Dr. Robert Sapolsky at Stanford University wrote this book explaining how our stress mechanism that was essential for our survival in the world of predators and prey, but is now killing us in our modern world of psychological stressors functions. The concept is:
You wake up from a resting heart rate and mild levels of nerve activity and a lion pounces on you. You jump immediately as nerve impulses shoot through your body spiking your heart rate and injecting tons of hormones into target organs. Your bladder relaxes, your leg muscles tighten, and you may vomit that heavy meal you ate. Your body is losing the dead weight as you start to run. A large chunk of your side is missing and it appears as if you can see your intestine but you can't feel any of it because receptors are being blocked. You can worry about pain when you're not going to die immediately.
Now you're staring at a blank word document. You may want to vomit but I hope you don't piss yourself. Tons of hormones that are toxic to your body at elevated levels are pumped to target organs. Your heart races and you break out in a cold sweat. The response is not nearly as severe but then it lasts for the 6-12 hours it takes you to crank out that 20 page paper. You don't increase your blood flow to increase circulation of oxygenated blood and blood containing elevated levels of hormones pools in various places in your body. This response is triggered by many common things throughout your day, causing your body to have little life-or-death crises. The sound of your alarm, the minute hand on the clock, forgotten assignment slowly accumulate as little detrimental effects to your health.
The activation of your sympathetic nervous system leaves levels of hormones that won't allow your parasympathetic nervous system to put you to sleep. You develop insomnia, your immune system struggles, and your body tries to store energy for a crisis in the form of fat. Now your heart labors harder every time you activate these stress responses and it either starts to try to build muscle causing a thickened septum wall and smaller chambers with less blood flow, or there are platelet globules that formed on tears in the artery due to heightened pressure and get stuck in the valves or the brain.
It's an extreme spiraling cycle of stress and increasing incapability with handling it.