In order to get to grips with why smoking is bad for health, it is useful to understand the respiratory system and how cigarette smoke affects it. 

When someone takes a drag on a cigarette, tobacco smoke is inhaled through the mouth, pharynx and larynx into the trachea. The trachea splits into two bronchi, one for each lung, and these tubes divide further until they are just 1mm wide. These are the bronchioles, which are composed of smooth muscle.

The bronchioles split further until they are only one cell thick. At this point – which marks the end of the breathing tubes – are alveoli. Here, gaseous exchange takes place via the large network of tiny capillaries that carry deoxygenated blood from the heart via the pulmonary artery to the lungs and newly oxygenated blood to the heart via the pulmonary vein. This is also the point at which the components of tobacco smoke, which include nicotine, carcinogens, tar and other toxins, are able to get into the blood and the body.