| MadSci Network: Anatomy |
Brian:
A most difficult question to approach, and one that reminds me of the classic: " How many angels can dance on the head of a pin?"
MUCUS is a moist, sticky, clear solution formed from the hydration (the addition of water) of MUCIN. The latter, a glycoprotein (protein polysaccharide complex) secreted either by unicelluar glands called goblet cells or by MUCOUS glands. Mucous glands can be pure glands producing only mucus, or they may be mixed glands, which secrete a mixture of mucus and certain enzymes. These glands are the defining feature of the lining of most hollow structures in the body, called MUCOUS MEMBRANES or the MUCOSAL LINING: they are particularly characteristic of the gastrointestinal and respiratory tracts, where they provide moistening and protective functions.
The only information I can readily find regarding mucus volume is the basal (or moistening) rate at which SALIVA is secreted. Saliva is a mixture of mucus, water, salts, and the enzymes including amylase which break down carbohydrates. In an awake individual, saliva is secreted at a rate of ~0.5 milliliters per minute. Multiplying this figure by 960 minutes in an average 16 waking hours/day gives an estimate of 480 milliliters of saliva/16 hours. But one must remember that excercise, eating, drinking, and speaking all will tend to increase the salivary volume. Acidic solutions are the most powerful salivary stimulant and can result in 4.0 milliliters of saliva/minute!!
By way of spelling: mucus and mucin are nouns, while mucous is the adjectival form of mucus. Best of luck in your learning endeavors.
Sincerely,
Scott Dietert
Admin Note: Within the human gastrointestinal tract, the gut secretes an average of *7 liters* of fluids for every 2 liters of ingested foodstuffs. The 7 liters correspond to mucus, hydrochloric acid secreted in the stomach, bile secreted from the liver, and enzymes secreted by the pancreas, liver and cells types in the stomach and small intestine. Much of the 7 liters consists of mucus (though I don't have an exact figure). Under normal conditions, 8 liters of the 9 are reabsorbed by the gut, leaving ~ 1 liter of wastes.
Added 2/6/2005
There was no answer available at that time, but more recent work
based on rodent models allows a rough guess to be made at the present time,
so that the question can be at least partially answered.
Here is the answer I came up with and the literature citations:
The key role played by mucus in our interactions with our physical environment is reflected by the prevalence of mucus in the respiratory, urogenital and gastrointestinal tracts. Mucus is by far the most abundant substance secreted by mammals, with saliva in a distant second place. Studies in rodents have shown that, in the gastrointestinal tract alone, the mucus layer ranges from 120 to 830 microns thick depending on the location (Atuma et al., 2001), and the mucus is completely regenerated about every 20 hours (Faure et al., 2002). Given the large surface area of the gastrointestinal tract, a substantial amount of mucus must be continually secreted to maintain this mucus layer: To maintain a comparable mucus layer and turnover rate in the human gastrointestinal tract would require the daily production of more than 4 liters of mucus per day, with more than 300 ml per day secreted in the large bowel alone.
References:
Atuma, C., Strugala, V., Allen, A., Holm, L., 2001.
The adherent
gastrointestinal mucus gel layer: thickness and physical state in vivo.
American Journal of Physiology Gastrointestinal & Liver Physiology. 280.
Faure, M., Moennoz, D., Montigon, F., Fay, L.B., Breuille, D., Finot, P.A.,
Ballevre, O., Boza, J., 2002.
Development of a rapid and convenient method
to purify mucins and determine their in vivo synthesis rate in rats. Anal.
Biochem. 307, 244-251.
Best wishes,
William Parker, PhD Assistant Professor Department of Surgery Duke University Medical Center