Breathing and Exchange of Gases — BIOLOGY STD 11 Science — Question

2. Respiratory system.

Explanation:

Smoking is a practice in which a substance is burned and the smoke is breathed in and absorbed into the bloodstream. Most commonly used substance is the dried leaves of the tobacco plant. Smoking is primarily practised for recreational drug use.

The combustion of the dried plant leaves vaporizes and delivers active substances into the lungs where they are rapidly absorbed into the bloodstream and reach bodily tissue. 
Smoking generally has negative health effects, because smoke inhalation may cause effects on physiologic processes such as respiration.