白杨挺拔向上,正气昂然,自古赞誉多矣。而人更应当昂首挺胸。昂首挺胸不仅显得人精神焕发,而且使人延年益寿。人体是靠骨骼系统支撑体重的。人到老年,由于内分泌激素发生变化,呼吸肌力减弱,胸廓萎缩,造成弯腰驼背,就会使肺活量减少,肺泡毛细血管血流量也减少,肺换气功能下降,血 Poplar upright, arrogant, since ancient times, more praise. And more should be held tall. Chest stand tall not only appears full of energy, but also people longevity. The body is supported by the skeletal system weight. People to the elderly, due to changes in endocrine hormones, respiratory muscle weakness, thoracic atrophy, resulting in hunched hump, it will reduce lung capacity, alveolar capillary blood flow also decreased, decreased pulmonary ventilation, blood