Blood pressure needs to run in an optimal and restricted range. Over that, you get kidney problems, optical nerve degeneration....it's just not good. If the BP is too low, the kidneys can't do their job to filter your blood and you produce too much ADH (anti-diuretic hormone) which keeps your fluids and electrolytes in the restricted range that THEY'RE supposed to be in. Bottom line, your body is a conglomeration of specialized cells that agree to work together to keep you upright and smiling. Those various cells comprise a 'system', in the true sense of that complicated concept. Systems must be in balance. When one component goes missing, or is running poorly, it affects several other components, and almost always not in a complementary way.
The term for the accumulation of fluids in the abdomen is properly called 'ascites'. Pronounced (sorry) ass -SITE-eez. A sure sign that either the heart, or the kidneys, or regrettably both, are not working well is ascites, although it can also be the endocrine system (all those important hormones that regulate our bodies, including ADH that I mentioned).
Some of you reading may recall, if it ever happened, that your cardiologist had you up on the examination table and asked to listen to your chest, and then had you lie on your back. In my case, he lifted my trouser cuff to bare my ankle and lower leg and poked my tissue there. He was looking for swelling in the lower extremities which is an easy tell-tale for cardiac deficiency, among other problems.
It's no wonder we need to have specialists in all these fields. After getting a medical degree, there are years ahead specializing in what is essentially yet another PhD!