How come after having a can of soft drink you feel the massive need to pee?

23 January 2012

The urge to pee is something we all experience. Usually it's no big deal. If everything is working okay, we get enough warning to make it to the bathroom in time. Of course, there are situations where you may just have to "hold it" a bit longer than you might like... sitting in a movie theater, riding on a bus, etc.

But what if you're someone who... feels a need to pee very suddenly and can't get to a toilet in time; laughs so hard you wet your pants; has to urinate when you hear running water? These are all signs of urinary incontinence (involuntary loss of urine) due to an overactive bladder or by weak bladder muscles.

To address this topic, we need to look at how our bladder works. And it all starts with our kidneys.

Production of Urine in the Kidneys

One function of the kidneys is to filter the blood and maintain a healthy balance of water in our body (the other is to remove waste material).

Did You Know? About 180 liters (720 cups) of fluid from the blood passes through the kidneys each day, but only 1-2 liters (4-8 cups) of urine are produced.

Let's say you've just finished off a bag of chips. The first thing that happens is the salt level in the blood increases. This causes the pituitary gland to secrete a chemical called antidiuretic hormone (ADH).

When blood containing ADH enters the kidneys, it's like a signal saying "Oh-oh. There's too much salt in this dude's body. We need to dilute it." In response, the kidneys will remove very little (if any) water from the blood. This prevents the salt from becoming even more concentrated.

The next thing that happens is we become thirsty. So we react by taking a drink. This helps to reduce the salt concentration in the blood and also causes less ADH to be produced. Now when the kidney sees lower levels of ADH, it begins removing more water as urine.

Did You Know? If you go to the movies and eat a bag of salted popcorn along with that super-sized soft drink, you have a better chance of not having to get up to use the washroom before the movie is finished.

The Bladder and the Urge to Pee

After urine is formed, it travels down two tubes called ureters. The muscles in the wall of the ureters constantly tighten and relax to force the urine into the bladder.

Did You Know? Small amounts of urine are squirted from the ureters into the bladder every 10-15 seconds.

The bladder is a flexible sac with detrusor muscles lining its walls. These muscles expand and contract, depending on whether the bladder is filling or emptying.

At the exit of the bladder are two sets of sphincters (ring-like band of muscles) to keep the urine from leaking out. The internal sphincter is an involuntary muscle, meaning we have no control over it. The external sphincter, however, is a voluntary muscle that we can control.

We get the first sensation to pee when about 200 mL ( 1 cup) of urine has been collected. After the volume reaches 400-500 mL (1.5-2 cups), the detrusor muscles in the bladder wall contract and the inner sphincter opens.

Once this happens, we now have to make a conscious effort to hold the urine in by squeezing the outer sphincter. When we've really got to hold it, we use muscles in our pelvic floor to help keep the sphincter closed until we can get to a toilet.

Urinary Incontinence

We started this article by mentioning involuntary bladder control (i.e. urinary incontinence). One type is urgency incontinence (or overactive bladder)... the bladder contracts for no apparent reason causing a strong need to urinate. Another is stress incontinence where a weak sphincter or pelvic floor muscle causes you to lose urine when you laugh, sneeze, etc. A third type is overflow incontinence... the bladder does not empty properly and frequent urination results.

Final Word...

Drinking a can of soda shouldn't trigger a massive need to pee any more than drinking the same amount of water. Although it was once thought that the caffeine in soft drinks had a diuretic effect (i.e. causes the kidneys to remove more water), this has now been disproved.

If you do find yourself urinating frequently or you're having trouble in holding your pee, you should discuss this with your family doctor.

Learn More!

  • The Association for Science Education: The kidneys and body balance.
  • Body basics for teens: Kidneys and Urinary Tract.
  • Should the toilet seat be left up or down? Check one mathematician's solution here.
  • Maughan, R.J. and J. Griffin. 2003. Caffeine ingestion and fluid balance: a review. J Hum Nutr Dietet (16): 411—420
  • Porth, C. M. 2004. Pathophysiology: Concepts of altered health states. (7th ed). Chapter 37 - Disorders of Urine Elimination. J. B. Lippincott. Philadelphia.
  • CRAM Science would like to thank Dr. Stan Megraw for his help and expertise in answering this question. Stan is a writer/researcher specializing in science, technology and medicine. He has more than 30 years experience as contributing author for research institutes, universities, government agencies and non-profit organizations.


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