Breathing unites respiration with ventilation. While respiration occurs in all organisms, ventilation does not. The coupling of respiration to ventilation increases an organism’s capacity for complexity. The price of complexity, however, is constrained adaptation—limitation to a specific environment where respiration and ventilation remain possible.
Breathing removes gaseous metabolic wastes produced by respiration—mostly, but not exclusively, carbon dioxide and water. Breathing moves nitrogen and oxygen into the body: nitrogen to hold open alveolar spaces required to match ventilation to blood perfusion; oxygen to be sequestered in blood before transfer into tissues for reduction to water during adenosine triphosphate (ATP) generation by mitochondrial electron transport. As all gas transfers follow pressure gradients, differences in partial pressure determine how a gas moves throughout a body.
A body’s internal ecology— its milieu intérieur—depends on respiration coupled to ventilation: respiration to generate energy inside mitochondria, ventilation to move gasses around. Aerobic biology ultimately depends on this sequence: oxygen intake, oxygen sequestration, oxygen diffusion, reactive oxygen species (ROS) generation from electron transfers in mitochondria during carbohydrate and lipid oxidation, oxygen reduction during ATP generation, carbon dioxide and water production, carbon dioxide and water elimination, antioxidant correction of any collateral damage.
So imagine you’ve taken your last breath. What happens? The simple version goes something like this:
As ventilation ceases, carbon dioxide accumulates in tissues and blood. Tissues become more acidic and electrochemically unstable. Respiration shifts from the efficient aerobic to less efficient anaerobic form. Nitrogen- and oxygen-filled alveoli accumulate carbon dioxide; they also collapse as the last oxygen is extracted and diaphragm motion ceases. As oxygen intake ceases, blood oxygen reservoirs decrease. As long as the heart keeps beating whatever oxygen is sequestered in the blood will be extracted by perfused tissues. After oxygen drops below the supply threshold required to support ATP formation at Complex IV in mitochondria, electron transport chain efficiency collapses and ROS production increases. As oxygen is too low to receive any generated electrons, the all-important electrochemical or chemiosmotic gradient that supports electron transfer and hydrogen ion transport across the inner mitochondrial membrane collapses. With hydrogen ion accumulation mitochondrial acidity increases and calcium regulation is disrupted. Generated reactive species overwhelm any remaining antioxidant capacity. Unchecked, chain reactions with fats, proteins, and nucleic acids further destroy tissue capacity to regulate and repair damage.
The net effect of stopping ventilation, then, is cessation of respiration. Artificial ventilation must be imposed if any possibility of salvaging undamaged tissues exists. In the absence of such an intervention, biological collapse ensues for us—but not our microbiomes. Our microbiomes, much of which is anaerobic to begin with, continue to respire until their unique metabolic needs can no longer be met either.
