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.
Persistent gas bubbles able to traverse capillaries can be prepared from a slowly permeating gas or with a mechanical structure surrounding a gas phase. If they are permeable to gases, such bubbles will carry O2 from the lungs to the tissues via the blood stream. Using a mathematical model based on physical laws, we present simulations of the behavior of bubbles stabilized by a slowly permeating gas (gas X). We show that the bubble persists longer if the tissue and venous blood contain N2 to dilute gas X and slow its outward diffusion. A 6-microns -diam bubble carries 0.11 pl of O2 during the breathing of pure O2, so 4.6 x 10(8) bubbles/ml in the blood will supply a normal arteriovenous difference. In conditions used for hyperbaric O2 therapy, a bubble carries approximately 0.26 pl of O2. Stabilized bubbles have the potential to transport O2 efficiently; they release O2 to tissue at high PO2 and require injection of only small amounts of a foreign substance.
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