blood that leaes the heart is bright red. it travels through arteries and capillaries. when it reaches the veins on it way back to the heart, it is darkishblue. why is that

Question

aaronq · Answer

Blood is never blue, it changes from bright red to a darker red.

It is that colour because of hemoglobin molecules, specifically the iron atom of Hb, within red blood cells. Its bright red when bound to oxygen and darker red when it has "dropped off" the oxygen where it needed to go, e.g. oxygen-deficient tissues.