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. 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. Smoking has a variety of undesirable effects. When smokers inhale, the chemicals in the smoke irritate the air passages and lungs. Smoker's cough can trigger back pain, and smoking can speed the deterioration of the disks that cushion the spine.