Why does the stomach growl when we are hungry?

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Suppose you haven't had any breakfast and are taking part in an important meeting. Suddenly, with no warning whatsoever, your stomach emits that embarrassing sound, reminding you that it is empty. It is a situation that must have happened to most of the people. The stomach, like many parts of our body, doesn't behave as we wish. This growling, which is most often linked with hunger, is one of those moments when our body makes a decision of its own.

The stomach growling has been a subject of interest for the scientific world for a long time. The ancient Greeks did even come up with an interesting name for this phenomenon. They called it borborygmi which literally translates to rumbling. The word is actually an onomatopoeia.

Even though we notice the rumbling of stomach usually when we are hungry, it can happen any time, no matter the stomach is empty or full. When the stomach is empty, this sound is rather louder because there is nothing to muffle it. And stomach is not the only source of this sound either. It can come from the small intestine as well.

The origin of the rumbling is the muscular activity in our digestive system. The digestive system can be considered a long hollow tube that extends from the mouth to the anus. The wall of this tube is made up of layers of fine muscles. The digestion of food takes place with the contracting of these muscles. It is called peristalsis. These muscle contractions churn the food, liquids and the enzymes together, to form a mix called chyme.

The peristalsis triggers a rhythmic fluctuation of electric potential in the muscle cells, called 'Basic Electrical Rhythm' or BER. This rate of BER is about three times per minute in the stomach, and 12 times per minute in the small intestines.

In the absence of food in stomach, a different pattern of electric waves will form, called Migrating Myoelectric Complex (MMC). This results in muscle contractions in stomach and pass down to the large intestine. The purpose of the MMC is to clear out all the residual food, mucus, bacteria and other debris down the stomach to the colon in preparation for cleaning before the next meal. This sound is what we hear as the grumble of stomach. Just like how a half filled bottle makes more sound than a full bottle when sloshed, our stomach also makes more sound when it is only slightly filled, or rather empty.

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