This line graph shows the results from a study of cigarette smoking over 7 hours and the effect this has on plasma blood levels of nicotine. Nicotine levels were measured over 7 hours of smoking cigarettes (1-2mg nicotine) in a single subject under standardised conditions. 

This shows an average increase in nicotine levels of 27-28 ng.ml within just 2 minutes of completing each cigarette, followed by a rapid drop in plasma nicotine levels. When you compare this to the previous slide, it becomes obvious how cigarettes create the physical dependence and need for nicotine.