Blood Vessels, Selective Permeability of

the mechanism that regulates exchange between the general internal environment of the body—the blood—and the immediate nutritive environment of the organs and tissues—the interstitial, or extracellular, fluid.

The anatomic basis of blood-tissue exchange is the endothelium of the capillaries and precapillaries. The term “blood-tissue barrier” was introduced by the Soviet physiologist L. S. Shtern in 1929. Selective permeability of the blood vessels also performs a protective function by preventing the transfer of harmful and foreign substances from blood to tissues and from tissues to blood. This explains the unequal distribution of many substances in the body, as well as the ineffectiveness of treatment with certain medications. The adaptability of blood vessel permeability to conditions of the external and internal environments is one of the most important conditions for maintenance of homeostasis (constancy of the internal environment), the stability of physiological functions, and protection from infections and intoxications.