The HPA axis activates cortisol secretion and the renin–angiotensin system, resulting in problems such as hyperglycemia, inflammation, and hypertension, while the SNS stimulates the secretion of the neurotransmitters epinephrine and norepinephrine, worsening HRV and blood viscosity [52,53,54,55]. The gene discussed is REN; the disease is Hypertension.