Why is Digestion Important - Part III
The Small Intestine
The most exciting place to be in the entire digestive system - this is where the final stages of chemical digestion occur and where almost all nutrients are absorbed.
The small intestine is the portal for absorption of virtually all nutrients into blood. By the time the food and drink ingested, called ingesta, reaches the small intestine, it has been mechanically broken down and reduced to a liquid by mastication and grinding in the stomach. Once within the small intestine, the ingesta is exposed to pancreatic enzymes and bile which enables digestion to molecules capable or almost capable of being absorbed. The final stages of digestion occur on the surface of the small intestinal epithelium.
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The net effect of passage through the small intestine is absorption of most of the water and electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules
The small intestine is the longest section of the digestive tube, roughly 3.5 times your body length, and consists of three segments forming a passage from the pylorus to the large intestine:
* Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts.
* Jejunum: considered to be roughly 40% of the small gut in humans.
* Ileum: empties into the large intestine; considered to be about 60% of the intestine.
It is within the small intestine that the final stages of enzymatic digestion occur, liberating small molecules capable of being absorbed. The small intestine is also the sole site in the digestive tube for absorption of amino acids and monosaccharides. Most lipids are also absorbed in this organ. All of this absorption and much of the enzymatic digestion takes place on the surface of small intestinal epithelial cells, and to accommodate these processes, a huge mucosal surface area is required.
If the small intestine is viewed as a simple pipe, its surface area would be on the order of one half of a square meter. But in reality, the absorptive surface area of the small intestine is roughly 250 square meters - the size of a tennis court! How is this possible? At first glance, the structure of the small intestine is similar to other regions of the digestive tract but the small intestine incorporates three features, which account for its huge absorptive surface area:
* Mucosal folds: the inner surface of the small intestine is not flat, but thrown into circular folds, which not only increase surface area, but aid in mixing the digesting matter by acting as baffles.
* Villi: the mucosa forms multitudes of projections which protrude into the path and are covered with epithelial cells.
* Microvilli: the membrane of absorptive epithelial cells is studded with densely-packed microvilli.
Movement through the small intestine
The small intestine cycles through two states, each of which is associated with distinctive patterns of movement:
* Following a meal, when the small intestine contains chyme, two types of movement predominate: segmentation contractions chop, mix and roll the chyme and peristalsis slowly propels it toward the large intestine.
* The interdigestive state is seen between meals, when the small intestine is largely devoid of contents. During such times, socalled housekeeping contractions propagate from the stomach through the entire small intestine, sweeping it clear of debris. This complex pattern of movement is the cause of “growling”.
Food spends up to 5 hours in a healthy small intestine.
The large intestine is the last attraction in digestive tube and the location of the last phases of digestion:
* Recovery of water and electrolytes from ingesta: By the time ingesta reaches the large intestine, roughly 90% of its water has been absorbed, but considerable water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large gut.
* Formation and storage of feces: As ingesta is moved through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces.
* Microbial fermentation: The large intestine of all species teems with microbial life. Those microbes produce enzymes capable of digesting many of molecules that to vertebrates are indigestible, cellulose being a premier example.
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Within the large intestine, three major segments are recognized:
* The Cecum is a blind-ended pouch that in humans carries a worm-like extension called the vermiform appendix.
* The Colon constitutes the majority of the length of the large intestine and is subclassified into ascending, transverse and descending segments.
* The Rectum is the short, terminal segment of the digestive tube, continuous with the anal canal.
The large intestine does not produce its own digestive enzymes, but contains huge numbers of bacteria which have the enzymes to digest and utilize many substrates. Two processes are attributed to the microbial flora of the large intestine:
*Digestion of carbohydrates not digested in the small intestine. Cellulose is common constituent in the diet of man, but none of our cells is known to produce a cellulase enzyme to break it down. Several species of bacteria in the large bowel synthesize cellulases and digest cellulose. Importantly, the major end products of microbial digestion of cellulose and other carbohydrates are volatile fatty acids, lactic acid, methane, hydrogen and carbon dioxide. Fermentation is thus the major source of intestinal gas. Volatile fatty acids generated from fermentation can be absorbed by diffusion in the colon.
* Synthesis of vitamin K and certain B vitamins. Synthesis of vitamin K by colonic bacteria provides a valuable supplement to dietary sources and makes clinical vitamin K deficiency rare.
Three patterns of movement are observed in the colon:
*Segmentation contractions which chop and mix the ingesta, presenting it to the mucosa where absorption occurs.
*Antiperistaltic contractions propagate back toward the ileum, which serves to retard the movement of ingesta through the colon, allowing additional opportunity for absorption of water and electrolytes. Peristaltic contractions, in addition to influx from the small intestine, facilitate movement of ingesta through the colon.
*Mass movements constitute a type of movement not seen elsewhere in the digestive tube. Known also as giant migrating contractions, this pattern of movement is like a very intense and prolonged peristaltic contraction which strips an area of large intestine clear of contents. In periods between meals, the colon is generally calm and quiet. Following a meal, colonic movement increases significantly, due to signals passed through the nervous system. The signal seems to be stimulated almost exclusively by the presence of fat in the small intestine. Additionally, distension of the colon is a primary stimulator of contractions.
Several times each day, mass movements push feces into the rectum, which is usually empty. Distension of the rectum stimulates the defecation reflex. This is largely a spinal reflex and results in reflex relaxation of the internal anal sphincter followed by voluntary relaxation of the external anal sphincter and defecation. Defecation can be prevented by voluntary constriction of the external sphincter. When this happens, the rectum soon relaxes and the internal sphincter again contracts, a state which persists until another bolus of feces is forced into the rectum.
Normal feces are roughly 75% water and 25% solids. The bulk of fecal solids are bacteria and undigested organic matter and fiber. The characteristic brown colors of feces are due to the bacterial degradation of bilirubin, the orange/yellowish pigment in bile. Fecal odor results from gases produced by bacterial metabolism.
Food spends 12 to 36 hours in a healthy large intestine.
So, there you have it. As you can see the digestion process is a complex one and we have only scratched the surface in detail. I hope you have enjoyed this series and that it has helped you to understand the process of digestion and assimilation. Hopefully, it has triggered a few "ah ha's"
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