Cardiorespiratory fitness refers to the ability of the circulatory and respiratory systems to supply oxygen to skeletal muscles during sustained physical activity. The primary measure of CRF is VO₂ max. The cardiorespiratory system consists of the heart and blood vessels, which work with the respiratory system (the lungs and airways). These body systems carry oxygen to the muscles and organs of the body, and remove waste products, including carbon dioxide. Activities such as running, swimming, and bicycling improve cardiorespiratory endurance.  You will usually hear the term shortened to just cardio or aerobic. You might also hear these activities called cardiorespiratory fitness, aerobic fitness, aerobic endurance, cardiopulmonary fitness, or a cardio workout.