Bubbles in circulating blood: stabilization and simulations of cyclic changes of size and content

Surface tension, blood pressure, and inherent unsaturation due to O2 metabolism promote diffusion of gases out of bubbles in the bloodstream. We review the mechanisms that can overcome the absorptive tendencies so small spherical bubbles can persist. One general type of stabilizer is a mechanical structure at the gas-liquid interface that can support a negative pressure so that gases inside can be in diffusion equilibrium with their counterparts outside; one possibility for mechanical stabilizers are surfactant films. We show that a slowly permeating gas is analogous to a mechanical stabilizer; it allows equilibration of other gases inside-to-outside by diluting the gases inside. By using numerically solved equations based on physics of diffusion, we demonstrate how nonrigid stabilized bubbles change size as they move through the circulatory system. In small pulmonary vessels, the bubbles enlarge because blood pressure is low, there is no inherent unsaturation, and O2 and N2 diffuse from lung gas into the bubble; these gases diffuse out again in the systemic circulation.