VITAL CAPACITY :::
It is the volume of air that can be breathed out by forced expiration after the deepest possible inspiration Normal vital capacity is 4.3 litres in male and 3.1 litres in female Spirometer is used for measuring the vital capacity vital capacity depends upon the following factors :
Size ::
Vital capacity depends upon the size of the subject and the extent of respiratory excursions
Sex ::
It is less in female as compared to male
Posture ::
Vital capacity is greater in the upright position
Surface Area ::
It is 2.5 litres per square meter of the body surface in male and 2.0 litres in female Surface area in male is about 1.8 sq meter In female it is much less
Sleep ::
During sleep vital capacity decreases
Disease :::
In diseases like pneumonia tuberculosis and heart failure vital capacity is reduced vital capacity is very useful index for the prognosis of respiratory disease in clinical practice
Respiraotry Efficiency Tests ::
Vital capacity measurement is one of the respiratory efficiency tests it gives us an idea of the efficiency of nervous as well as muscular mechanisms of respiration other important tests are (1) Breath holding test and (2) The 40 mmHg endurance test In breath holding test the subject is asked to hold the breath after the normal expiration The time for which one can do this manouver without discomfort gives the breath holding ability The normal time is about one minute This test gives the idea about the stability of respiratory centre to increased tension of CO2
In the 40 mmHg endurance test a clip is applied to the nose and subject is asked to inspire as deeply as possible after a deep expiration Then the subject blows the mercury upto 40 mm and maintains it steadily at that level for the maximum possible time The normal time in healthy persons is about 50 sec This test gives the idea of the diaphragm and abdominal muscles to withstand capacity of fatigue
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