At rest, a person inhales and exhales about 0.5 liters of air each time. However, after exhaling these 0.5 liters, you can exhale about 1.5 liters more air by contracting the abdominal muscles. After this, about 1 liter of air remains in the lungs, which can no longer be removed.
Thus, during normal breathing, there is approximately 2.5 liters of reserve air in the lungs, with which the inhaled 0.5 liters are mixed. After the usual inhalation of 0.5 liters of air, you can take about 3 liters more into the lungs with a deep breath, so that during physical work a person can increase the amount of inhaled and exhaled air from 0.5 to 5 liters. But even with intense muscular work, this possibility of a tenfold increase is rarely fully realized; instead, increased breathing usually occurs.
If a person inhales as deeply as possible and then exhales as much air as they can into some kind of volume measuring device, they will be able to exhale about 4.5 liters of air. This value, called the vital capacity of the lungs, is usually increased in trained athletes; in some diseases of the heart and lungs it can be significantly reduced compared to normal. If at least a small amount of air has once entered the lungs, enough of it remains so that the lungs removed after the death of a person do not drown in water.
But the lungs of a stillborn child who has not taken a single breath do not float on the surface of the water. Although we inhale about 500 ml of air each time, approximately only 350 ml reaches the alveoli, since the last 150 ml remains in the wider airways, where no gas exchange can take place between air and blood. During subsequent exhalation, this air comes out first.
The last 150 ml leaving the alveoli with each exhalation also remains in the respiratory tract; this air, although saturated with carbon dioxide, is the first to enter the alveoli during the next inhalation. Thus, each time only about 350 ml of fresh air reaches the alveoli and mixes with the 2500 ml already there. The 150 ml airway space is called dead space.
If the dead space is increased (if, for example, you breathe through a long tube), then the supply of oxygen in the air entering the lungs will quickly dry up and death will occur.