How do we breathe?
The process of breathing in and out
Breathing in at rest is an active process (costs energy) – the diaphragm contracts (moving it downwards) and the external intercostal muscles help pull the ribs up and outwards. The cerebral cortex (this is some brain stuff for anti-nervous system people) facilitates this process. There is also accessory muscles (helpers) working, being the sternocleidomastoid (just think neck), scalenes group (neck) and pectoralis minor (chest) – these are way more active during heavy breathing.
Breathing out at rest is a almost entirely a passive process. This is due to muscles relaxing (for example the diaphragm, moving it upwards), elastic recoil of the lung alveoli (this is where gas exchange of oxygen, CO2 etc occurs between the lungs and the rest of the body) and stretched elastic tissues in the chest wall that pushes the air out.
Breathing in is basically like throwing a ball downtoward the ground, and it bouncing right back up again.You are controlling the throw, but you cannot control the fact that it wants to bounce up again (physics stuff).
During heavy breathing energy is used during both situations. During active expiration the internal intercostals (except interchondral part) will pull the ribs down and the abdominal muscles will help the diaphragm to be pushed superiorly (reducing the cavity volume and thereby force air out).
A little physics
During inspiration what we call the intrapulmonary pressure (pressure inside the lungs) is reduced by 1 compared t the standard atmospheric pressure at 760 mm Hg, this because of the increase in volume. During expiration the pressure is increased by 1 compared to the standard, because of the decrease of volume. This is explain by Boyle’s Law, which is C1*V1=C2*V2, but we will get more into all the physics stuff later and further explain the pressures inside the lung while doing that as well.
Fun Fact: It is also impossible to commit suicide through trying to stop breathing (please do not attempt this though) as when oxygen levels drops, the CNS will shut down and you will pass out, this is followed by it making you start breathing again automatically which is stimulated by the breathing centers – located in the brain stem.