Blow to the kidney

Not too bad an injury at all. He'd be passing blood for a couple of days after no more than a swift kick placed just about at waist level, on the back. They'd keep him under observation, probably do a CT scan of the abdomen just to make sure the "capsule" isn't ruptured. If they find out it is, then they might keep him longer. I believe from what I do in my day job that it'd have to be a fairly major disruption before they'd operate.

Kidneys do sustain mild injury rather easily with blunt trauma and bleeding is about the only symptom other than the expected pain.

HTH

Deb's right. The degree of visible blood in the urine does not correlate with the degree of blunt injury to the kidney itself. In a mild renal contusion it would be reasonable that there is visible blood (typically looking like cherry kool-aid) intermittently plus or minus some casts for a day or three and microscopic blood for about 6 weeks. (here a cast is a blood clot that has formed to the shape of the inside of the ureter, so that they look like tiny earthworms). When you start passing large amorphous clots or urine looking like tomato juice, that's more typical of a bladder or prostate bleeding.

enters thread, realizes quickly that this does not have the porn intent he thought it might, and leaves, virtually unnoticed.

You a take a fist or boot to the lower back hard enough, or a bullet into the body armor from the back, and you WILL piss cherry koolaid for a day or two. Unless the pain doesn't back off in an hour or so, you can pretty much ignore it and keep on trucking.

The pain can be so bad that some people just drop to the ground and writhe on the ground unable to do anything, some people can take those hits and keep right on coming, just getting madder and meaner from the pain.

A hard enough blow to the lower back can kill from wound-shock, and a knife with a half-twist has an even better chance.

But if you're character is a street monster, that kidney shot will probably force him to his knees at the most, or just enrage him further, and the pissing blood will more than likely either be handled by drinking booze and/or doing drugs, or he'll head to a local street doc to get it checked.

Just my .05