appendicitis, surgery, medication, and even very large meals. On the other hand, there are certain things that can increase peristalsis such as laxatives and certain kinds of illness or toxicity.As anyone who has experienced food poisoning or stomach flu would know, peristalsis is quite capable of shooting food through the intestinal tract when required.
In simple terms, the body responds to toxins in the intestinal tract by adhering to the old bromide, "The solution to pollution is dilution. " In effect, the body pours fluid into the intestines and increases peristalsis to eject and weaken toxins in cases such as bacterial contamination. In extreme situations such as presented by cholera, victims may actually die of dehydration from massive diarrhea.Note: in cases of massive diarrhea, you cannot drink enough water to compensate for the loss of fluids. Without the use of massive IV's, you will die of dehydration.
It should also be noted that in the period between meals, when the small intestine is for the most part empty, peristaltic contractions continue throughout the entire small intestine. Think of it as housekeeping activity, designed to sweep the small bowel clear of debris. This movement is the cause of "growling" that can be heard when people have not eaten for awhile.Chemical digestion By the time chyme reaches the small bowel, it is a mix of partially digested carbohydrates, lipids, and proteins -- not yet ready for absorption.
Digestion must be completed in the small intestine, because the colon will not absorb nutrients to any significant degree. As I mentioned earlier, the ratio of digestion to absorption changes dramatically as the chyme moves through the small intestine and is exposed to ever more chemical digestion. Specifically, digestion for each type of nutrient proceeds as follows. ProteinsProteins are denatured (unwound) by acid and broken down by pepsin in the stomach.
For the most part, they arrive as polypeptides (short-chain amino acids) in the small intestine. The extent of breakdown into polypeptides is dependent on several factors such as: * The amount of proteases that arrive undamaged with the food to significantly break down proteins before being neutralized by the release of stomach acid (about 45 minutes after food enters the stomach) -- or the use of supplemental digestive enzymes to make up the difference. The ability of the stomach to produce sufficient stomach acid to denature the protein. If the protein is not unwound from its tight ball-like structure into a long chain, pepsin won't be able to work on it. * Sufficient pepsin production to chop up the protein into its smaller component chunks. * Any use of antacids or proton pump inhibitor drugs, of course, totally compromise the ability of the body to break down proteins in the stomach since they suppress the stomach acid required to unwind the protein.
Any breakdown not accomplished in the stomach must now be compensated for in the small intestine -- in addition to the small intestine's role in breaking down short-chain amino acids into even smaller molecules capable of being absorbed into the bloodstream. In either case, after proteins leave the stomach, breakdown continues in the small bowel by activated pancreatic enzymes, including trypsin, chymotrypsin, and elastase (which breaks down elastin fibers). All three are necessary because they each act at different places in the amino acid sequences.In addition, brush border cells of the small bowel excrete more peptidases -- enzymes such as aminopeptidase and dipeptidase -- that complete the splitting of the amino acids into ever smaller components. Ultimately, this creates molecules small enough to transport across the brush border cells and into the bloodstream.
Lipids (fats) Some lingual and gastric lipases (fat digesting enzymes) have already been at work, but the major job of fat digestion takes place in the small bowel.Again, if fats are consumed uncooked or unprocessed or if supplemental digestive enzymes are consumed with the meal, the equation changes. But in lieu of that, at this point in the process, fats are composed mainly of triglycerides (three fatty acids bound to glycerine). It is the action of pancreatic lipase in the small bowel that breaks them down into smaller, potentially absorbable components.
Specifically, pancreatic lipase splits off a monoglyceride, leaving two of the lipids still attached to the glycerine.To a significant degree, the ability of pancreatic lipase to break down lipids is regulated by how soluble those fats have become. It should be noted that lipids in their natural state are not water-soluble (that is, they do not dissolve in water). This is where bile, regulated by the gallbladder, comes into play. Bile salts (from the liver and gallbladder) emulsify (break into small droplets) the fat for easier entry into water solutions -- or more technically, into water suspensions.
If you have gallstones, or have had your gallbladder removed, you will tend to have incomplete breakdown of lipids in your small intestine, resulting in fatty stools and a tendency to intestinal discomfort. In addition, and even more important, malabsorption of lipids prevents the body from receiving any of the nutrients dissolved in the fat. We're talking about vitamins A, E, and D, tocotrienols, and Omega-3 fatty acids to name some of the more familiar ones. Carbohydrates Unless you chewed your food properly (to pick up amylase from your saliva), r took supplemental enzymes with your meal, carbohydrates, for the most part, enter the small intestine intact.
Once there, however, they are cleaved into sugars by pancreatic amylase. Further down the small bowel, maltase, sucrase, lactase, isomaltase and alpha dextrinas, secreted by the brush border cells, act on the remaining carbohydrates, cleaving off the component simple sugars one sugar at a time.For example: * Maltase acts on maltose -- cleaving it into its component parts, glucose and glucose. * Sucrase acts on sucrose -- cleaving it into glucose and fructose. Lactase acts on lactose -- cleaving it into its component parts, glucose and galactose. Note: if lactase levels are insufficient, lactose intolerance develops.
Bacteria ferment the unbroken lactose, and excess gas is produced. Note: pancreatic lipase and amylase in the blood are used to measure abnormal function of damaged pancreatic cells. Absorption Again, everything we've talked about so far is about preparing the chyme for absorption into the bloodstream. Ninety to ninety-five percent of nutrition is absorbed in the small bowel.By the time chyme has reached the small intestine, it has been mechanically broken down and reduced to a liquid by chewing and by mechanical grinding in the stomach.
In addition, partial chemical digestion may already have taken place as the result of enzymes in the food itself and enzymes found in saliva. As discussed previously, the effect of those enzymes can be extensive (up to 70% of total digestion) or virtually non-existent depending on how cooked and processed the food is and how much it is chewed. The use of supplemental digestive enzymes, of course, can change that equation dramatically.And finally, the action of stomach acids and pepsin serve to denature proteins and begin the process of breaking them down, making them readily amenable to final breakdown in the small intestine. Thus, once inside the small intestine, the "partially" digested chyme is exposed to pancreatic enzymes and bile, which ultimately break down the chyme into "component" forms of protein, carbohydrates, and fats capable of being absorbed. By the end of its passage through the small intestine, virtually everything of value to the body has been extracted from the chyme.
