[PHysiological function of veins and their intrinsic regulation].

Physiological functions of the veins include: 1. Blood collectors with flow-rectifying valves; 2. Selective barrier function, venular reabsorption of fluid; 3. Capacity function and redistribution of the blood volume, pressure-damping; 4. Maintaining the filling pressure of the heart by adequate venous return at varying cardiac output; 5. Increasing orthostatic tolerance of the body; 6. Postcapillary resistance; 7. Angiogenesis (in venous side of the microcirculation); 8. Secretion of bioactive substances by the endothelial and smooth muscle cells; 9. Lymphocyte homing (high-endothelial venules); 10. Cooperation between venular endothelium and the polymorphonuclear leukocytes (margination and rolling); 11. Enhanced inhibition of platelet aggregation and thromboembolic reactions; 12. Other special functions (e.g. facial vein: contribution to cranial thermoregulation; internal jugular vein: antisiphonage "device", etc.). Recent results suggest: 1. that intrinsic biomechanical adaptation of the systemic vein wall may occur in response a) to fast pressure changes within the physiological range by myogenic capacity autoregulation of the lumen; b) to long-term physiological haemodynamic loading by increase in total capacity and in short-term myogenic autoregulation range without marked changes of the wall thickness; c) to chronic arterialization with compensatory increase in wall-thickness reducing the extreme mechanical stresses induced by arterial pressure (such changes also imply fibrotic transformation of the wall-structure); 2. EDRF physiologically released by venous endothelium seems to be an important factor protecting the veins against non-desirable increases of smooth muscle tone (e.g. at low Mg-ion level of extracellular fluid, or in case of axial overstretch of the vein, etc.).