4. Circulatory and Respiratory system

What is red blood and blue blood?

All blood is red. Hemoglobin, an iron-rich protein in red blood cells, mixes with oxygen to give blood its red color. Blood that’s rich in oxygen is known as red blood.

Your veins carry oxygen-poor blood. This is sometimes called blue blood because your veins can look blue underneath the skin. The blood is actually red, but the low oxygen levels give veins a bluish hue.

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