Sentence:  The original electrocardiograph primarily indicates irregularities in the heartbeat , but today 's techniques allow exact measurements of the flow of blood through the aorta , dimensioning of the <WSD>heart</WSD> and its chambers , and a much more detailed study of each heartbeat . 
Meaning of the word marked with <WSD> tags:  the hollow muscular organ located behind the sternum and between the lungs; its rhythmic contractions move the blood through the body