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In a healthy body, sudden stress or fear (and to a lesser degree, standing up) causes the body to secrete norepinephrine and epinephrine (aka adrenaline) for that classic "fight or flight" response, causing increased heart rate and contraction of the blood vessels in the lower body to help keep blood circulating adequately to the heart and brain. Those excess hormones are quickly cleared out of the bloodstream once the danger is past. In these illnesses, though, the hormones are not released in the right amounts and are not cleared away quickly. Instead, the vessels in the lower extremities don't contract as they should, with blood pooling in the feet and legs, leaving too little blood flow to the heart and the brain, and heart rate stays high long after it should have returned to normal.
Hormones also regulate blood volume so that people with these illnesses don’t have enough blood in their bodies, which further contributes to OI (10). Because of these dysfunctional hormones, those with OI have low levels of sodium and cannot hold onto fluids. Even if you drink water constantly, your blood volume remains low. And when you do get dehydrated, that triggers the release of more epinephrine, making OI even worse.
All of this is a part of the autonomic nervous system, the way the body regulates automatic functions, like heart rate, blood pressure, digestion, and temperature. With the endocrine system not responding correctly, these functions that are normally automatic don't work as they should. As a result, most patients with these illnesses struggle with poor temperature regulation, poor vascular control (i.e. the way the veins should contract and expand automatically), and poor regulation of BP and HR (OI).