How does a sedentary lifestyle cause hypertension?

Being active increases blood flow, and rate at which the heart beats. This is controlled by several physiological processes, but the ultimate goal is to bring more oxygen to muscles that are doing work. Naturally, once the activity subsides, the physiology returns back to normal. So while blood pressure rises, it eventually returns back to normal.

Hypertension, on the contrary, is a medical condition whereby one's blood pressure is too high for an extended period of time; this eventually cause damage to blood vessels. People with this condition likely stems from damaged blood vessels -- due to diet, lifestyle, or even genetics.