BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.5. Gas Exchange in Mammals

9.5.6. Lung Volumes and Capacities

The Lungs of a young adult female hold on average about 4 liters of air, while those of a young adult male hold about 5 liters. Fig. 9.29 illustrates several parameters characterizing lung function in a young adult male. For females, these values are approximately 20% lower, reflecting differences in overall body mass.

1. Tidal volume is the volume of air exchanged in a single breath (inhalation and exhalation). At rest, a person inhales and exhales approximately 450 ml of air. During maximum physical exertion, the tidal volume reaches about 3 liters.

2. Beyond the 450 ml, a person can inhale an additional 1500 ml or so. This is known as the inspiratory reserve volume.

3. Following a normal exhalation, an extra 1500 ml can be exhaled. This is the expiratory reserve volume.

4. If a maximum inhalation is followed by a maximum exhalation, the total volume of air exhaled gives a value called the vital capacity.

5. Even after a maximally deep exhalation, about 1500 ml of air still remains in the lungs and cannot be expelled; this is called the residual volume.

During inhalation, out of 450 ml of inspired atmospheric air, only about 300 ml reaches the alveoli, while approximately 150 ml remains in the Airways and takes no part in gas exchange. Upon the subsequent exhalation, this air is expelled unchanged, its composition identical to that of atmospheric air. It is therefore referred to as dead space air. The air that reaches the alveoli mixes there with 3000 ml of air already residing within them. The newly arrived portion is small compared to the volume to which it is added; consequently, the complete renewal of all the air contained in the lungs is necessarily a slow process. This gradual exchange between atmospheric and alveolar air affects the alveolar air so slightly that its composition remains virtually constant (13.8% oxygen, 5.5% СО₂, and 80.7% nitrogen).

Class="center">

Fig. 9.29. Proportions of lung air volumes (spirometer trace). By convention, spirograms are presented upside down, as shown here. In reality, during inhalation, the kymograph pen moves downward (air leaves the inner vessel, causing it to descend), whereas during exhalation, it moves upward (air enters the inner vessel, causing it to ascend).

It is interesting to compare the composition of alveolar air with that of inspired and expired air (Table 9.4). The table shows that the body retains about one-fifth of the incoming oxygen for its metabolic needs, whereas The amount of expired СО₂ is 100 times greater than the amount taken in during inhalation. Alveolar air comes into intimate contact with the Blood. Compared to inspired air, it contains less oxygen and more СО₂.

Table 9.4. Comparison of the composition of inspired, alveolar, and expired air (in percentage by volume)