Comparative facilitated transport of oxygen.

Numerical solutions were obtained for a model of facilitated transport of O2. The dependence of the facilitated flow on the concentrations of O2 at the two boundaries of the transport path was studied. The numerical values of the parameters correspond to the adult Ascaris lumbricoides and to vertebrate red striated muscle. A global control principle is formulated. This states that for every fixed O2 concentration at the low concentration boundary there exists an O2 concentration at the high concentration boundary for which the facilitated flow is maximum. The collection of these maxima makes possible the existence of a global adjustment of the facilitated transport in contradistinction to the mere presence of a local maximum. The ranges of the pairs of boundary O2 concentrations thus defined and for which the facilitated flow is within 70% of its attainable maximum were found to coincide with the physiological ranges of boundary O2 concentrations for the Ascaris and vertebrate striated muscle. This phenomenon has the character of a graded compensatory mechanism to hypoxia. It is an intrinsic property of the carrier transport system and does not depend on sensors for hypoxia.