The scale shows 450 gms but you say only 350 ml blood is collected. Why?
The bag in which blood is collected contains a solution which prevents blood from clotting. The total weight of the bag, before blood collection, is about 80 Gms. Besides, blood being heavier than water, the weight of 350 ml blood is about 370 Gms. The total weight of the bag, after blood collection is, thus, 450 Gms.