Without actually dying, I recently acquired some more data on this question, and have come to my own conclusion.

On the fresh water side of things, I've many times had this experience, as I'd guess you have. You're in the shower, you bend down to pick up the soap or wash between your toes, and the water streams down from your chin into your inverted nose. Not a good feeling, the minor sting that it provides. Most of us aren't doctors, but we've probably heard that there aren't many places in the body where the blood comes closer to the outside world, and we see that illustrated so close to home, many a morning while the coffee is brewing in the kitchen.

I had the opportunity to compare this with salt water the other day when I was at the Tri-Counties Blood Bank for my monthly platelet donation. When I first started donating platelets, they used small, portable Haemonetics machines, except for one big Kobe monster that looked like it belonged in the control room of a power plant. The smaller machines are less efficient than the Kobe, and usually the donation process takes longer on one of them, while ending up with less usable platelets and plasma. Eventually, I asked what the criteria were for putting a donor on one kind or the other, and was told that the staff had to learn about each individual, and whether their veins could stand up to the higher pressure that the Kobe used to return the mixture of plasma, red cells, saline solution, and anticoagulant to the donor's circulatory system. They told me of the rare occasions when the vein couldn't handle it; the word blowout was used. I think I probably heard more of a horror story than they intended. After about ten donations, they put me on the Kobe one day, but then I was back on the Haemonetics after that. Though there was no problem, I guess I didn't measure up.

I was quite happy with that situation, but the staff didn't like the smaller machines, and they were being incrementally replaced with new, incredibly smart units from Baxter. Arriving for my recent appointment, I found that the TCBB is now a Baxter-only shop, as far as platelet donation goes.

So I was hooked up, and the first drawing cycle commenced. My whole blood was diverted out through the needle, around and around the transparent spiral plastic tubing, and into the heart of the machine which, after having its way with it, dripped the plasma component into one bag, the platelets in another, and was ready for the first return phase. For some reason, there were three technicians huddled around the status display, including John, the most senior. As the direction of the flow in the tubing reversed, they asked me "Does that hurt?". I was, and I assume they were, thinking along the lines of excessive pressure. I said no, and they seemed suprised at that, as though whatever they were reading made them think that it should be. "Are you sure?". "Yes". A few more seconds, and then "Oh — that hurts. Ow!" They jumped into action, stopping the process, and told me that what I was feeling was saline solution being pumped into my muscle and other tissues, rather than into the vein. Despite the blood having been drawn quite without incident, they were concerned that the needle was badly placed.

I assented to an attempt to use my left arm instead and they brought out the special machine they use on the rare occasions when they need to continue a donation in the middle using a new needle. The tube was hermetically sealed, the needle removed and a new one emplaced, shortly to be jabbed into another vein, this time by a different nurse. The return process was resumed, and within seconds I felt the same pain as before. They stopped the return, and said I was done for the day.

I asked them why it hurt so much. I had always thought of saline solution as being essentially neutral in the body. I don't remember their explanation, but I was amused that even while I was talking with them, I was remembering this node on E2.

Let me tell you, it burns! I was amazed at how much it hurt. The other writeups here attempt learned speculation based on osmosis and intracellular pressures and whatnot. I'll leave them to it. But I've tried both, and if it ever comes to drowning, give me the fresh water!