cardiorespiratory fitness cardiorespiratory fitness cardiorespiratory fitness (crf) refers to the ability of the circulatory and respiratory systems to supply oxygen to skeletal muscles during sustained physical activity. scientists and researchers use crf to assess the functional capacity of the respiratory and cardiovascular systems. these functions include ventilation, perfusion, gas exchange, vasodilation, and delivery of oxygen to the body's tissues. as these body's functions are vital to an individual's health, crf allows observers to quantify an individual's morbidity and mortality risk as a function of cardiorespiratory health.