Tidal volume, simply stated, is the amount of air that is displaced during a standard inhalation or exhalation, in other words, the amount of air breathed in during a normal breath. It, along with all other measures of capacity in respiratory physiology, are measured using a device called a spirometer. In order to truly comprehend tidal volume, however, it is important to develop a more thorough understanding of respiratory physiology.
The act of breathing functions to exchange used air from within the body with new air from the outside. The new air, which is dense with gases, is then used for respiration. Respiration, which is often incorrectly used interchangeably with the term breathing, differs in that it refers to the actual exchange of gases rather than the movement of air.
Breathing occurs due to a differential pressure gradient between the body and the atmosphere. This occurs physiologically through the diaphragm muscle located at the base of the lungs contracting. It contracts and increases lung volume, hence decreasing pressure. When the internal pressure falls below external pressure, air flows into the lungs until pressure is equalized. This air is then used during respiration, which is necessary for sustaining life.
If an individual were to inhale as deeply as he or she could, the volume of air held in the lungs would be termed inspiratory capacity (IC). A person's IC is composed of the tidal volume plus the extra volume of air, known as inspiratory reserve volume. Tidal volume is generally around 500 ml (16.9 ounces) but may vary based on body size and other physical conditions. The reserve volume may be voluntarily acquired by an excessive contraction of the diaphragm or may be utilized during physiological stress to aid in obtaining more oxygen.
Exhalation, similar to inhalation, consists of the standard tidal volume as well as the potential for an expiratory resolve volume (ERV). The ERV can be thought of as the amount of air that is pushed out after a normal breath if you were to exhale as much as possible. Another commonly used term is vital capacity (VC). VC is equal to tidal volume plus expiratory reserve volume plus inspiratory reserve volume. VC is used as a measure of the potential volume of air that would flow into and out of the lungs if they were stressed to maximum capacity.
Other terms regarding lung capacity are residual volume and total lung capacity. Residual volume describes any lung volume that does not benefit breathing or gas exchange, such as dead space between the alveoli in the lungs. Air may fill these spaces, but the air is not in a position to be utilized for any physiologically beneficial functions. Total lung capacity, as its name suggests, is the vital capacity plus the residual volume.