What causes "POTS" (postural orthostatic tachycardia syndrome) in EDS? 

"Physicians propose that these syndromes occur together due to abnormal connective tissue in dependent blood vessels in those with EDS, which permits veins to distend excessively in response to ordinary hydrostatic pressure. Simply put, this connective tissue abnormally allows excessive amount of blood to pool in these patient’s lower limbs when they stand." (Rowe.et.al. 1999)