Early this morning after a night of drinking I woke up and really had to pee. I’m sure you know what I’m talking about. But what happens if I don’t? Is there a long-term health risk if I regularly choose to hold it? If alcohol is thrown into the equation, are there other effects or considerations?

'Matt Groves, Valdez, Alaska

I have to level with you, Matt. When I wake up after a long night with the needle pegged on F, my first thought isn’t: What if I held this in? But that’s OK'a willingness to think outside the box is the mark of a true scientist. Luckily, you needn’t experiment on yourself. Many folks who really are scientists sometimes enjoy a cold one, and as often happens following alcohol consumption, they’ve had occasion to study their bodily functions up close.

When you drink, liquid passes from your stomach and small intestine into your bloodstream. Your body monitors how dilute your blood is, and varies urine production depending on salt content and volume consumed. A night of tippling gives you a lot of liquid to unload, and while you’re in bed your kidneys are going to be busy.

As you sleep, your bladder fills up. It’s a tough, muscular bag that stretches as it fills, and as it does so, stretch-receptor nerves tell you that you’re approaching capacity and it’s time to head for the can. Maximum bladder volume varies, ranging from about a pint to nearly a quart (imagine the patient research so somebody could plot that on a spreadsheet), with the urge to go kicking in at anywhere from 5 to 13 fluid ounces. Your bladder has two sphincters on the bottom that keep the contents in, one controlled automatically by your nervous system and a second controlled voluntarily by … well, it’s still your nervous system, but higher up in the chain of command. With luck, you can control that second one well enough to wake up dry.

Now for the complications. The ureters, the tubes that feed from each kidney to the bladder, don’t have valves to prevent reverse flow. If your bladder gets too full, the fluid can back up into the kidneys, leading to potentially serious medical issues. Kidneys work by a pressure differential system involving a fine network of capillaries and tubules, and fluid backup will make them less effective at filtering your blood. Depending on how high the backpressure gets, your kidneys could be damaged. The area at greatest risk is the kidney’s central collection region, the pelvis and calyces, which may become dilated, a condition called hydronephrosis. Another risk from high bladder pressure is kidney infection due to bacteria forced up from below, called pyelonephritis.

But don’t worry. Unless there’s some physical obstruction, or you’ve got a lingering case of childhood voiding dysfunction (see below), you’ll let go long before backpressure can do any real harm. If you do have a blockage, such as from kidney or bladder stones, different story, but should that happen you’ll know it'the pain caused by stones is legendary.

Generally speaking, voluntary urine retention won’t hurt your bladder either. It’s rugged and well suited to the purpose, so it’s not going to explode just because you’re determined to hold out. Some cite astronomer Tycho Brahe, of golden nose fame, who reputedly died of a burst bladder after he refused to leave a banquet table for fear of being rude. However, most theories on the cause of his death now focus on a bladder infection, mercury poisoning, or both.

That’s not to say bladders can’t break. A full bladder can be ruptured during a car accident'impact with the steering column or an improperly worn seat belt is often what does it'since there isn’t time for the sphincters to open and relieve the pressure. Other complaints you’re likely to have at the same time, such as a broken pelvis, can make surgical treatment difficult. So keep that belt low across your hips, not snugged around your middle, and if you feel a traffic accident coming on, go to the bathroom first.

Alcohol can complicate the retention issue, mainly because like caffeine it’s a diuretic, meaning it stimulates urine production. It also has anesthetic properties, so you may not realize how full your bladder is getting, but again like caffeine it’s an irritant, making it feel more urgent for you to urinate.

In the last analysis, however, the issue isn’t physiological but psychological. I mentioned voiding dysfunction, which sometimes develops in children who refuse to go to the bathroom for extended periods, leading to incontinence (hey, there are physical limits), infection, and other problems. Freud made a career out of people who held in what they shouldn’t have. Spare yourself years of therapy, Matt'when your body tells you to go, go.

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