Citation Hunt

The Wikipedia snippet below is not backed by a reliable source. Can you find one?

Click I got this! to go to Wikipedia and fix the snippet, or Next! to see another one. Good luck!

In page Arteriovenous oxygen difference:

This text has been unreferenced on Wikipedia for a very long time. If you can't find a source, be bold and remove it!

"

Alternatively, in order to find the efficiency of the lungs in replenishing blood oxygen levels, the a-vO2 diff may instead be taken by comparing blood from the pulmonary artery and the pulmonary vein; in this case a negative value for a-vO2 diff would be obtained as the oxygen content of the blood would have increased.[citation needed]