Bladder cancer

Bladder cancer begins when cells in the bladder start growing out of control, because the way they work, grow, and divide changes. And it does not arise in just any cell: it almost always arises in the urothelium, the inner lining of the urinary tract. That is why by far the most common type is called urothelial carcinoma, and those same urothelial cells also line the urethra, the ureters, and the renal pelvis, the part of the kidney where urine collects.

That is what explains why tobacco matters so much. Smoking is a major risk factor for bladder cancer, and the path is this: the substances in the smoke are absorbed into the blood, the kidney filters them, and they end up concentrated in the urine. The bladder is precisely the container where that urine waits, so its lining sits bathed in those substances, and that contact can damage the DNA of its cells, the instruction manual the cell works from. Having the risk factor does not guarantee the disease, but it explains why that particular wall is the one that takes the damage.