Respiratory system

The respiratory system is the biological system of any organism that engages in gaseous exchange. Even trees have respiratory systems, taking in carbon dioxide and emitting oxygen.

The human respiratory system consists of:

• nasal passages, in which air is moistened and in which the sense of smell operates
• the pharynx, a common area in the back of the mouth through which air, food and drink passes
• the larynx, or voicebox
• the trachea, an air tube that connects with the bronchi
• bronchi, tubes that carry air to and from the lungs
• bronchioles, branches of the bronchi which distribute air to the alveoli
• alveoli, terminal sacs in which gas exchange occurs

The alveoli and bronchioles form the lungs. Air is moved in and out of the lungs by the movements of the rib cage and diaphragm, which expand the lungs to draw in air and compress the lungs to drive out air. A simple model of how the lungs are inflated can be built from a bell jar.

There are four basic measured lung volumes:

• The tidal volume (TV): the volume of air an individual is normally breathing in and out.
• The inspiratory reserve volume (IRV): the maximum volume of air that can be inspired in addition to the tidal volume.
• The expiratory reserve volume (ERV): the maximum volume of air that can be expired in addition to the normally expired air.
• The residual volume (RV): the amount of air that is always in the lungs and can never be expired (i.e.: the amount of air that stays in the lungs after maximum expiration).

From these volumes, several important capacities are also calculated:

• The total lung capacity (TLC): the total volume of the lung (i.e.: the volume of air in the lungs after maximum inspiration).

 TLC = IRV + TV + ERV + RV

The functional residual capacity (FRC): the amount of air that stays in the lungs during normal breathing.

 FRC = ERV + RV