If the total volume of expired air be 500 ml and the dotted area represents the alveolar air and the hatched area be dead space air – 8.17 the steady N 2 level is shown to be at 60%. In the last part of the tracing the N 2 percentage level off and remains more or less steady because it is pure alveolar air and in an ideal lung all the alveoli are contracting synchronously unloading their N 2 content in the expired air at a uniform rate.

The second part of the curve shows rising N 2 concentration because it is a mixture of O 2 from the dead space and N 2 from alveolar air. The first part of expired air contains ‘zero’ % of N 2 because it is pure O 2 from dead space. The subject takes a deep breath of oxygen and then expires through a N 2 meter which records graphically the percentage of N 2 in the expired air from moment to moment. This ratio increases with age but decreases on exercise. The normal ratio of dead space to tidal volume is in the range 0.2 to 0.35 during breathing at rest. In some disease of the lungs the physiological dead space may amount to 1 to 2 litres producing great respiratory insufficiency. In normal subjects the volumes are very nearly the same. Measurement of dead space in these cases will give a value much higher than anatomical dead space and constitutes what is known as physiological dead space.

These are, therefore, partially dead space areas. In diseased conditions the situation may be worse-there may be many alveoli which are hyperventilated but have very poor blood flow. Most of the ventilation in these alveoli, therefore, is wasted and produced partial dead space effect. In fact, in normal subjects the apical alveoli are ventilated adequately but have got a poor blood supply. But such an ideal condition is seldom obtained. In an ideal lung all the alveoli are evenly ventilated and adequately perfused with exactly the required amount of blood. Adrenaline dilates the lower airways by relaxing smooth muscles and upper airways by vasoconstriction which shrink the nasal mucosa. Reflex broncho contraction due to stimulation of the parasympathetic nerves diminishes the anatomical dead space. But the particles about 2.0 µm entering the alveoli are removed by lymphatics.įactors Influencing the Anatomical Dead Space : Removes the particulate matter in sizes more than 2.0 µm from the inspired air before it is delivered to the alveoli. Inspired air is saturated by water vapour before it reaches the alveoli of the lungs.

What is the total dead space in lungs