Hello Tarah
When you do a backbend you are opening up your heart - in many ways. On an emotional level we clear *stuff*. Being more emotionally light is a way to regulate and normalize your blood pressure (over time with practice).
On a physical level we are stretching the heart muscle giving it exercise. Cobra also requires a very strong or intense lower body muscle engagement from the hips down through the toes. To contract muscles means that you work harder because your muscles need some fuel to keep up, and of course the effects on your circulation around your working areas is like a tourniquet which means there is more work being done by the heart to pump your blood around. Your cardiac output increases and so does your stroke volume.
Other ways that blood pressure is normalized is by the effect on the body’s systems. In this pose as in many others there is deep work occurring at a cellular level (to cleanse and open up the circulation) and also at a systemic level normalizing regulatory hormones for better functioning.
All these benefits happen in some way, shape and form with most yoga poses.
I hope that helps steer you in the right direction.
Namaste
Gabrielle 
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