Question

why blood changes its colour after sometime when kept exposed in a paper??????

Answer 1

when blood is in human body it functions as a living thing but when it comes out of the body the red blood cells which are in the blood may not be working and they changes its colour by turning its red colour into other pale colour.

so to that we can observe that when blood is changes its colour when it is expposed in paper .

hope this answer is clear to you.

Answer 2

As we know blood is a connective tissue which contains RBCs which is only responsible for red color of blood, WBCs as natural army against diseases, platelets and plasma.
The main thing we have to focus is that when the blood remain in body it function properly as a living tissue but when keep it out at paper then it it become dead and also one more phenomenon take place i.e. absorption by paper. This is why blood chages its color.