It is no doubt that you see a lot of people who smoke on a daily basis and sometimes when you aren't even a smoker, you decide to chill and hang out with them.
Let me give you this breaking news that even when you are not smoking but are exposed to secondhand smoking fume you are liable to certain health problems which could include coronary heart disease , lung cancer , stroke as a result of thrombosis which could reduce the amount of blood and oxygen being carried to the brain , low birth weight , and in children , it can lead to an asthma attack in the infants and other respiratory infections, and Sudden death syndrome. This is what you get for smoking (although you might have them and worse), this is what you get for sitting with someone who smokes regularly.
With my explanation above, I do not think we need a soothsayer to tell us that smoking kills but for the sake of this question, let me explain what happens when you smoke. When you smoke a cigarette or any other type of harmful material, toxic gases go straight to your lungs (let me quickly remind you that the lungs are responsible for oxygenating the blood that goes back to your heart and removing the CO2 from it) When they get into your lungs, they no doubt mix with your blood and goes into your bloodstream where they begin to go to every organ in your body. The neurotransmitters in your brain also relate this to the brain, and that is why after you smoke these substances, you feel the effect in your body and most especially in your brain.