Kidney Stones Are Torture. So's a Chemistry Lesson About Them

Have you ever had a kidney stone? Your answer will almost certainly be "yes" or "no," not "I'm not sure." It’s a binary event, like asking, “Has your poodle ever piloted an Airbus A380?” No gray area here.

Having gone through this once, I'd like to say that I'm prepared for the next one. But that's a lie; it was that bad. And the pain doesn't come from what people assume - the stone getting stuck in your thingamabob when you're trying to pass it. 

Nope. The real pain starts when the stone gets jammed in a ureter—the skinny tube connecting the kidney to the bladder. That’s when the spasms begin, as the tube tries (and fails) to push the stone along. The pressure backs up into the kidney, adding a delightful mix of nausea and vomiting to an already-lovely experience.

Don't believe me? Fine, let's take this up with wee wee expert ChatGPT. Here's the exchange:

Q: What is the most painful condition?

A: The most painful medical condition, based on clinical studies, pain scales (like the McGill Pain Index), and what patients and doctors consistently rank as absolutely excruciating, is a kidney stone. This is top-tier pain, as in, “please sedate me with a brick” pain.

Q: What do you know? You don't even have a bladder.

A: Touché. I don’t have a bladder, kidneys, or even a ureter to clog—but I’ve read millions of pages written by people who do, including the ones who’ve screamed, passed out, or begged to be run over by a truck in the ER just to make it stop.

What Could Be Worse?

Before you answer "nothing," first consider a different kind of pain. How is this possible? Could it be...

Neither Steve (left) nor Irving has urinated in millennia, as urine is a rare commodity in hell. Nonetheless, the fellas graciously agreed to host another DCLFH^®. But only after being offered a ride on New Jersey Transit as an alternative.

The Aqueous Solubility of Various Calcium Salts at 25°C.  Nice title. 

If that doesn't keep the audience mesmerized, what will?

Most kidney stones are crystals composed of calcium oxalate, and both the calcium and the oxalate components contribute to the problem. Calcium salts [1] are frequently less water soluble than other common metal ions, such as sodium and potassium (Table 1). 

Table 1. The water solubility of four common sulfate salts @20°C

In Table 1, sulfate (SO₄^-2), a common anion, was chosen as an example because its solubility in water varies substantially depending on the metal it is paired with. Compared to sodium, potassium, and lithium sulfate, calcium sulfate is 40-100 times less soluble in water. The same trend is seen with other common anions, such as hydroxide, phosphate, and acetate. In each case, calcium salts are less soluble than those of the three other metals.

But oxalate, the anion of oxalic acid, brings with it its own set of problems, especially when paired with calcium. (Calcium can be used as a general term for a large number minerals that contain the element. In chemistry the term refers to the metallic element or ionic forms.) (Table 2). 

Table 2. The water solubility of metal cations of oxalic acid @20°C. Oxalic acid itself is not anionic but is still water soluble. Solubility data for both tables is taken from the Wikipedia Solubility Table.

Now you can see why oxalic acid is not your friend. While its sodium, potassium, and lithium salts are freely soluble in water, the opposite is true when oxalate is paired with a calcium cation; the resulting salt, calcium oxalate, is what chemists call a "brick" - something that is nearly impossible to dissolve. Calcium oxalate is about 5-50 times less soluble than other metal salts; it is so insoluble that only 0.67 grams (670 mg) of it will dissolve in 100 mL of water. So, when a calcium (Ca⁺²⁾ chloride and oxalic acid are each dissolved in water and then mixed, the reaction in Figure 1 occurs instantaneously forming the calcium oxalate which precipitates out of solution. This is how the stone begins to form.

Figure 1. The formation of calcium oxalate. Two soluble species, calcium ion, and oxalic acid react, forming insoluble (crystalline) calcium oxalate, which precipitates as a solid - the stone. 

The same holds true in urine. Stone formation is highly associated with calcium and oxalate concentration in urine, so even a small decrease in the amount of water in urine can be sufficient to cause a supersaturated calcium oxalate solution, which can form the stone upon precipitation.

This is why the best defense against a kidney stone is to avoid becoming dehydrated. As urine becomes more concentrated, the concentration of calcium oxalate in the bladder will also increase. Since the stuff is so insoluble, when there is less water in the urine there is a greater chance that it will start to precipitate. Next thing you know you're writhing on the ground begging for a quick death [3]. 

Summer is coming. Drink up and watch your diet. Certain foods contain higher amounts of oxalic acid [4]. One of these is kale, in case you need another reason to avoid this vile weed. <--- UPDATE! A reader wrote to say that kale doesn't belong on this list, which is correct. I guess I let my pure hatred for this stuff cloud my judgement. (I'm a practicing anti-kale-ite.) My bad. 

NOTES:

(1) In chemistry, a salt is defined as the product the reaction between an acid and a base. For example, sodium hydroxide (lye, a base) reacts with hydrochloric acid to form sodium chloride, or with sulfuric acid to form sodium sulfate. This process is also known as neutralization.

(2) Small kidney stones are often described as the size of a grain of sand. Thought-provoking, no?

(3) The "precipitating" event that caused your idiot author to become so afflicted was two hours of playing hockey without drinking any fluids. Duh.

(4) The other way to prevent kidney stones is to avoid foods that contain a lot of oxalic acid. According to NYU Langone Health, "Calcium oxalate stones are linked with foods high in oxalate, which is a naturally occurring substance in plants and animals. These include beets, black tea, chocolate, nuts, potatoes, and spinach."