Nicotine in small doses acts as a s

Nicotine in small doses acts as a stimulant to the brain. In large doses, it's a depressant, inhibiting the flow of signals between nerve cells. In even larger doses, it's a lethal poison, affecting the heart, blood vessels, and hormones. Nicotine in the bloodstream acts to make the smoker feel calm.
But visible smoke contributes only 5-8% to the total output of a cigarette. The remaining bulk that cannot be seen makes up the so-called vapor or gas phase of cigarette "smoke.

As a cigarette is smoked, the amount of tar inhaled into the lungs increases, and the last puff contains more than twice as much tar as the first puff. Carbon monoxide makes it harder for red blood cells to carry oxygen throughout the body. Tar is a mixture of substances that together form a sticky mass in the lungs.

Most of the chemicals inhaled in cigarette smoke stay in the lungs. The more you inhale, the better it feels—and the greater the damage to your lungs. You can ask anyone working on bachelors degree in any medical field and they will be able to tell you what damage smoking does to the lungs.