Where does blue blood come from before entering the heart?

Answer 1

There is no "blue blood," only red blood. Deoxygenated blood is dark red and oxygenated blood is bright red. Deoxygenated blood (dark red) picks up oxygen at the lungs in the alveoli. The high concentration of oxygen in the lung cavities diffuse into the low concentration of oxygen in the blood. The oxygen binds to the hemoglobin and becomes "oxygenated," and continues to the systemic circuit.

Answer 2

There is no blue blood in the human body. In anatomy pictures, the peripheral veins are colored blue to help visualize the difference between oxygenated and deoxygenated blood. That being said, deoxygenated blood (usually colored blue in anatomy pictures) comes from the tissues of the body. It will have relatively less O2 and more CO2 than arterial blood (which is generally colored red in anatomy pictures). The actual color difference is this: Oxygenated blood will appear bright red, deoxygenated blood will be a more dull red. Interestingly enough, the pulmonary arteries are the only arteries in the body that are supposed to carry deoxygenated blood. Also, the pulmonary veins are the only veins that are supposed to carry oxygenated blood from the lungs back to the heart. Remember that when you are coloring in your anatomy diagrams, or you will likely be marked down. The difference between vein and artery is whether the vessel carries blood to or away from the heart. If the vessel carries blood toward the heart, is it a vein, if it carries blood away from the heart, it is an artery. This is not to be confused when considering the cardiac arteries and veins. The heart also needs everything that other muscle cells need (oxygen, sugar, calcium, etc.). So the heart also has arteries that technically carry blood "toward" the heart, but they are there to feed the heard muscle.