Sentence:  An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic the way real kidneys cleanse <WSD>blood</WSD> and eject impurities and surplus water as urine. 
Meaning of the word marked with <WSD> tags:  A vital liquid flowing in the bodies of many types of animals that usually conveys nutrients and oxygen. In vertebrates, it is colored red by hemoglobin, is conveyed by arteries and veins, is pumped by the heart and is usually generated in bone marrow.