This is about oxygenated an de-oxygenated blood. Our red blood cells carry oxygen around our bodies to organs that need it. there is around 5 litres of blood in our bodies. It leaves our heart bright red, full of oxygen, then goes to the organs that need oxygen and is extracted. Oxygen is used in the brain too! When it dumps all of the oxygen it picks up some carbon dioxide. Then its colour changes from bright red to dark red. Eventually it flows back into the heart and goes through the lungs for more delicious oxygen. It’s always dark red though, never blue. When we see it blue it’s because it’s come through lots of layers of skin. The skin changes the way the light can get to the dark red blood inside. IT has lots of different layers, and the layers all make the light bounce around and move at different speeds. That means the light bends. Eventually it comes back out and we see blue.