In pulmonary thromboembolism experiment, we gave thrombin in mice through the tail vein, it blocks the artery of the lung, not any other vein (tail vein). What would be the possible cause of it?
The anatomy of the mouse vascular system is not so different from humans. If you inject the pro-coagulant stimulus into the tail vein, the blood flow will take the emerging embolus towards the heart. The diameter of the blood vessels increases towards the heart so the embolus will not occlude the vessel. Once it passes the right atrium and ventricle, it enters the pumoary arteries where the diameter decreases until the embolus is to large to pass any further.
The only way that I can imagine, that the embolus could get stuck in the venous system is if it got lodged behind a venous valve.
More interesting though, the rate of embolus formation could be a variable of importance in your model. And it could be dependent on the amount of thrombin initially injected.
Thrombin is not always a pro coagulant in vivo. It is one of the factor activating Protein C to activated Protein C. So, it is instead an anticoagulant substance in vivo in most cases. Concerning your model, I have no clear explanation, but believe me I did inject massive loads of thrombin to animals and invariably got them quite bleeding despite the fact all catheters end so on were clogged.