Sentence:  The original electrocardiograph primarily indicates irregularities in the heartbeat , but today 's techniques allow exact measurements of the flow of <WSD>blood</WSD> through the aorta , dimensioning of the heart and its chambers , and a much more detailed study of each heartbeat . 
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