The body responds to stressful situations by activating what is known as the flight-or-fight response. When faced with perceived danger our body prepares either to fight that danger or to run from it. The metabolism speeds up, the heart beats faster, the rate of breathing increases, the muscles contract, the digestive system begins to shut down and your blood will clot faster.
These responses have evolved over millions of years to help us survive danger. A good example of the flight or fight response working at it's best is if you are crossing the road and you notice a speeding car approaching - you quickly run out of harms way. After the danger has past, your heartbeat should return to normal within a few minutes. This is acute stress.
Unfortunately, the same mechanism can also lead to chronic stress. If you are constantly exposed to stressful situations throughout the day (even low level stress) your body does not have enough time between them to return to normal and the very mechanism that is designed to protect us can become harmful. Your arteries may remain constricted, with a consequent increase in blood pressure and blood clots are more likely to form in your coronary arteries. Additionally certain hormones are released into the blood stream when you are under stress such as cortisol and adrenaline which cause artery blockages to build up more quickly and impair your immune system.
Of course it's virtually impossible to avoid stressful situations with the ever increasing pace of modern life, but one key element to improve your health is the ability to respond to stress quickly and efficiently and then be able to relax afterwards to get your body responses back to normal.