one red blood cell too many

rbc mass is the total number of rbc's in entire body in mL/kg body weight.
rbc count is the total number of rbc's per microliter of blood (amount of rbc in a certain volume of blood).


so why is understanding these definitions a big deal?

say for example, you went running today and ended up being volume depleted. your rbc count would increase, because there's a decrease in plasma volume, BUT your rbc mass would be unchanged (no additional synthesis of rbc). this is called a RELATIVE POLYCYTHEMIA (it only looks like there's more rbc per microliter of blood because of the decreased plasma volume).

now, an ABSOLUTE POLYCYTHEMIA, involves an increase in the rbc count as well as an increase in the rbc mass, which can be either a appropriate increase or an inappropriate increase.

when would it be appropriate for the rbc mass to increase? when there is a need for more rbc's to be synthesized because of TISSUE HYPOXIA (lung disease, hypoxemia, COPD, high altitude). erythropoetin is released from the kidneys in response to hypoxia, and it stimulates rbc proliferation in the bone marrow.

but what if there is an increase in the rbc mass, with normal blood gasses (no tissue hypoxia)? this would constitute an INAPPROPRIATE absolute polycythemia. usually it is caused by polycythemia rubra vera, a neoplastic disease of the bone marrow (the stem cells proliferate unchecked).

an inappropriate response can also be caused by an non physiological (excess) production of erythopoetin from a tumor, renal cyst, or renal adenocarcinoma. it could also be due to an exogenous intake of erythropoetin, usually seen in athletes who do this to increase their performance.

so to sum it up: polycythemia can be relative or absolute. relative means you just lost plasma volume, so your rbc count would be increased, but your rbc mass is normal (looks like you have more rbc's, but actually you are just volume depleted, so your rbc count is hemoconcentrated). absolute: is it appropriate? anything that is a hypoxic stimulus for eythropoetin. if there isn't a hypoxic condition (normal blood gasses), then it's an inappropriate increase due to an ectopic production of erythropoetin (from a tumor, cyst, renal cancer) or a myeoloproliferative disorder that causes proliferation of rbc's (polycythemia rubra vera).