and a simple one to answer. our blood circulation system consists of arteries which carry oxygen-rich blood to the tissues and veins which return oxygen-depleted blood back to the heart and lungs. oxygen-rich blood tends to look bright red. blood carrying less oxygen tends to look dark red. veins carry dark red blood. the reason your veins look bluish is because they lie under skin and fatty tissue which distorts the color of the blood to our eyes. the blood in those veins is indeed red but because of the overlying tissue the veins appear bluish. i hope this answers your question!
why are our veins blue