Hematuria (blood in the urine)

Hematuria means there's blood in the urine when there shouldn't be. Sometimes you can see it with the naked eye, and that's called gross hematuria. Other times the urine looks normal, and red blood cells, the cells that give blood its color, only show up under a microscope. That's microscopic hematuria. It doesn't change the color of the urine and usually doesn't cause any discomfort.

So where does the blood come from? It can come from the glomerulus, the tiny filter in the kidney that blood passes through to get cleaned. That filter has a thin layer called the basement membrane, which works like a fine mesh. When inflammation or the body's own defense system damages it, blood usually leaks through. That's glomerular hematuria. Or the blood can come from the urothelium, the layer that lines the inside of the urinary tract, the route urine follows. A stone (like a tiny pebble), a tumor, or trauma such as a blow can scrape that lining and make it bleed. That's nonglomerular hematuria.

And it doesn't take much blood to notice it. Picture a few drops of red paint falling into a bucket of water: the whole bucket changes color. Urine works the same way. Just a little blood is enough to make it look pink, red, or brown.