Breathing

Pumping Air.

    The role of breathing is to bringing oxygen (O₂; some kind of fuel) to our blood, and dispose of carbon dioxide (CO₂; some kind of waste). To achieve such a task, we need a pump to force the air into our lungs and we need a control center, the brain, to sense and adjust how much we need to breath.

    The main muscle for breathing is the diaphragm. Contraction of the diaphragm lowers the floor of the rib cage which increases its volume. This produces a negative pressure in the lung. Because the thorax is a rigid and sealed cage, the only opening allowing air intake to equilibrate this pressure change is the upper airway.

    During inspiration, other muscles may participate, to the expansion of the rib cage: the external intercostal's, the sternocleidomastoids, the anterior serrati, the scaleni and the scapular elevator. They play a minor role during normal breathing, but are strongly solicited during deep inspiration.

    If inspiration is an active process, expiration however is mostly passive. Relaxation of the diaphragm makes the rib cage to return to smaller volume, its rest volume. This increases the pressure in the lung and pushes the air out. Like for inspiration, other muscles may be involved in forced expiration. These are mostly the abdominal muscles and the intercostal's.

Respiratory Capacity.

    In the graph bellow, we see an outline of the changes in lung volume during normal breathing, two maximal inspirations and two maximal expirations. These volumes are an assessment of the respiratory capacity for an average-size man; in woman, these values could be 20 to 25% lower; in athletes, on the other hand, they could be higher.

    The tidal volume (~ 0.5 L) is the volume of a normal breath. If we add the additional air that we can add at maximal inspiration (~ 3 L), we have an inspiratory capacity (~ 3.5 L). If we add the maximum expiratory volume (~ 1.1), we reach a vital capacity of ~ 4.6 L, which is the maximum air exchange one can perform . To these number, we can add the residual air remaining in your alveoli (~ 1.2) and we obtain a total lung capacity of  ~ 5.8 L. This residual air volume includes a dead space (~ 0.15 L), contained in the upper airways, that does not participate in gas exchange.

    In addition to the tidal volume, there is also the frequency of breathing that will determine air exchange. If we multiply the tidal volume by the frequency of breathing, we obtain the respiratory rate. For example, if a normal tidal volume is 500 mL, minus the volume of the dead space, 150 ml, we obtain an alveolar ventilation of 350 mL and if we breath at a frequency of 12 breaths per minute, we will have an alveolar ventilation of 4.2 L per minute.