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Why Beer Makes Us Pee

Why Beer Makes Us Pee

By B. James McCallum, Stuart M. Smith and Arabindra B. Katwal

A young man sits at the end of a bar in a trendy downtown night spot. He is nursing his third beer of the evening in an effort to fill his empty life and wondering how he got to this spot. That is until he spots a comely young lady through the haze of cigarette smoke1.

She's new in town and is looking for a bit of companionship. Their eyes meet. She smiles, he smiles. They start making their way towards each other through the crowd, when, suddenly, he has the overwhelming urge to urinate. Unable to tolerate the sensation any longer, he yells "stay right there" and bolts for the water closet. By the time he returns to the floor, she is gone, as is his one chance at happiness in life. Ah, why did it all go wrong?

Sadly, physiology was stacked against our hero. How much urine someone produces, and therefore how much and how often they have to urinate, depends on a complex series of interactions regulated by stretch receptors, osmoreceptors, and hormones in the human body.

Alcohol and ADH at a glance. Click to enlarge
Blood is of course a solution with what amounts to water as a solvent and various sundry electrolytes, proteins, sugars, etc as solutes. As with any solution, the concentration of blood can be varied by changing the amount of solvent present or the amount of solute present. Our bodies work the best within a relatively narrow band of concentrations – somewhere between 270-290 mOsm/L (that’s milliosmoles – a posh way of measuring concentrations).

However, we are constantly taking on, using or losing the water and solutes in our blood, which could cause massive changes in concentration levels. Therefore our bodies have developed sophisticated mechanisms to regulate this balance.

Among these mechanisms is a system involving a hormone called ADH (antidiuretic hormone), also known as arginine vasopressin. Specific cells in the brain called osmoreceptors monitor the concentration of the blood. If this concentration becomes too high, then they stimulate the release of ADH. The hormone then travels through the bloodstream to the kidneys. As we all know, urine is produced in the kidneys – this is done by passing blood through a complex series of filters and tubes that ultimately lead to the long tube connecting the kidneys to the bladder – the ureter.

The last major set of microscopic tubules the urine passes through before reaching the ureters is called the collecting ducts. ADH acts on these collecting ducts by inserting proteins called aquaporins in them. These aquaporins allow free water, but not solute, to cross back in to the blood stream, which increases the concentration of the urine, whilst decreasing the concentration of the blood. What this means is with more ADH, you produce small volumes of very concentrated urine. With less ADH, you produce copious quantities of dilute urine.

As it turns out, both alcohol and caffeine inhibit the release of ADH. Therefore, when our hero was downing his pints of beer, not only was he pouring a lot of liquid into his body, but he was inhibiting the production of ADH. The result? Our hero has a full bladder and misses out on the love of his life.

Find out how other stuff works:

- Weird - The Brazil Nut Effect
- Cool - How electric eels work
- Interesting - Can you catch a yawn?
- Make your own mind - Genetic modification

1Note that this article has been written by three Americans who live in a country where free choice is still allowed (to a certain extent). In the UK the man and woman would be gazing at each other through a haze of pretentious chitchat.

Image: Stephan Wagner

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15 May 2010
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