Why is blood that flows from the lungs to the heart bright red rather than dark red?

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Oxygen makes it red.

Gastric juices produce the red colour of the blood.

The lungs add a pigment (dye) to blood as it flows through them.

Carbon dioxide makes it red.

answer is A.

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Detailed Solution

The blood that flows through the lungs into the heart is arterial blood.
It has a bright red colour because it contains oxygen. While the blood which flows to the lungs is venous blood.
It has a dark red colour because it contains carbon dioxide.
The red colour of the blood is the result of haemoglobin. Haemoglobin is associated with oxygen.
The carbon dioxide in the deoxygenated bloodstream makes it darker.
This is because of the difference in colour between deoxyhaemoglobin and oxyhaemoglobin.
The correct response is therefore A.

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