It’s the kind of call you never want to have to make; discerning whether that pressure you’re feeling is the need to pass gas, or something more. Misjudging the moment can ruin your entire day—and pants. Fortunately, the human body is a marvel of biology that has evolved to make these decisions for you.
According to Matt Barton, PhD, a YouTuber and medical researcher at Australia’s Griffiths University, telling the difference between a fart and a poop all comes down to the pectinate line, which separates the upper two thirds and lower third of the anal canal—and it starts before we’re even born.
“In the embryo, there is a long tube known as the gut tube,” he explains in a new video. “This will eventually become the digestive tract. This tube can be broken into the foregut, the midgut, and the hindgut.”
At around the seventh week, the hindgut and urinary system separate, leading to the formation of two separate exit points, one of which is the anal pit, or proctodeum. The proctodeum accounts for the lower third of the anal canal, ending at the pectinate line.
“When wind or faeces enters the anal canal, it will stretch the walls of the intestine, activating pressure receptors and taken back by visceral nerves, telling you that something is entering the canal,” says Barton. “But it’s not until it reaches the pectinate line that the stretch receptors become cutaneous receptors, like the skin, and from here onwards you can distinguish between wind and faeces. Therefore, you can decide whether you’re safe or you need to use the bathroom.”
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