Intraluminal resistance to gas transport between the microcirculation and tissue was neglected for a half-century following the early work of Krogh. In recent years it has come to be understood that this neglect is seriously in error. This paper reviews the background for the long period of misdirection, and progress in placing the simulation of gas transport processes on a more accurate, quantitative basis.
In this article, we consider a parametric survival model that is appropriate when the population of interest contains long-term survivors or immunes. The model referred to as the cure rate model was introduced by Boag 1 in terms of a mixture model that included a component representing the proportion of immunes and a distribution representing the life times of the susceptible population. We propose a cure rate model based on the generalized exponential distribution that incorporates the effects of risk factors or covariates on the probability of an individual being a long-time survivor. Maximum likelihood estimators of the model parameters are obtained using the the expectation-maximisation (EM) algorithm. A graphical method is also provided for assessing the goodness-of-fit of the model. We present an example to illustrate the fit of this model to data that examines the effects of different risk factors on relapse time for drug addicts.cure rate, long-term survivor, generalized exponential distribution, EM algorithm, goodness-of-fit,
An in vitro artificial capillary system has been developed for use in examining the O2 transport properties of free hemoglobin and erythrocytes. The artificial capillary was constructed by casting a thin film of transparent silicone rubber around a strand of tungsten wire that was 24 micron in diameter. After the rubber had polymerized, the wire was removed. Typical dimensions of the silicone rubber film were 170 micron thick, 1 cm wide, 5 mm long in the direction of flow, and a 27-micron lumen diameter. The artificial capillary bed was mounted on a microscope and perfused by either hemoglobin solutions or cell suspensions. Fractional saturation was measured as a function of axial position by a dual-wave-length microspectrophotometer, and the flow rate was regulated precisely by a syringe pump. O2 release experiments were carried out by suffusing the gas space surrounding the artificial capillary film with 100% N2 and perfusing with an oxygenated sample. O2 uptake experiments were carried out by suffusing the gas space with O2-N2 mixtures and perfusing with deoxygenated samples. The axial velocities were varied from 3 to 15 mm/s. The residence time (the time a particular red cell or hemoglobin molecule has spent in the capillary) for 50% oxygenation of a 4 mM (heme) deoxyhemoglobin solution was approximately 0.05 s at 37 degrees C when the gas space surrounding the capillary contained air. The corresponding time for 50% oxygenation of an equivalent red cell suspension was approximately 0.25 s.(ABSTRACT TRUNCATED AT 250 WORDS)
O2 transport was examined by measuring the fractional saturation of concentrated hemoglobin solutions flowing through an artificial capillary that was approximately 27 micron in diameter and embedded in a silicone rubber film approximately 170 micron thick. The effects of pH, hemoglobin concentration, O2 tension, temperature, and organic phosphate were measured and analyzed quantitatively by a rigorous mathematical model that included the geometry of the capillary in the silicone film, parabolic flow velocity distributions inside the lumen, and cooperative O2 binding by hemoglobin. The rates of both oxygenation and deoxygenation were limited by diffusion and governed by the magnitude of the O2 gradient between the intracapillary fluid phase and the external gas space. In uptake experiments, O2 flux is determined primarily by the external O2 tension (16-160 mmHg in our experiments) because the internal O2 pressure is kept small due to chemical combination with hemoglobin. In release experiments, the external O2 tension is maintained at zero, and the transport rate is determined by the intracapillary partial pressure of O2 that is proportional to the O2 half-saturation pressure of hemoglobin value of the hemoglobin sample. As a result, factors that change the affinity of hemoglobin for O2, such as pH, temperature, and organic phosphate concentration, influence strongly the rate of O2 release but have little effect on the rate of O2 uptake. These properties are physiologically advantageous, since a decrease in pH or an increase in temperature during exercise increases both the rate and extent of deoxygenation while not altering the kinetics of oxygenation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.