Oooh, something I can answer!
So basics: what you're defining is 'hypovolemic shock', meaning the body goes into a compensatory state because there is less blood in the body- 'reduced organ perfusion'. Doctors just say shock and most know what that means. There are other types- neurologic, septic, blah...
It can either be due to open trauma, or as the poster above me said, internal bleeding. Usually open trauma is easier to treat, faster to notice (it's there, man).Internal bleeding, depending on what artery is involved is a bit different, but usually goes at a slower pace (say 4-6 hours before death in spontaneous hemoperitoneum, that is described above)
So first *rubs hands happily* vitals and systemics! Blood pressure will be low, heart high, maybe a fast respiratory rate (so hypotensive, trachycardic, taychpnea). Conjuctival pallor (pale lower eyelids, works for any ethnicity) reduced capillary refill (a test you do, press on the fingernail and see how fast the white turns to pink, again, works for any ethnicity)... then there are a lot of other symptoms but no doc is gonna hang around to check them all out. If the patient had head injuries, esp for road accidents, we will do a Glasgow Coma Score.
We will first start IV isotonic (0.9%) saline solution, or Ringer's lactate. Stop any open bleeders, using sutures or compression bandaging. We will take blood for cross matching, and send blood for CBC and electrolytes. We will also put a catheter and check for any reduced urine output (which is bad). All this will also prep him for surgery if need be, and usually we will send for HIV and Hep B testing as well (in case we operate). We don't infuse blood until the results return- unless of course we get the CBC first and the hemoglobin (Hb) is terribly low (I'd say 5mg/dl, but for a tall white guy, make it 8mg/dl to be safe. Normal adult male: 16mg/dl) and then we'd raid our fridges for O negative blood.
Other values we look at are total RBC count (reduced), MCV, MCH MCHC, and hematocrit. All would be normal except hematocrit, which will be low
Like someone pointed out, you got lots of blood in your body. A rule of thumb is 1l per 10kg of body weight. Assuming our guy is 60-70kgs, he'll have a min of 5l of blood. Even then, the physiology of our body is designed to compensate for this loss, until you get help.
Usual symptoms of shock- lightheadedness, fatigue, confusion, being aware of your heartbeat, and I've personally heard a drumming, pulse like sound in my ear before I passed out. If internal bleeding- usually dull, generalised abdominal pain (or wherever blood is being collected. even if blood is collected in the pelvis they describe it as lower abdominal pain.) Sometimes if they lie down they might feel a sharp, shoulder tip pain. This usually happens in women, in ruptured ectopic pregnancies)
1). A character loses two pints – we’ll say around one litre as I know UK and US pints are different – of the red stuff (no sparkly vampires involved). He’s forty years old and in good health, around six feet tall and of medium build. Within twenty-four hours he has a blood test carried out for something unrelated. Even if he appears perfectly healthy after all this, would the results of the blood test show that he’d lost this specific(-ish!) amount of blood recently? As I understand it, the plasma and white cells are replaced quite quickly but the red cells take many weeks to get back to normal levels – hence the rules for the minimum intervals between voluntary blood donations.
No, maybe a very slight decrease in Hb, but really negligible- you don't have any previous reading to compare with anyway! And I think that answers the rest of your questions, except for Q4
2). If there is a detectable difference, what sort of values would be detected? Imagine two doctors are discussing the character’s blood test results. Ideally, I'd like them to be a bit confused by some sort of discrepancy.
3). The character appears none the worse for wear after losing a litre of blood.
a). Would he feel different? Maybe a little light-headed or tired?
b). Would he look paler than usual? He’s a white Englishman, and they tend to look pretty pallid even when healthy!
4). My googling has supplied plenty of information about what would happen if he then lost another litre of blood (no giant leeches involved either) soon after. I can write the hypovolaemia scenes clinically, but if anyone has any authentic snippets they could share it would be a massive help. Either from a doctor, nurse, paramedic or patient perspective is fine.
I must disagree about the immature blood cells. This would indicate a chronic pathology, not something that would happen in a day. Blood cells are produced everyday, and the cells die every 120 days, so you got time. If you have defective erethropoesis (defect in producing red blood cells), then yes, you'll have different lab values, the reason for it being anything from a lack of vitamin B12 in the diet to cancer- and that's another kettle of fish.
Sorry for the long post!