Horizon of Breathing, Reflections on Key Meditaiton Text

Sometimes, we are not aware that we are breathing, we just breathe. This form of breathing is automatic.  About 500 milliliters of air comes in and out without our awareness and it is known as tidal volume.  When these 500 milliliters of air is out, about 2300 milliliters of air still remains in the lungs and when it is so, the author refers to it the horizon of breathing, Sekida, 59.

An additional 1100 milliliters of air can be forced out from the lungs, and it will be clear given what I have stated before this additional air refers to the expiratory reserve volume.

The author refers to horizon of breathing again, but since we are not aware of it, when air is in or is out, that is the tidal volume, and how can we use it as reference point, and say above or below the horizon of breathing?  for a reference ought to be, isn’t?, sort of things that we know, or aware of it, ?

Reference
Katsuki Sekida, Zen Training: Methods and Philosophy
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