Residual Volume - Respiration
No matter how hard one may try, the lungs are never empty of air; there is still 1650 cm3 of air left in it. This is known as residual volume. During inhalation, 350 (500-150) cm3 of fresh air enters and this mixes with the 1650 cm3 of air already in the lungs. The renewal of air is only one part in five. Therefore, the lungs have a constant composition of 15% oxygen and 5% carbon dioxide. This remains same even during exercise as increased ventilation during exercise supplies the extra oxygen requirements of the body.