A cigarette contains more than 4,000 chemicals.  The main three are:

Nicotine

• Highly addictive component which reaches the brain in 10–19 seconds.
• It causes emotional changes, including relaxation and decreased tension
• However, it also increases heart rate and blood pressure and constricts arteries. 

Tar

• Tar is inhaled when the smoker draws on a burning cigarette.
• It is a collective name for the many chemicals that form the thick, sticky residue of tobacco smoke.
• It is linked to heart disease, lung disease and cancer.

Carbon monoxide

• Carbon monoxide reduces the ability of blood to carry oxygen around the body.
• It puts additional strain on the heart because it has to work harder to transfer oxygen.
• Up to 15% of a smoker’s blood can be carrying carbon monoxide instead of oxygen.

Cigarette smoking damages the lining of the blood vessels, increases your blood pressure and makes your blood stickier.  Recent studies have shown that changes occur to fat distribution, so smokers – especially women – tend to put more weight on around the waist than a non–smoker.  All of the above increase the risk of coronary heart disease and stroke.

Cigarette smoking also changes the structure of the lungs, which causes air to become trapped in the air sacs, impairing the exchange of oxygen and carbon dioxide.  This is emphysema, commonly known as COPD.