Worms and Washing Machines:
How the Intestine Works

by Dr. W. Daniel Jackson, M.D., Gastroenterologist

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:

Top-Loaders

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.

 

Normal nutrition and growth depends on:

Adequate intake of nutrients.

Digestion of nutrients: enzymatic hydrolysis(breakdown) of polymers(molecular chains)

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) regulation

No "dumping": too fast transit causes diarrhea

No stasis: too slow transit allows bacterial colonization and fermentation

Membrane Transport:  specialized transport processes

Monosaccharides: (sugars: glucose, galactose, fructose)

Amino acids, di- and tri-peptides (protein molecules)

Fatty acids:  Pinocytosis(absorption by engulfment)

Specialized segmental absorptive functions of the GI tract:

Mouth: glucose

Esophagus: none

Stomach: minimal

Duodenum: iron, Ca, Mg, Vits A, D, E, K

Jejunum: water soluble vitamins(C and B vitamins); Carbs, Fats, Proteins [net secretion of water]

Ileum: Jejunal functions plus: Vitamin B12, bile acids(95%); [net absorption of water]

Colon: short chain fatty acids from fermentation, water

Loss of Jejunum: consequences:

Loss of absorptive surface area

Reduced absorption of Vitamins and trace minerals, especially iron and zinc

More nutrients presented to the ileum for digestion and absorption

Note: the Ileum may adapt to compensate for most digestive and absorptive functions

Loss of Ileum: consequences

Reduced absorptive surface area

Loss of absorptive efficiency

Loss of specialized absorptive functions

Bile acid malabsorption

Choleraic (bile salt induced) diarrhea

Diminished bile acid pool

Vitamin B12 malabsorption

Altered Motility- rapid transit

Loss of ileal brake: malabsorbed fat slows motility

Loss of ileocecal valve:

bacterial contamination of distal small bowel from colon

more rapid transit to colon?

Normal Bowel Segment Length based on Gestational Age:

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

 

Read about this topic on the web

http://www.seattlechildrens.org/clinics-programs/transplant/intestine/how-the-small-intestine-works/ - How the Small Intestine Works, Seattle Children's Hospital

http://www.cmaj.ca/content/vol166/issue10/images/large/21ff1.jpeg Diagram of absorption in the intestine, Canadian Medical Association Journal

http://kidshealth.org/parent/general/body_basics/digestive.html Body Basics: Digestive System, KidsHealth