Arterial occlusion. Coloured chest X-ray angiogram showing an occlusion in the left subclavian artery (red, upper centre). An occlusion is a closing or obstruction of a hollow organ. The occlusion is a thrombosis, which is a blockage caused by a blood clot. The arch of the aorta is at centre, with the subclavian arteries (black, far left and upper centre) branching off it. The subclavian arteries supply the arms with blood. The artery on the right is thinner and fainter than the one on the left. This is because a thrombosis at its base has blocked normal flow of blood. If this is not treated and the clot moves to the heart or brain the patient may suffer a heart attack or stroke.