Lots of reasons.
Firstly on the heart - because it's pumping at such an increased effort to sustain the pressure the main problems are left ventricular hypertrophy
- a big heart that's working too hard. You can get abnormal rhythms, heart failure
(fluid overload), aneurysms, ischaemic heart disease
(leading to heart disease). You can get stroke, kidney disease, metabolic syndrome
and many other complications.