Mark A. W. Andrews, associate professor of physiology and associate director of the Independent Study program at the Lake Erie College of Osteopathic Medicine, provides the following explanation.
Though stomach growling is commonly heard and associated with hunger and an absence of food in the stomach, it can occur at any time, on an empty or full stomach. Furthermore, growling doesn't only come from the stomach but, just as often, can be heard coming from the small intestines. Growling is more commonly associated with hunger because it is typically louder when the stomach and intestines are empty and so the organs' contents don't muffle the noise.
This growling has been of interest for so many years that the ancient Greeks came up with the rather interesting name for it: borborygmi (the plural of borborygmus). The etymology of the term relies on onomatopoeia; it is an attempt to put the rumbling sound into words. Borborygmi actually translates as "rumbling."
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The physiological origin of this rumbling involves muscular activity in the stomach and small intestines. In general, the gastrointestinal tract is a hollow tube that runs from mouth to anus and its walls are primarily composed of layers of smooth muscle. When the walls are activated and squeeze the tract's contents to mix and propel food, gas and fluids through the stomach and small intestines, it generates a rumbling noise. This squeezing of the muscular walls is termed peristalsis and involves a ring of contraction moving aborally (away from the oral cavity) towards the anus a few inches at a time.
The generation of these waves of peristalsis results from a rhythmic fluctuation of electrical potential in the smooth muscle cells, which, all other conditions being appropriate, will cause the muscle to contract. This fluctuation is called the basic electrical rhythm (BER) and is a result of inherent activity of the enteric nervous system, which is found in the walls of the gut. The BER causes the muscle cells of the stomach and small intestines to activate at a regular rhythm (three and 12 times per minute, respectively), in a manner similar to, but slower than, the rhythmicity of cardiac muscle in the heart. The autonomic nervous system and hormonal factors can modulate this BER.
Though the rate and force of peristalsis typically increases in the presence of food, activity also increases after the stomach and small intestines have been empty for approximately two hours. In the latter case, receptors in the walls of the stomach sense the absence of food, causing a reflex generation of waves of electrical activity (migrating myoelectric complexes, or MMCs) in the enteric nervous system. These MMCs travel along the stomach and small intestines and lead to hunger contractions. Such hunger contractions start in the antrum, or lower region, of the stomach and propagate along the entire length of the gut, sweeping to the terminal ileum. They clear out any and all stomach contents¿including mucus, remaining foodstuffs and bacteria¿and keep them from accumulating at any one site.
The contractions also produce vibrations and the rumbling noise associated with hunger. Hunger contractions may continue for 10 to 20 minutes once initiated, and then repeat every one to two hours until the next meal is ingested. These are not the same as hunger pangs, which start 12 to 24 hours after the last meal and may continue for a few days before gradually subsiding. (It is possible such pangs are important in the hunger sensation that drives animals to eat.) Low blood sugar enhances this activity, which can also be induced using an intravenous infusion of the hormone motilin. After feeding, the MMCs subside.