The lecithin-sphingomyelin ratio is a test used to assess fetal lung maturity. Lecithin and sphingomyelin are both substances that make up surfactant, a soap-like substance that lowers the surface pressure of the alveoli of lungs. When dealing with premature babies, it is important that the lecithin-sphnigomyelin ratio is at least 2.0, which suggests fetal lung maturity. This ratio is obtained by acquiring amniotic fluid through a needle and helps doctors to assess if a premature baby is ready for delivery.

Tidal volume is typically 500 milliliters, and is the amount of air that is moved by normal respiration; however, some of this air does not make it to the alveoli in order to take part in gas exchange. This air may simply remain in the trachea, bronchi, and bronchioles, where gas exchange cannot take place. As a result, a portion of the tidal volume is considered "dead space" volume.

Dead space volume is a small fraction of the tidal volume, and is usually around 150 milliliters of air.

The vital capacity is defined as the maximum amount of air that a person can exhale after a maximum inspiration. This value does not incorporate the amount of air that remains in the lungs after the maximum expiration. This remaining volume of air is called the residual volume, and is included in the total lung capacity.

Tidal volume (TV) is the amount of air moved with each unconscious breath. Inspiratory reserve volume (IRV) is the volume of additional air that can be forcefully inhaled, and expiratory reserve volume (ERV) is the additional volume of air that can be forcefully exhaled. After a forced inhalation, the lungs contain a volume equal to the lung capacity. After a forced exhalation, a volume equal to the vital capacity has been exhaled and a volume equal to the residual volume (RV) remains in the lungs.

To sum it up using equations:

The inspiratory reserve volume can be thought of as the amount of air that can be brought into the lungs consciously after an unconscious inhalation. This value does not include the tidal volume that is brought into the body by normal breathing. The maximum volume of inhaled air from rest is equal to half the tidal volume plus the inspiratory reserve volume.

Total lung capacity is the sum of the tidal volume (normal breathing), inspiratory reserve volume (additional volume from forced inhalation), expiratory reserve volume (additional volume from forced exhalation), and residual volume (air that cannot be forcefully moved from the lungs).

Depending on physical ability and gender, the inspiratory reserve volume is between 1900 and 3000 milliliters of air. Women typically have a lower inspiratory reserve volume compared to men.