The purpose of this study was to prove the mechanism of mineralization, when hydroxyapatite (HAP) is formed in blood plasma. These observations were substantiated by in vitro simulation of HAP crystallization in the plasma of healthy adults in a controllable quasi-physiological environment (T = 37°C, pH = 7.4) and at concentrations of dissolved Ca and P ions that resemble those in the blood of healthy adults. Another objective of the study was to investigate the role of Mg2+, Na+Cl - and serum albumin, the main blood protein, in the formation of calcium phosphate. Mineralized aortic and mitral valves of the heart were obtained in operations on patients with rheumatic or septic acquired valvular disease. Macroscopic and histological analyses of these samples were conducted. Primary nanocrystals of HAP formed in the blood may take part in the mineralization of both cardiac valves and vessels. The penetration of HAP nanocrystals may be caused by breaks in the endothelium of blood vessels. The lowering of pH may result from substantial degradation in the valve tissue. The experimental data make it possible to conclude that albumin and Fetuin-A are structural proteins that are responsible for subsequent incorporation of HAP nanocrystals in newly formed bone tissue.