Sentence:  Theoretically , they are capable of extracting their required oxygen either from the surrounding air ( Ghoreyeb and Karsner , ' 13 ) or from pulmonary arterial <WSD>blood</WSD> ( Comroe , ' 58 ) . 
Meaning of the word marked with <WSD> tags:  the fluid (red in vertebrates) that is pumped through the body by the heart and contains plasma, blood cells, and platelets