SummaryThis study was conducted in order to examine possible anticoagulant properties of the lungs during tissue thromboplastin-induced intravascular coagulation. Rabbit brain tissue thromboplastin (n = 17) or saline (n = 6 + 3) was infused above the right atrium (n = 11 + 3) of the heart or in the arcus aorta (n = 6) for a period of 120 min in non-pregnant New Zealand rabbits. Rabbits infused with tissue thromboplastin responded with significantly (p <0.05) more excessive changes in a number of haemodynamic variables (heart rate, paO2> paCO2, blood pH etc.) compared with rabbits infused with saline.Similarly, the prothrombin time (p <0.05) and the activated partial thromboplastin time (p <0.05) were significantly more prolonged in rabbits receiving tissue thromboplastin compared with control animals. Also the concentration of blood platelets (p<0.05), plasma fibrinogen (p <0.05), antithrombin (p <0.05), and protein C (p <0.05) decreased significantly in thromboplastin-treated animals compared with control animals. In all these haemostatic variables there was a common trend that animals infused with tissue thromboplastin in the arcus aorta responded more excessively than animals infused in the right atrium of the heart, and these deviations were statistically significant for fibrinogen (p <0.05) and prothrombin time (p <0.05). Similarly, animals infused with tissue thromboplastin in the arcus aorta had an increased number of microthrombi in the lungs and kidneys compared with animals receiving tissue thromboplastin above the right atrium.As the lungs are the first pass organ when you infuse above the right atrium the results from this study suggest that the lungs play a key role in protecting the organism against excessive tissue thromboplastin-induced activation of coagulation.