THRESHOLD BEHAVIOR OF THE BLOOD-COAGULATI ON SYSTEM AT CHANGING CALCIUM-CONCENTRATION

Dependence of the citrate human blood plasma clotting kinetics on free calcium concentration under its titration with calcium has been studi ed in vitro. Activation of factor XI is shown to be independent on cal cium, while of thrombin concentration increases non-linearly at calciu m concentrations in the range 0.2-0.3 mM. Kinetics of the thrombin gen eration fits well by the exponential function. Power indexes of the ex ponents rise steeply as calcium concentration increases from 0.2-0.5 m M and reach plateau at higher concentrations. At free calcium concentr ations under 0.2 mM the thrombin level does not increase and remains l ower than 10 pmol/ml as seen by our measuring system whose sensitivity threshold is surely less than 10 pmol/ml. Thus, the blood coagulation system behaves in a threshold manner under changes of calcium concent ration. The threshold concentration of free calcium is equal to 0.2 mM .