The function of the gastrointestinal tract is to get the nutrient molecules in our food into our body to be used in metabolism. In some ways we are fancy worms with a tube running through our middle. This tube has specialized segments, each with its own function that works in sequence to digest our food down to the molecules that our body can use for fuel and building material. One way to look at the functional anatomy of our GI tract and understand the consequences of losing certain parts as in short bowel syndrome is to compare it to a washing machine. In many ways we are:
Fill Wash Tub: Food or formula entering the mouth is mixed with salivary amylase enzyme to begin starch(carbohydrate) digestion, then swallowed into the stomach, where it meets acid and pepsin and gastric lipase enzymes to begin breaking down protein and fat.
Agitate/Mix: The contractions of the stomach and the intestines help mix the food with the digestive enzymes and liquid to physically break down the food particles.
Add Detergent: The food entering the first part of the intestine, the duodenum, passes by the little tubes or ducts that squirt in bile from the liver which acts like a detergent (green and slippery, just like Palmolive liquid) to emulsify fat, and enzymes from the pancreas which chemically break down the large food molecular chains into smaller molecules that can be absorbed. There are special enzymes for different protein chains of amino acids, carbohydrate chains of glucose sugar, and fatty acid chains.
Wash: The jejunum [upper 2/5 (40%) of the small intestine after the short duodenum loop] is the wash cycle where the liquefied food is agitated with the enzymes to allow complete breakdown or digestion into small molecules that can be absorbed through the lining to go to the liver to be used by the body for energy and to growth. Only part of the fluid absorption occurs here and the "clothes" or digested foods are still quite wet as they leave the jejunum.
Spin: The ileum [lower 3/5 or 60% of the small intestine emptying into the large intestine] is the spin cycle where the majority of nutrients and liquid in the digested broken-down liquefied food or chyme is absorbed before being allowed to go into the large intestine through the ileo-cecal valve lips that separate the ileum from the cecum or beginning of the large intestine. The ileum can compensate and do all the digestive functions of the jejunum if necessary. The last part of the ileum or terminal ileum has unique functions of absorbing Vitamin B12, important for nerve functions, and for immune function, including tolerance to foods. The ileum also reabsorbs the bile acid detergent for recycling back to the liver.
Dryer: The cecum or beginning of the intestine has lots of bacteria to break down or ferment food that was not fully digested or absorbed by the small intestine (especially the short bowel) including indigestible fiber. This fermentation process produces fatty acids that can be salvaged and absorbed for additional nutrition as well as gas and acidic stools. Mostly water absorption occurs in the slow moving large intestine.
Intra-lumenal: salivary amylase,gastric lipase, pancreatic enzymes
Mucosal brush-border enzymes: disaccharidases, peptidases
Absorption of nutrients across epithelial membranes(intestinal lining):
Sufficient mucosal surface area:intestinal length and radius dependent villi and microvilli (absorptive fringe) increase surface area dramatically.
Transit time: motility (movement) regulationNo "dumping": too fast transit causes diarrhea No stasis: too slow transit allows bacterial colonization and fermentation
Membrane Transport: specialized transport processesMonosaccharides: (sugars: glucose, galactose, fructose) Amino acids, di- and tri-peptides (protein molecules) Fatty acids: Pinocytosis(absorption by engulfment)
Loss of specialized absorptive functionsBile acid malabsorption Choleraic (bile salt induced) diarrhea Diminished bile acid pool Vitamin B12 malabsorption
Altered Motility- rapid transitLoss of ileal brake: malabsorbed fat slows motility Loss of ileocecal valve: bacterial contamination of distal small bowel from colon more rapid transit to colon?
|Gestational Age||Total Bowel||Small Bowel||Jejunum||Ileum|
|19-27 weeks||142 cm||115 cm||45 cm||70 cm|
|27-35 weeks||218 cm||170 cm||70 cm||100 cm|
|> 35 weeks||304 cm||250 cm||100 cm||150 cm|
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