'Something was pouring from his mouth. He examined his sleeve. Blood!? Blood. Crimson copper-smelling blood, his blood. Blood. Blood. Blood. And bits of sick.'
Garth Marenghi, Slicer
Thank you, Garth. Blood. We all have it (unless you're a 13th generation cyber-mind reading this in the thirty-fifth century, but I can only cover so many angles with my writing) and we all have an idea of what it's for: to take oxygen out of your lungs and transport it to all your other bits to prevent them from being dead. It has other uses too, but this is the one you will be most concerned about if you start to run out.
Let's do a bit of jargon de-tangling here - us medical types love our jargon (see this post on the subject). The medical prefix for blood is 'haem' (or heme if you prefer; you know who you are. Go on then, just this once, I will do you a favor and leave the 'u' out of that favour. Happy now?). So: haemophobia - fear of blood; haemorrhage - blood loss; haematuria - bloody pee - and so on. Lack of blood - specially red blood cells, the Werther's original-shaped cells packed with the oxygen-loving haemoglobin that do the donkey work of moving oxygen around - is known as anaemia (presumably because anhaemia is awkward to say), and in its severe forms is what is going to make us reach for the blood bags.
Anaemia (and, so y'know, there's a lot more to know about anaemia, but let's keep it simple) is the most likely reason that you're going to find yourself at the business end of a blood giving set, and my extensive study of human literature has revealed that commonest form of human blood loss is, of course, vampiric attack. For dogs and cats it's a little different. We are not, at least in general practice, ER (my close resemblance to George Clooney to the contrary). If an animal has suffered such severe blood loss in an accident that it is in immediate need of a blood transfusion, then I'm afraid it is very unlikely to survive - principally because in general practice we are not allowed to store animal blood products, and so the dog or cat is going to have to wait for us to find a donor animal and drain it before it gets a snifter of the good red stuff, by which time it is highly likely to have given up waiting in a terminal fashion.
So, for vets in general practice at least, the most likely reason we are going to want to give a blood transfusion is either due to a slow semi-controlled bleed (like a ruptured splenic tumour) or expected blood loss (which reminds me of one of my lecturers telling us about attempted surgery to remove a tumour from a dog's aorta - the main artery from the left side of the heart; the vessel ruptured and our lecturer, a master of understatement, told us that the dog 'experienced brisk haemorrhage' which later had to be scrubbed off the ceiling of the op theatre), or because of IMHA.
IMHA stands for 'Immune-mediated haemolytic anaemia' - please don't switch off, we can get through this, don't worry. Haemolytic anaemia we can already work out - lysis is the medical term for cells breaking down, so 'haemolytic anaemia' just means 'red blood cells breaking down, leading to a lack of blood cells'. I think HA is punchier, though.
As for Immune-mediated - well, as it sounds, this just means the disease is mediated by (i.e. caused by) the immune system going a bit tits up. Lots and lots of diseases are immune-mediated - all allergies, for instance. Just like The Force, the immune system is a powerful tool, but it can be used for evil as well as good. In IMHA, the immune system has decided that red blood cells are invaders and must be destroyed without mercy. Anything that stimulates the immune system - infections, drugs, and yes, sadly even vaccinations can trigger the immune system off in the wrong direction. Regardless of the In a grim parody of the Russian Revolution, all throughout the blood vessels, red cells square off against white. And red always loses.
The treatment for IMHA, like many of the immune mediated diseases, will be familiar to IT professionals. Switch the immune system off as quickly as possible, then let it come back online slowly and hope it's calmed down a bit. Steroids are the cheapest and most-frequently used drugs to do this, though there are a number of others. The problem is, it takes time for the immune system to stop popping the red-blood cells like an overactive needle-armed toddler in a balloon shop, and during that time the anaemia can reach severely (and frequently fatally) low levels.
Another definition for you (sorry) -Anaemia is - or at least can be - measured by the Packed Cell Volume - or PCV. If you spin down blood in a centrifuge, all the red cells will squish up together at the bottom of the tube, and the PCV is just the percentage of tube that is filled by these red cells - the lower the PCV, the fewer red blood cells you have. Dogs and cats normally have a PCV in the 30-40% range. Unless they're planning to run a marathon any time soon, they're going to be fine until it drops below 20%, where they're going to start looking a little tired. By the time it drops below 12% they're heading into trouble and are probably going to need a transfusion very soon. For dogs, if it drops much below, say, seven or eight percent, they are in serious danger of death. Cats seem to tolerate very low PCVs a little better, and I've know a few survive even with PCVs of five or six - but it's still brown-trousers territory for the clinician involved.
So, for whatever reason, we have a dog or cat with a worryingly pale gum colour and a PCV that is making us sweat. What now?
Let's talk about that in part two (although - spoilers - it involves sticking more blood into the animal).