Blood clots. A blood vessel could possibly be blocked by an embolus. This is the very small mass of debris that moves in the bloodstream. Or it may be blocked by a thrombus. That is a blood clot. Most important lymphedema (exceptional): Many people are born with no sure lymph vessels https://www.sgvascularctr.com/a-comprehensive-approach-to-lymphoedema-treatment-a-vascular-surgeons-perspective/