Cigarettes and unhealthy foods both contain a high fat content, and also contain other chemicals that can irritate the lining of your blood vessels & arteries. Once irritated, the fats in your blood can collect in these areas which restricts the channel diameter for the blood to flow through.
Over time these deposits also cause a degeneration of the elastic fibres in the vessel walls, which means they're less stretchy (the elasticity is used to maintain a constant blood pressure as the heart pumps blood further away from itself, and between beats).
The combination of a smaller surface area for flow (due to the restriction from the fat deposits) and the lack of elasticity increases the overall pressure of the system.
