|From Pheasant to Faeces|
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My senses register the coming meal before Jim tells me that it's ready - the smell from the kitchen sets off a parasympathetic response. Pavlovian conditioning stimulates acini of salivary glands - parotid, submaxillary and sublingual. A soup of water, electrolytes, mucus and enzymes dribbles along the collecting ducts inside my mouth. The saliva mixture flows from smaller to larger ducts, re-absorbing sodium and secreting bicarbonate, until it reaches a single large duct and empties into my oral cavity.
My stomach too is preparing for action, entering a cephalic phase. Acetylcholine is released, binding to receptors on the G cells, stimulating gastrin secretion and in turn eliciting a few small acid secretions from my parietal cells. The gastric motor is turned on, but idling.
I cut up small pieces of food and pass them through my mouth, out of sight. My teeth cut and grind fragments. With the help of my tongue, lips and cheeks, they masticate them into a bolus. Papillae, small pegs of epithelium on my tongue, hold the taste buds, which are excited by different chemicals present in the mix of pheasant, vegetables and sauces. Receptors report back the oral concentration of salty, sour, sweet, bitter and umami constituents.
My tongue presses the slippery bolus back into the pharynx, towards my oesophagus. The lumen of my larynx squeezes shut, my epiglottis swings back, and the entrance to my oesophagus is instantly enlarged. One last peristaltic contraction shunts the food-ball through the upper oesophageal sphincter towards my stomach. Further jets of saliva flush and lubricate my mouth. My dinner has now entered the digestive tube - the long one-way tunnel from mouth to anus.
Stratified squamous epithelium along the lining of my oesophagus wall prevent lesion by stray bits of bone or crisp potato. Strong peristaltic contractions send the bolus of food down to the lower oesophageal sphincter and into my stomach within four to five seconds. Lubricating secretions from my mucus glands speed it down. A particularly large piece of food is left in the oesophagus, so secondary waves of peristalsis emanate from the point of distension, clearing the blockage. My oesophagus is ready to take another chewed-up glob of food.
The bolus of meat and vegetables, crushed and impregnated with saliva,
enters my stomach. The landscape has changed ? the epithelium transformed
abruptly from the stratified lining of the oesophagus to the columnar,
secretory epithelial cells of the stomach wall, designed not for coaxing
food downwards but for vigorous crushing and grinding. The mood has
changed too, the stomach's moved from the cephalic phase to the
gastric phase, secretions intensify. Hydrochloric acid, pepsin, gastrin
and mucous are fired from cells all around. Motor function increases
in the surrounding muscles.
At the bottom of my stomach, the pyloric gatekeeper bars the way to solids of more than a couple of millimetres diameter. These reflux back into my stomach to undergo further liquefaction. That penny I ate along with the pheasant, as a gotcha test-case for my internals, will come out eventually. But it has to wait for the rest of the meal to be digested before the migrating motor complex makes a sweep of my gastrointestinal tract.
None of my meal has been absorbed by the stomach lining, although the alcohol component of the wine with which I washed it down has filtered through receptor cells and into my bloodstream. Meantime, the substances secreted within my stomach have begun the job of breaking down macromolecular nutrients into their constituent parts.
My small intestine is beginning its work. It has a payload of chyme fresh from the stomach, it needs time to neutralize the acids and absorb the nutrients. The enterogastric reflex sends inhibitory nervous and endocrine signals back to my stomach, telling it to temporarily go into a slower, intestinal phase.
In my pancreas, close to the stomach's exit, the exocrine ascinar cells secrete some of the digestive enzymes they've been synthesizing. Together, the proteases, pancreatic lipase, amylase, ribonuclease, deoxyribonuclease, gelatinase, elastase and other digestive juices reduce virtually all digestible macromolecules into forms that are capable, or nearly capable, of being absorbed. Bile leaves my liver, travelling along the common bile duct before bursting through the Sphincter of Oddi. Never forget that bile acids are facial amphipathic, two-faced hydrophobic hydrophilic little bastards. Meanwhile, the epithelial cells lining my pancreatic ducts secrete bicarbonate and water to neutralize the acidic chyme. These secretions flow into my duodenum, the initial short section of the small intestine.
Virtually all the nutrients that I have ingested are absorbed into my blood by epithelial cells during their trip along the six metre length of the small intestine. The mucosal surface area of my small intestine is huge due to the mucosal folds, villi and microvilli that form its epithelium. The crypts of Lieberkuhn invaginate between the villa and secrete additional fluids into the mix. The cells of the villi die young, and are shed into the lumen to be cannibalistically absorbed along with my pheasant and vegetable meal.
Past the jejunum and into the ileum, at the end of my small intestine, where bile is leeched from the food mix travelling down the tube and is piped back into my liver. Here it is cleaned and 95% of it, having been recycled, will re-enter my intestine to help break down the next part of my meal. By the time it's reached my ileum, my former dinner is stained stercobilin-brown by the metabolism of bilirubin. The vast quantities of water that travelled with it, either entering as part of my meal or joining on the way due to my body's secretions, are almost all re-absorbed leaving a sludgy, tightly-packed mix in my tubes.
The job of the large intestine is to squeeze even more water out of this brown sludgy ingesta, mix it with bacteria and mucus, and craft it lovingly into faeces. Microbes living in this stretch of my body happily ferment and digest some of the few components of my meal that my body couldn't cope with, such as cellulose. As it ferments, bad-smelling gases such as skatole, mercaptans, and hydrogen sulphide are given off. The raft of waste material moves slowly from cecum to colon and finally to rectum.
I have gorged myself once again. My stomach and intestine full, the
vagus nerve relays my satiety back to the hypothalamus. Somewhat reluctantly,
I stop eating this richly filling gloop and squeeze in a little refreshing
fruit and frozen cream to soothe my overworked gastric nerves. Meanwhile,
my distended rectum is bringing on a defecation reflex. Soon it will
be time to leave the room, position myself squarely over the toilet,
and wait for a giant migrating contraction to push what is left of
the meal through my internal and external anal sphincters and out into
the void beyond.
© Dan Sumption, March 2003
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