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| Respiratory Volumes |
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| There are totally about 700 million alveoli in the two lungs of an adult human being. This increases the surface area enormously. The total surface area of the lungs is 70 square metres which is almost the size of the tennis court. It is nearly 100 times the surface of the human body (skin). |
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| Thus, the lungs can hold a lot of air, about 6000 ml. This lung capacity is defined as the maximum air which can be held in the two lungs at any given time. |
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| However, during one breath in and out, the volume of gas exchanged is called the tidal volume. It is about 450 mL during quiet breathing. |
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| The volume of air that can be drawn in after normal inspiration is about 1500 mL and is called the inspiratory reserve volume (complemental air). |
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| The volume of air that can be expelled out after a normal expiration is about 1500mL and is called the expiratory reserve volume (supplemental air). |
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| Even after forced expiration, some amount of air remains in the lungs. This is called residual air which is about 1500ml. |
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| Some amount of air remains in the various parts of the respiratory tract also. The air in the trachea and bronchi (where no diffusion occurs) is called dead space air (350ml). The air remaining in the alveoli or air sacs is alveolar air (150mL). |
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| The maximum volume of air that can be exchanged in one breath in and out is called the vital capacity. It is about 5000mL. |
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| The above figures are represented in the given graph: |
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| Lung Volumes and Lungs Capacities |
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| The instrument that is used to measure the lung volume is called spirometer. |
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