Question

why blood is a mixture and graphite an element

Answer 1

blood- 1) blood is made up of different types of constituents 
          2) these constituents can be separated.
          3) each constituents of the blood shows its own property
          4) the composition of blood is not uniform 
Graphite  -1) it is made up of only one kind of particle i.e. carbon
               2) the constituents can't be separated 
               3) it is made up of only one kind of particle so it only shows property of                    carbon
              4) the composition is uniform throughout 

Answer 2

as blood contains different types of particles it is called mixture.Blood can be separated into different compounds. the atoms present in an element can not be break into miner particles.As graphite has only carbon atoms it is called an element