Heart disease often relates to coronary artery disease in this context. Coronary artery disease is a condition where a major blood vessel which leaves the heart with oxygen-rich blood is constricted. This means a clot or closure is more likely and either of these can cause a heart attack. That is where the phrase "having a coronary" comes from.

Being a heavy smoker for a long time increases the odds enormously of that happening to you.

One reason for this is that cigarette smoke contains carbon monoxide, which binds with hemoglobin. Hemoglobin is the molecule found in red blood cells which transports oxygen through the body, including to the heart. The less oxygen the heart receives, the more chance of a heart attack.

Nicotine is another substance which reduces blood oxygen and contributes to other potentially harmful conditions. It increases blood clotting which can put you at risk of a heart attack. Nicotine is also responsible for more subtle, dangerous effects. It encourages the growth of fatty deposits on the arteries. This constricts blood flow and hardens the blood vessel.

Smoking decreases the amount of high density lipoprotein, or HDL, which is the "good" type of cholesterol. Losing this encourages fatty deposits to grow. This condition, which is a major factor in heart attack risk, is called atherosclerosis.

If an artery is reduced in diameter, this increases blood pressure, making it more likely that a weakness in the artery wall, or "aneurysm", will rupture. That in turn causes a stroke, when the brain starved of oxygen. This is why you often hear about strokes and heart disease in the same sentence.

If an artery is hardened, it is not so able to withstand the usual stresses and strains of its function. A blood vessel acts like a hose but is different too. A blood vessel carries fluid like a hose but unlike a hose it is on all the time. The fluid can only be carried if there are no holes in the blood vessel. If the blood slows or stops, this can cause immediate health issues, even if it is only a temporary slowing down or stoppage. Tissues need to receive blood constantly, else they quickly die.

Cigarette smoking also has the overall effect of contributing to heart disease. It causes physical effects that lower health. Shortness of breath, reduced oxygen and other effects which make exercise unpleasant and harder. Lifestyle choices associated with smoking reduce overall fitness.

Lack of exercise resulting in increased body fat percentage and weight gain increases the risk of heart attack and heart disease even more. The body is unable to cope with strains which would otherwise be minor. It cannot withstand serious biological shocks when a heart attack occurs. This means the attack is likely to be fatal.

Long term, one pack a day smokers have up to four times the chance of developing coronary heart disease than non-smokers. Quitting immediately improves your odds. Your circulation improves after three months and after a year the odds are half that of a smoker. After five to fifteen years, you have the same odds as someone who has never smoked.

It's never too late to stop smoking.