Why do our stomachs grumble? Because the muscles in the stomach and small intestines are contracting and there is no food in there to muffle the sound.
Our stomachs rumble a lot of the time, but we usually cannot hear it because the food in our stomachs and small intestines soak up the sound. The rumbling is the natural sound of our digestion and doesn’t necessarily mean that we are hungry.
Stomach rumbling, or grumbling, is known as borborygmus. The word comes from Greek and was, to them, the sound that the stomach makes when it is rumbling.
The rumbling is caused by the muscles as they push food and liquid along the gastrointestinal tract. The gastrointestinal tract is divided into the lower tract and the upper tract, but it is basically one tube that goes all the way from the mouth to the anus. It starts with the esophagus, goes through the stomach, continues with the small intestine, then the large intestine, before passing through the colon.
Food and liquids obviously start by going in through the mouth. We chew them up into smaller pieces and swallow them. Our tongues push the food to the back of our throat and we swallow it, covering our windpipes with a flap of skin so that we don’t choke. Saliva helps the food move down the throat and starts to break the food down.
Once the food is in the esophagus, the muscles start to work. All of the intestinal tract is made of layers of smooth muscle. This intestinal tract is about 9 meters long when removed from the body, but it is constantly kept under tension so it isn’t that long in the body. The brain sends signals to the muscles and they contract and relax in sequence to push food and liquid along the tract. This movement is called peristalsis. The contractions happen between three and twelve times a minute. And all of this goes on without us feeling a thing.
The food goes through the sphincter, which is a seal at the top of the stomach and is to stop food coming back up into the esophagus. In the stomach, stomach acid starts to break down the food. The stomach also has muscles lining it and these muscles move to mix the food with the acid to dissolve the food and make it easier for enzymes to break it down. These digestive juices can break down carbohydrates, proteins, and fats.
After the stomach has done what it can, the food and digestive juice is passed onto the small intestine. Peristalsis continues to push the mush along and the walls of the small intestine absorb water and nutrients out of the food. Whatever is left passes into the large intestine and more water is absorbed, leaving stool, which we pass at our convenience. (Or in our convenience.)
Peristalsis is the source of the rumbling (grumbling?) in our stomach. Peristalsis is a completely unconscious action. When there is food in the gastrointestinal tract, it pushes on the walls, stretching it. That causes serotonin to be released, which activates the myenteric plexus, which is a group of nerves in the muscles of the tract. The myenteric plexus releases two different groups of chemicals. It releases a neuropeptide and acetylcholine, which make the muscle behind the food contract, and it releases nitric oxide and vasoactive intestinal polypeptide, which make the muscles in front of the food relax. This pushes the food forwards and it continues all the way along the tract.
The muscles in the gastrointestinal tract push the food and liquid along, but they also push air and gas along as well. This makes a vibrating sound in the tract, which is the sound of the stomach rumbling. When there is food being moved along, it muffles and absorbs the sounds, but when the tract is empty, the sound echoes and builds, becoming much louder and more obvious. This is why we associate the sound with hunger.
Peristalsis also happens several hours after we have eaten and digested our food. When the stomach is empty, the walls of the stomach sense the absence of food, which starts the peristalsis going again. This time, the contractions are used to clear out any food, gas or liquid that might be left in the intestinal tract. They remove the stomach, remove mucus, any leftover food, and bacteria, preventing it from building up in any one place. This also causes rumbles, but these ones we hear because our stomachs and intestinal tracts are empty. And this is why our stomachs grumble and this is what I learned today.
Sources
https://www.scientificamerican.com/article/why-does-your-stomach-gro/
https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works
https://en.wikipedia.org/wiki/Stomach_rumble
https://en.wikipedia.org/wiki/Peristalsis
https://wonderopolis.org/wonder/what-makes-your-stomach-growl