The volume of blood a tick extracts from a human host is negligible; the spread of disease is the main cause of concern for humans regarding ticks.  Over the course of several days, a tick’s outer surface can grow to hold an amount of blood 200 to 600 times their bodyweight when fully engorged, but that still only amounts to a few drops’ worth.  The largest publicized amount of blood consumed by a tick would be the tick Hyalomma asiaticium tick consuming 8ml of blood.