When you take a shot to your scrotum, your testicles aren’t the only ones that feel the pain: Your stomach often joins in on it, too.

But why would an organ situated above the belt reel from a blow felt below it? (And why did the jerk who delivered the nut punch not realize you will strike back even harder when he least expects it?) 

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It all comes down to biology. 

Your testicles actually developed in your abdomen near your kidneys. From there, they descended toward the scrotum, pulling the sensory nerves with them.

That means you have sensitive nerves that stretch from your stomach to your scrotum. 

“So when the nuts are hit or squeezed, the nerve sends a signal to the upper part of the belly, which is why it hurts in the stomach when you get kicked down there,” said Dr. Darius Paduch, Ph.D., the director of sexual health and medicine at Weill Cornell Medicine.

Related: Why Your Balls Might Feel Like a Bag Of Worms

In addition to feeling vengeful, you might also feel dizzy, nauseous, or even vomit—all of which is to be expected. 

“It’s due to the vagal reflex, in which a nerve signal from your testicles travels up your spinal cord and brain stem and activates the nausea and vomiting centers in your brain,” Paduch said. 

There’s actually a pretty strange evolutionary benefit to this: It works to keep you safe if your blow to the balls comes during a physical altercation.

If the urge to vomit hits, you’ll likely step away from the fight, which prevents it from escalating.

Imagine two bulls are fighting and one taps the other in the balls, just like sadistic humans do. If the victim gets injured in the testis, Paduch said, he just gives up the fight instead of fighting to the death. 

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Writhing in pain for 10 to 15 minutes is to be expected. During this time, there’s not much you can do to dull the pain—though an ice pack might help take the edge off. 

But if you have persistent pain, nausea, bruising, swelling, vomiting, or dizziness that continues beyond that 15-minute mark—or any bleeding at all—you should visit your urologist or go to the ER right away, he says.  

Related: What It Means If You Feel a Lump On Your Balls 

In these cases, you’ll need a scrotal ultrasound as soon as possible to determine whether the testicle is damaged. 

The worst-case scenario is that your testicle is ruptured. If so, you’ll need to have it surgically repaired within 24 hours, or else you may need to have your testicle removed. 

That’s because a ruptured testicle exposes your immune system to your sperm. As a result, your immune system may consider your sperm as something foreign—so it attacks your testicle to try to keep you safe.

Related: The 10 Worst Things That Can Happen To Your Penis

Bottom line: If you’re hit in the balls and still have symptoms after about 15 minutes, seek medical treatment right away.   

This article originally appeared on MensHealth.com.