Blood flow was the passage of blood through the circulatory system, as regulated by the heartbeat.
In 2365, after Worf had fainted on the bridge of the USS Enterprise-D, Doctor Katherine Pulaski described the cause of fainting as suffering a dramatic drop in blood pressure, blood glucose level dropping, and having deficient blood flow resulting from circulatory failure. Despite his protests, she checked the computer and diagnosed him with rop'ngor. (TNG: "Up The Long Ladder")