I know the answer to this question, but I want to hear how you explain it. We once stumped our biology teacher by asking him why our stomachs growl when we are hungry. You should have seen him squirm. He finally admitted he didn't know. I eventually looked it up myself and thought the reason was very interesting. Since you're the doc, how about telling the story?
Don't be too hard on the guy. I've got a distinct advantage since I can pick and choose the questions I answer. If they're too hard, I can just blame an unreliable server for eating the e-mail. Or the query can simply be edited into a fun and easy one. I mean, "Why does your stomach growl?" is essentially the same thing as "Explain the biochemistry of Vitamin A," isn't it?
Your question (as far as anyone knows) about gurgling abdominal sounds can be answered by way of example. Think of how a whoopee cushion works: air is forced under pressure through a small, floppy space. A bowel sound, called borborygmus (bawr-buh-RIG-mus), is made in the same manner: gas and liquid are squeezed and mashed through the constantly changing spaces in your stomach and intestines. Since your digestive tract is lengthy (usually about five times as long as you are tall) and packed into a tight space, it contains many folds, twists and turns in which borborygmi may be produced. Typically, the sounds begin in a low pitch, then get much louder and higher pitched over about 30 seconds--kind of like a gastrointestinal Mariah Carey.
The ingenious method by which the intestines move our food to its ultimate destination has everything to do with the bands of smooth muscle lining every inch of the gut. Electrical signals firing in an organized sequence, like lights on a marquee, contract these muscles in waves. The result is either a churning in place or a push toward the exit. Called peristalsis, this function mixes in digestive juices and keeps food from accumulating in a single spot.
As for the juices, the secretion of these fluids is choreographed better than the fountains at the Bellagio. It actually takes a couple of gallons of the stuff to completely digest our daily intake of food's three macronutrients: carbohydrates, proteins and fats. Beginning in the mouth, saliva starts the process by breaking apart carbohydrates into simple-to-absorb sugars. Food then meets an unforgiving acid bath when it hits the stomach, where the harsh environment disassembles protein into individual amino acids. Next, this semi-liquid "chyme" moves into the small intestine where a few squirts of neutralizer take care of the acid (so you don't get an ulcer). And finally, more secretions from the pancreas and liver dismantle the remaining component, fat.
All this fluid, plus any swallowed air, churns around in your small intestine, where the bulk of these now individual nutrients will be absorbed. Our normally well-behaved tenants, bacteria, pay their rent by further breaking down foodstuffs into even better-assimilated substances. However, in the presence of incompletely digested carbohydrates like lactose, gluten or beans, the bacteria can produce gas. In excess, this additional air can raise the volume of your gurgling, making it audible to snarky co-workers even above the music in the elevator. Fortunately, the resultant case of flatus will have them exiting as soon as the doors open, whether it's their floor or not.
The growling does not always indicate hunger; these peristaltic contractions are present throughout the day whether there is food in the bowels or not. But they tend to be louder between meals for the same reason a half-filled water bottle in your backpack is louder than a full one--an empty stomach has more room for sloshing. Borborygmus often does increase in amplitude as a reflexive response to hunger or thoughts/smells of food. Increased sounds that are accompanied by severe bloating and cramping, however, can indicate an early intestinal obstruction, though usually benign. But abdominal pain with an absence of bowel sounds requires a fairly urgent evaluation by a medical doctor--infection of the pancreas or bowel inflammation need to be ruled out.
So, to contradict your biology teacher, borborygmi is not a name for the very short natives of the Pacific island of Bora Bora. Borborygmi are the normal sounds made by intestinal muscle contractions in the presence of food. OK, I admit it: This question was a bit too easy. Perhaps I'll tackle Vitamin A biochemistry next--just as soon as I finish lunch. Seems right now, the borborygmis are getting restless.