Question

check for understanding: why is the blood \blue\ after it passes by the organs? why is it
ed\ after it passes by our lungs?

Explanation:

Blood appears "blue" after passing through organs because it has delivered oxygen and picked up carbon dioxide. The deoxygenated blood has a lower oxygen - carrying capacity. Hemoglobin in deoxygenated blood has a different structure and absorbs more red light, making it appear darker (often misperceived as "blue" under certain lighting and skin conditions). When blood passes through the lungs, it picks up oxygen. Oxygenated hemoglobin has a structure that reflects more red light, so the blood appears red.

Answer:

Blood is "blue" (darker due to deoxygenation) after passing organs as it has delivered oxygen and hemoglobin structure changes to absorb more red light. It is "red" after lungs as hemoglobin binds oxygen, reflecting more red light.