Like most of you, I ate too much on Thanksgiving, and felt guilty about it afterward. But then I came up with the idea of writing this column on digestion, and presto! No more guilt. You see, I didn’t overeat, I conducted research.
You’ll have to find your own excuse.
When you sit down to eat, even before the first bite of food reaches your mouth, your digestive system is geared up for it: the sight and smell of food starts all the digestive juices flowing. (Why do you think we call a good spread of food “mouth-watering”?)
Once you’ve taken a bite of food, you start chewing, which grinds the food into small particles and mixes it liberally with saliva from the six glands inside the mouth. Saliva breaks down any starches into sugars, partially dissolves the food, stimulates the secretion of other digestive enzymes, and lubricates the food so it will slide easily down the next segment of the digestive system, the esophagus.
This 23- to 25-centimetre-long tube delivers food to the stomach. Except maybe for oysters, few foods are able to just slide down the esophagus, so the esophagus helps them along with “peristalsis,” rhythmic, coordinated contractions of the esophagus walls. At the bottom end of the esophagus, a sphincter opens to let the soft mass of chewed food (called a “bolus”) into the stomach.
Sometimes this sphincter doesn’t close properly, which allows stomach juices back up into the esophagus. This causes heartburn, because stomach juices are powerful chemicals: among other things, the stomach produces large amounts of hydrochloric acid. Fortunately, mucus protects the stomach walls. (The stomach has another fortunate ability: it can expand hugely from its normal size of about 20 centimetres in length. Otherwise, Thanksgiving and Christmas wouldn’t be nearly as much fun.)
Hydrochloric acid breaks down protein. Other stomach juices include enzymes such as pepsin (which also breaks down proteins), rennin (which separates milk into liquid and solid portions) and lipase (which acts on fat).
From the stomach, the partially digested food passes into the small intestine, a coiled organ an amazing five metres in length, with an interior surface area of a whopping 140 square metres–about the same as a small house. It’s in the small intestine that food is finally converted to a form useable by your body, and is absorbed into the bloodstream. Three more powerful digestive fluids come into play here: pancreatic fluid, intestinal juice and bile.
Pancreatic fluid, secreted by the pancreas, splits complex proteins into simpler components which your body can then use to build its own proteins; it also breaks down fat and starches. The secretion of pancreatic fluid is stimulated when you eat proteins and fats.
Intestinal juice, secreted by the intestine, completes the processes begun by pancreatic juice; its flow is stimulated by the pressure of food against the intestinal walls.
Finally, bile, secreted by the liver, aids in the absorption of fats by making them blood-soluble. Bile flows in response to fat in the stomach and upper intestine.
Some substances, including sodium, glucose and many amino acids, pass directly through the intestinal wall into the bloodstream. Water-soluble substances, including minerals and carbohydrates, flow directly to the liver.
The intestine keeps substances which the body can’t use out of the system; conversely, when your body really needs a particular nutrient, it will be more efficiently absorbed than at other times.
Anything which the intestine rejected or could not digest passes into the final section of the digestive system, the large intestine, where water is reabsorbed. The remaining waste product is (how shall I put this delicately?) expelled from the body.
After your big Thls and nutrients in your systems after eating interferes with normal brain function, but I don’t buy that
I figure it’s just the body’s natural defense mechanism against having to do dishes.
