A first principles calculation of the oxygen uptake in the human pulmonary acinus at maximal exercise

It has recently been shown that the acinus can have a reduced efficiency due to a “screening effect” governed by the ratio of oxygen diffusivity to membrane permeability, the gas flow velocity, as well as the size and configuration of the acinus. We present here a top to bottom calculation of the functioning of a machine acinus at exercise that takes this screening effect into account. It shows that, given the geometry and the breathing dynamics of real acini, respiration can be correlated to a single equivalent parameter that we call the integrative permeability. In particular we find that both ${\dot{V}}_{{\text{O}}_2\text{,max}}$ and $P{\text{A}}_{{\text{O}}_2}$ depend on this permeability in a non-linear manner. Numerical solutions of dynamic convection–diffusion equations indicate that only a narrow range of permeability values is compatible with the experimental measurements of $P{\text{A}}_{{\text{O}}_2}$ and ${\dot{V}}_{{\text{O}}_2\text{,max}}$. These permeability values are significantly smaller than those found in the literature. In a second step, we present a new type of evaluation of the diffusive permeability, yielding values compatible with the top to bottom approach, but smaller than the usual morphometric value.     

Correlation between longitudinal,circumferential, and radial moduli in cortical bone: Effect of mineral content

Previous studies indicate that changes in the longitudinal elastic properties of bone due to changes in mineral content are related to the longitudinal strength of bone tissue. Changes in mineral content are expected to affect bone tissue mechanical properties along all directions, albeit to different extents. However, changes in tissue mechanical properties along the different directions are expected to be correlated to one another. In this study, we investigate if radial, circumferential, and longitudinal moduli are related in bone tissue with varying mineral content. Plexiform bovine femoral bone samples were treated in fluoride ion solutions for a period of 3 and 12 days to obtain bones with 20% and 32% lower effective mineral contents. Transmission ultrasound velocities were obtained in the radial, circumferential, and longitudinal axes of bone and combined with measured densities to obtain corresponding tensorial moduli. Results indicate that moduli decreased with fluoride ion treatments and were significantly correlated to one another ($r^2$ radial vs. longitudinal = 0.80, $r^2$ circumferential vs. longitudinal = 0.90, $r^2$ radial vs. circumferential = 0.85). Densities calculated from using ultrasound parameters, acoustic impedance and transmission velocities, were moderately correlated to those measured by the Archimedes principle ($r^2=0.54$, $p<0.01$). These results suggest that radial and circumferential ultrasound measurements could be used to determine the longitudinal properties of bone and that ultrasound may not be able to predict in vitro densities of bones containing unbonded mineral.