In the event of excessive blood loss, AIUI, doctors/EMTs can deliver (cheap) saline IV fluid instead of (precious) blood in order to restore volume and pressure. This of course dilutes the life-sustaining components (red blood cells, platelets, etc.) that were already present in the body. Assuming the patient was basically healthy before needing fluids, how much can the blood be diluted with saline before the anemia becomes so severe that whole blood needs to be added?
About twenty years ago, I had a family member who was deathly ill and in an ICU. He was being given half a dozen (or so, it’s been a while) medications via half a dozen or so IV bags. He was in an induced coma and wasn’t urinating enough. So he looked quite bloated. He looked, frankly, like the people in movies do when they’re in a fat suit and made up to look overweight. As I remember, eventually they resorted to dialysis.
So in short, I think (as a layman) the answer to your question is that the person’s kidneys would be working overtime to process the blood and pull the excess water out of the body via urination.
But I also remember from a story about a fraternity that hazed pledges by forcing them to consume gallons of water that it’s possible to, essentially, overdose on water.
The Merck pages on hypernatremia are here: Hypernatremia - Endocrine and Metabolic Disorders - Merck Manuals Professional Edition
It doesn’t give the fatal dose, but perhaps our med people will be along soon.
Similarly, our other weird lab people will probably come along to tell us about the chemistry of blood ex-vivo and have us imagine sludge-salt blood.
I have diabetes, so it come up a lot. And, in the other direction, have had more than my fair share of being bagged in various emergency rooms for this and that.
It happened once that the first time I brined a chicken with an injector (previously I calculated my brine with a discrete solute percentage) the same day I had to take my dog to the vet, where he was given a yummy bag of Ringer’s solution (Ringer's solution - Wikipedia).
It amused me to try to figure out out to what level I had brined my dog, but lacked the necessary skills.
One of you is talking about hyponatremia and the other is talking about hypernatremia, but I wouldn’t think either should be an issue if isotonic saline fluid is used for the IV.
I’m more curious about anemia. If your normal concentration of red blood cells is X cells per liter, but you still need more fluid volume in your circulatory system (due to blood loss) how low can your red cell concentration get before the nurses/doctors say “no more saline for this patient, start using whole blood instead”?
You made me flash on the Soup Nazi: “No more saline for you!”
The salt content in the saline solution is specifically set at a level at which it won’t overdrive the kidneys, that’s what “isotonic” means.
Big oversimplification ahead:
The kidneys remove a lot of different things from the body; they will work harder to remove a particular item when there is more of that item. In the case of water, the physical parameter which indicates how much of it is in a solution (from the point of view of kidney mechanisms) is called osmotic pressure.
Osmotic pressure is a measure of how much solvent (in this case water) is in a solution (in this case the blood). You can have two solutions which have different solutes (which in this case includes “anything other than water”) and which have them in different concentrations, but whose osmotic pressure happens to be the same. You place a kidney in a saline solution with X osmotic pressure, in a sugar solution with X osmotic pressure and in a mixture of water and blood cells with X osmotic pressure, its water-removal mechanism will be at the same level in all three cases.
Correction: the actual preparation we and animals usually get is lactated Ringer’s: Ringer's lactate solution - Wikipedia
If you just add saline to reduce the ratio of blood to saline, then the excess water will get absorbed by the body’s cells even if you are using isotonic saline. At some point the cells get too much water in them and burst.
If you take blood out and put saline in to replace it, the best numbers I can find on google indicate that you run into trouble when you’ve lost around 40 percent or so of your blood. That’s the point where a transfusion becomes necessary or the patient will likely die.
One interesting thing I found while poking around on google is that doctors now sometimes use cold saline to quickly chill the body and slow its metabolism, giving them more time to repair whatever wounds caused the blood loss in the first place. Once they’ve got al the holes patched, they replace the saline with a blood transfusion. So temperature makes a big difference here.
Saline and Ringer’s lactate and the like are basically salts and water. They are collectively called “crystalloid”. They lack the colloids that real blood has (like albumin and all the other proteins floating about. They are good to bring up the blood volume of a bleeding patient in a pinch, but eventually you will run into issues as the salts diffuse out of the bloodstream and into the tissues (like the “fat man” described above).
Colloids are better. The osmotically active particles stay in the bloodstream better. Albumin can be used, but that’s a blood product again, and if you have access to that, whole blood or red cells would be a better replacement. There are synthetic things that are cheaper than real blood products, as well. Hetastarch can be used to mimic the colloidal effects. There may be better replacements now.
None of these things has blood clotting factors or oxygen carrying capacity, so there are real limits. Unfortunately, I don’t know them as I don’t manage this kind of thing.
Hopefully an ER doc or intensivist will check in.
What you seem to be asking is what is the minimum hematocrit that can sustain life. It’s been a far spell and while I wasn’t on the floor administering the stuff, I used to work in a blood bank and advise people. The answer is… it depends.
I vaguely recall case reports and war stories of people pulling through with temporary values in the single digits, but no one (I seriously doubt anyway) would want their patient to stay that low very long, all else equal. How fast the crit drops matters as well. In general the slower the better. And what’s been said about the other functions of blood - clotting factors, intravascular osmotic stability being two important things - is well said.
And FWIW nowadays “whole blood” is vanishingly rarely administered. Instead, donated whole blood is split into its components and the components are given as indicated.
There are various ways to measure this, one way is hemoglobin (Hb) in grams per deciliter. Normal Hb for an adult male is 13.5 to 17.5. People have survived with Hb levels of around 2 (using normal O2, not hyperbaric), although there is increasing mortality as it drops.
There is obviously significant excess oxygen transport capacity in a healthy human, else some could not climb Mt. Everest with no supplemental oxygen. At the summit the total atmospheric pressure is only about 4.8 psi (vs 14.7 psi at sea level) and partial pressure O2 is about 1 psi.
It is possible a human in an ICU could survive short term with a Hb level below 1.0 in a hyperbaric chamber and/or using therapeutic hypothermia, but I haven’t reviewed the literature:
I myself am incredibly pleased to learn that “intensivist” was not an Autocorrect for “internist.”