Harmful Effects of Smoking on Lungs
Smoking affects lungs severely due to the toxic nicotine present in tobacco of cigarettes. When nicotine enters into the lungs it accumulates in the bronchial tube near the Alveoli as a tar. Alveoli are tiny air sacs, which is filled with the oxygen during breathing. The deposited tar in bronchial tube and alveoli reduces the oxygen retaining capacity and cause shortness of breath. The presence of tar in the lung increases the production of mucus in lungs and that results in many respiratory diseases. The continuous accumulation of tar in the lungs makes it dysfunctional and induces chronic diseases like cancer, chronic obstructive pulmonary disease, etc. 

Harmful Effects of Smoking on Heart
Smoking is drastically dangerous for heart. Smoking directly affects the cardiovascular system of heart. Since, smoking reduces the capacity of lung to retain oxygen, the heart pumps more blood to maintain the availability of oxygen in the body. This results in increase blood pressure. Many times increase in blood pressure cause heart failure. Due to the inadequate amount of oxygen heart works faster and that leads to the stress on the heart and cause heart attack. Moreover, smoking induces narrowing and hardening of the arteries that result in the occurrence of atherosclerosis. Furthermore, the atherosclerosis results in coronary heart disease, artery disease and stroke.

Harmful Effects of Smoking on Brain
The brain is a vital organ of the body, and smoking is definitely perilous to the brain. The nicotine present in the cigarette reaches to the brain within few seconds of the smoking. The nicotine reaches to the brain and affects specialized receptor cells, which are responsible for the memory, mood and well-being. In simple words, the nicotine alters the chemical reaction of the brain. According to the research conducted by the University of Michigan it has been concluded that smoking reduces the IQ of a smoker. Moreover, it also reduces the thinking ability of a smoker. Moreover, the blockage caused by the accumulation of nicotine cuts off the blood supply to the brain cell that result in cerebral thrombosis.  

Harmful Effects of Smoking on Central Nervous System
When nicotine enters into the bloodstream, it reaches to the brain and affects Central Nervous System. The nicotine stimulates and alters the activity of neurotransmitters, which controls body functions. It alters the Central Nervous System and increases the respiration, heart rate and blood pressure. It also narrows the blood vessels, which results in many other chronic problems. Moreover, prolong exposure of nicotine usage leads to the muscular sclerosis. Addiction of nicotine also creates a problem during sudden cessation of smoking. In lack of nicotine, a person may feel depression, fatigue, mood swings and cravings. The lack of nicotine also increases anxiety, stress and appetite in a person.  

Harmful Effects of Smoking on Digestive System (stomach)
Smoking is dangerous for all part of the body, including stomach, which regulates the digestive system. Smoking can affect almost all parts of the body included in the digestive system. Often, the people who are addicted to smoking complain about gastric problems. Moreover, smoking weakens the Lower Esophageal Sphincter, which keeps the acids of the stomach out of the esophagus. This results in the heart burn when acids of stomach come in contact with the esophagus. Moreover, the presence of nicotine also increase the pH of stomach and causes digestive problems. Smoking is the reason of many diseases and infection such as peptic ulcer, liver disease, crohn's disease, etc.