Lungs evolved in early lungfishes as outpocketings of the digestive tract. Although lung structure has evolved considerably, lungs remain dead-end sacs in all air-breathing vertebrates except birds (and likely their extinct reptilian ancestors). Ventilation of dead-end sacs with common passageways for inflow and outflow cannot be constant and unidirectional but must be tidal: air flows in and exhaled gases flow out by the same route. Since the lungs and airways can never be completely emptied of air, they always contain dead space. We can easily measure the volumes of air exchanged during breathing, but we have to use an indirect method to measure the dead space contained in the lungs and airways. Measures of dead space are important in assessing lung health and disease.

A flowmeter measures the volume of air breathed in and out (Figure 48.9). Using a human as an example, the amount of air that moves either in or out per breath when at rest is called the tidal volume (TV) (about 500 mL for an average human adult). When we breathe in as much as possible, the additional volume is the inspiratory reserve volume (IRV). Conversely, if we forcefully exhale as much air as possible, the additional amount of air expelled is the expiratory reserve volume (ERV). The maximum capacity for air exchange in one breath, or the vital capacity (VC), is the sum of TV + IRV + ERV. The vital capacity of an athlete is generally greater than that of a nonathlete, and vital capacity decreases with age because of stiffening of the lung tissue.

Vital capacity is not the entire lung capacity because of the dead space, also called the residual volume (RV). We can’t measure RV directly with the flowmeter, but we can measure it indirectly using the helium dilution method. The flowmeter is attached to a closed reservoir of air of a known volume (V_res) so that a person can breathe in and out from that reservoir. If this reservoir has a known amount of helium (He) in it, and a subject breathes the air in that reservoir through several breathing cycles, the helium will become distributed in the reservoir and in the subject’s respiratory system. Helium is not absorbed into the body from the lungs, so the total amount of helium at the beginning of a test (He_i) will equal the total amount at the end of the test (He_f)—it will just be distributed over a larger volume and therefore be less concentrated. The following formula quantifies that relationship:

Initial amount of He = final amount of He

1030

We start the subject breathing from the reservoir at an end-tidal breath, and we end the breathing from the reservoir at an end-tidal breath. Since the V_res does not change, the decrease in helium concentration [He] enables us to calculate the subject’s functional residual volume (FRV), which is the ERV + RV.

V_res × [He]_i = (V_res + FRV) × [He]_f

Since we can measure the ERV with the flowmeter, we can subtract the ERV from the FRV to obtain the RV.

Why is the RV important? Referring to Figure 48.9, you will see that for a normal person the ERV is about 1,000 mL and the RV is 1,000 mL. Thus the FRV is 2,000 mL, but the tidal volume is only 500 mL; this means that the air that reaches the alveoli (the actual gas exchange surfaces) with each breath consists of only 500 mL of fresh air diluted by 2,000 mL of stale air. The maximumP_O2 in this mixed air is much below theP_O2 of the outside air, and because of the tidal ventilation pattern, the P_O2 in the alveoli is steadily dropping during the breathing cycle. The RV is important because it contributes to the FRV and to the dilution of the O₂ in the inhaled air. Any disease or condition that increases the RV (such as emphysema or pulmonary fibrosis) compromises a patient’s respiratory ability. Similarly, considering the mixing of fresh air with the FRV, you can understand why reductions in tidal volume can be a problem—and therefore why patients recovering from surgery are encouraged to breathe deeply, even if it hurts.