Why does the blood that appears to flow from the lungs to the heart appear to be bright red rather than dark red?
A) Oxygen makes it red
B) Carbon dioxide makes it red
C) Gastric juices produce the red colour of blood
D) The lungs add a pigment to the blood as it flows through them

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Hint: The blood contains a red colour pigment called the haemoglobin. The haemoglobin is the carrier of oxygen. The blood which flows from the lungs to the heart through the pulmonary vein is rich in oxygen.

Complete answer:
> To solve this question, we must know about the composition of blood from the lungs to heart.
> The blood contains haemoglobin which has a high affinity for oxygen (Statement D is incorrect). The heart is the main organ of pumping oxygenated blood to the whole body. It receives the deoxygenated (rich in carbon dioxide) blood from all the parts of the body through superior and inferior vena cava. The haemoglobin combines with carbon dioxide to form carboxy-hemoglobin. This blood is dark red in colour due the presence of carbon dioxide. (Option B is incorrect)
> The deoxygenated blood flows to the lungs for oxygenation. The pulmonary artery carries blood from the heart to lung for oxygenation. The blood gets oxygenated in the lungs and is carried back to heart after oxygenation through the pulmonary vein. The pulmonary vein appears to be bright red in colour due to the presence of oxy-hemoglobin (option A is correct)
> The gastric juices secreted in the stomach are not transported through blood.

Hence, the correct answer is option (A)

Note: The arteries carry blood away from the heart except the pulmonary artery. They are rich in oxygen so are red in colour. The veins carry blood towards the heart except pulmonary vein so are dark red in colour due to carbon dioxide rich blood.