Red blood cell countRBC · Erythrocyte count
How many oxygen-carrying cells are in a given volume of blood.
The red blood cell count is the number of erythrocytes per litre of blood. These are the cells that carry haemoglobin, and therefore oxygen, from the lungs to every tissue.
It matters as one of three related views on the same thing. The count says how many cells there are, haemoglobin says how much oxygen-carrying pigment, and haematocrit says what proportion of blood volume they occupy. Anaemia can show up in any of them, and the cell-size indices explain what kind.
In practice the count alone is rarely the interesting number, because cells vary in size and haemoglobin content. It is read with MCV, MCH and haematocrit, and it rises with dehydration simply because the plasma has shrunk.