Final answer:
Thrombosis in the axillary or subclavian vein is typically caused by venous stasis, thrombophilia, or endothelial damage that leads to thrombus formation, potentially impeding blood flow and increasing local blood pressure.
Step-by-step explanation:
Thrombosis of axillary or subclavian vein usually occurs due to several factors including venous stasis, which is when blood in the veins remains stationary for long periods. This condition can occur during long airplane flights or situations where mobility is reduced. Another cause can be thrombophilia, also known as hypercoagulation, where there's a tendency to form thrombosis. This may be due to genetic factors or can be acquired from conditions such as lupus, immune reactions to heparin, polycythemia vera, thrombocytosis, sickle cell disease, pregnancy, or obesity. Furthermore, damage to the vessel's endothelial lining can activate the clotting mechanism, leading to the formation of a thrombus. A thrombus that adheres to the vessel wall and decreases the flow of blood is known as a mural thrombus, while one that totally blocks the flow is an occlusive thrombus. These thrombi can impede blood flow and cause a local increase in blood pressure.