Inside of a blood clot: a delicate meshwork of fibrin strings traps many red blood cells. The smallest discoid particles deep down in the dark background are blood platelets, the smallest cells in our blood.

How small? About 5000 times smaller in diameter than that of the smallest fill for your automatic pencil. When the blood clots, fibrin is formed. The fibrin meshwork forms within a few minutes, yet in this short time 13 different blood components have to interact biochemically, like a precise clockwork, to make these fibrins solid and eventually stop the flow of blood. People who lack any of these clotting factors may dangerously or even fatally bleed upon the slightest injury. Your blood donation can save these lives, because your blood contains the clotting factors that these people need.