The human body is no different from an internal combustion engine. In that, it burns carbohydrates in the presence of oxygen to release energy, producing carbon dioxide as a byproduct. The only difference is the body’s use of enzymatic reactions, which makes the burn much more controllable and puts Spontaneous Human Combustion (SHC) in the realm of pseudoscience.
Humans’ respiratory system is responsible for uptaking oxygen from the environment and eliminating carbon dioxide.
Oxygen and carbon dioxide have different water solubility, which determines their behavior and factors influencing their interactions with the respiratory system.
Carbon dioxide is highly water-soluble. Consequently, it can penetrate alveolar-capillary (AC) membrane with ease, not requiring a high pressure difference (gradient) across the membrane. As a result, most of the carbon dioxide delivered to the lungs by venous blood easily crosses to the alveoli. Its removal from the body is limited by factors affecting the movement of the gas from the alveoli to ambient air (ventilation). The amount of air moving through the lungs, determined by breath size (tidal volume), frequency (respiratory rate), and amount of wasted ventilation (dead space), would affect carbon dioxide transport. Oxygen’s behavior is different. Unlike carbon dioxide, it is not very soluble in water, making passing the alveolar-capillary membrane a limiting step. Consequently, fractional oxygen concentration (FiO2), meaning airway pressure (MAP) – the force driving the oxygen through the membrane and the surface available for the gas transfer are variables affecting oxygen transport in the lungs.
While the concepts discussed above may appear removed from clinical practice, they are the foundation for understanding different types of respiratory failure and ventilator management.