system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how doesn't high blood pressure from exercise destroy our bodies

Everyone says that high blood pressure is destructive on blood vessels and can lead to heart attack and stroke. When you do physical exercise, your blood pressure rises for relatively long periods of time. However, physical exercise... reduces your chance of having a heart attack? How does that work?
gpt
A healthy body with healthy blood vessels is designed to accommodate relatively large increases in pressure and flow from time to time. This is the whole basis of "fight or flight", if your body breaks down because it can't withstand the blood pressure and heart rate when you're running from a bear, well that's not very good for humans as a species under natural selection. 

The problem arises when you have damage to your heart and vessels from chronic problems. The main culprit is atherosclerosis due to high cholesterol and fats. Without going into too much detail, cholesterol and lipids will slowly mess up the flexibility of your cardiovascular system. It makes your vessels physically narrower, it will make arteries less stretchy, it will decrease blood flow to your heart itself. Eventually, your whole cardiovascular system will be under stress and high pressure even at rest. Ultimately your heart gives up, or you do something suddenly intensive and your heart can't keep up anymore. This is why lots of people die from heart failure in old age, or really unhealthy people just drop dead when they suddenly exert themselves to shovel snow in their driveways.

Exercise will lead to better energy management by your body in the long term. It will promote better utilization of fats for energy and prevent the accumulation of cholesterol in your small vessels. Good exercise and nutrition will change the fundamental energy balance processes in your body and reduce the risk of cardiovascular events overall. High blood pressure at rest is one of the earliest markers of coronary artery disease, but thankfully also the most manageable with diet, exercise, and pharmacologic therapy.