Why transfuse ffp and not ff serum?Why smoker's hematocrit higher than normal?

Blood serum is blood plasma without clotting factors (i.e.—whole blood minus both the cells and the clotting factors). In other words, serum is the liquid part of blood AFTER coagulation, therefore devoid of clotting factors as fibrinogen. plasma is the liquid, cell-free (by centrifugation, for example) part of blood, that has been treated with anti-coagulants.

Smoking can cause this in a couple of ways.

First, it is associated with biochemical changes which can cause a mild degree of dehydration. This can cause a relative increase in the cellular component (the hematocrit) relative to the plasma (water) component, thereby raising the hematocrit %.

Secondly, smoking poisons the red blood cells (RBCs) with carbon monoxide, rendering those cells incapable of bonding oxygen. your body senses this and creates more RBCs (more hematocrit in other words) to carry more oxygen (these of course are poisoned to but that's another story).

Chronic smoking leads to a lowered oxygen state which causes the body to produce more red blood cells to meet the oxygen demands of the body.

Carbon Monoxide attaches preferentially to the hemoglobin (red blood cells). This is very difficult to replace with oxygen (need a barometric chamber OR a MUCH higher % oxygen in the inhaled air). This means that up to 45% of the hemoglobin can be affected.