ANSWER: (A) 1/3rd of tidal volumeREF: Guyton physiology 11th edition page 478, http://en.wikipedia.org/wiki/Dead_space_%28ph ysiology%29Indirect repeat in December 2010"The normal dead space air in a young adult man is about 150 millilitres. This increases slightly with age1'Some of the air a person breathes never reaches the gas exchange areas but simply fills respiratory passages where gas exchange does not occur, such as the nose, pharynx, and trachea.This air is called dead space air because it is not useful for gas exchange.TOTAL DEAD SPACE (also known as "PHYSIOLOGICAL" DEAD SPACE) can be divided into anatomical dead space and alveolar dead space.ANATOMICAL DEAD SPACEAnatomical dead space is the gas in the conducting areas of the respiratory system, such as the mouth and trachea, where air does not come into contact with the alveoli of the lungs. Birds, which, relatively, have a far longer and wider trachea than mammals, have a higher proportion of dead space.It is normally equal in milliliters to your body weight in pounds. A150 lb (68 kg) male would have an anatomical dead space of about 150 mL (1 mL per lb or 2.2 mL per kilogram of body weight). This is the same conversion of kilograms to pounds, except the final unit is in mL. This is about a third of the resting tidal volume (450-500 mL).Anatomic dead space is the volume of the conducting airways. It may be measured by Fowler's method, a nitrogen washout technique.ALVEOLAR DEAD SPACEAlveolar dead space is caused by air contacting alveoli without blood flow in their adjacent pulmonary capillaries, i.e. ventilation without perfusion. As a result, no gas exchange can occur. Alveolar dead space is negligible in healthy individuals, but can increase dramatically in some lung diseases