Endurance<br > The basic element offitness: building<br > up the heart and lungs<br > O all the elements of fitness，<br >the most crucial is that of cardiovascular endurance -- the sustained<br >ability of the heart and lungs and blood to take oxygen from the air and<br >deliver it througl}out the body. Every cell in the body requires oxygen<br >to fimction. The stronger the heart and the more elastic the lungs， the<br >better the cardiovascular system delivers oxygen. And the better the<br >cardiovascular system works， the livelier we feel， the sharper our<br >minds are and the greater the likelihood that we will find ourselves<br >with reserves of energy at the end of the day.<br >How is endurance built?<br >Since there is no way to exercise the heart and lungs and blood vessels<br >directly， you must do an exercise that places a demand on the ;，ardio-<br >vascular system -- an exercise that summons large quantities of o~ -<br >gen into the system.<br > In essence， any form of exercise that lasts more than 90 seconds<br >