Which vessels are exceptions to the general rule that veins carry oxygen-poor blood and arteries carry oxygen-rich blood?

• A. Pulmonary artery and pulmonary veins
• B. Aorta and Vena Cava
• C. Coronary arteries and veins
• D. Hepatic arteries and veins

Answer: (D)

This question tests the idea that not all vessels follow the simple idea that arteries carry oxygen-rich blood and veins carry oxygen-poor blood. The special case is the pulmonary circulation, where the pattern reverses. The pulmonary artery carries blood that is low in oxygen as it travels from the right side of the heart to the lungs for gas exchange. After picking up oxygen in the lungs, the blood returns to the heart via the pulmonary veins, which carry oxygen-rich blood back to the left side of the heart. So the vessels that don’t fit the usual rule are the pulmonary artery and the pulmonary veins.

Other vessels follow the common pattern: systemic arteries, like the aorta, carry oxygen-rich blood away from the heart to the tissues, and systemic veins, like the vena cava, return oxygen-poor blood to the heart. Coronary arteries deliver oxygenated blood to the heart muscle, and the coronary veins drain deoxygenated blood back toward the right atrium. Hepatic arteries carry oxygenated blood to the liver, and hepatic veins drain deoxygenated blood away.