Smoking is the number one cause of preventable death in the United States. Approximately one in every five deaths is due to complications related to smoking. The harmful effects of smoking are due to several factors including the smoke from tobacco and nicotine. Given below are the effects of nicotine and tobacco on the body.

Metabolism
Nicotine has a profound effect on metabolism. On a molecular level, nicotine promotes the release of neurotransmitters called catecholamines which activate the autonomic system. Nicotine enhances the breakdown of glycogen in the blood through stimulation of adrenoceptors, thus reducing blood sugar levels. Nicotine also increases the breakdown of fat, resulting in weight loss. However, nicotine can also have negative effects on metabolism as it alters the body’s response to insulin, which can promote metabolic syndrome.

Cardiovascular System
Cigarette smoke consists of two forms: tar and gas. The smoke contains 8% tar and 92% gas. Both types of smoke have billions of free radicals per puff. While the free radical in the tar can last up to months, the free radicals in the gas portion have a relatively short lifespan on the side of seconds. The tar also contains nicotine.
Cigarette smoke increases susceptibility to atherosclerosis (a thickening and narrowing of the arteries) and related syndromes like angina, stroke, and acute coronary syndrome.

Nicotine also promotes the release of catecholamines resulting in an increased heart rate, blood pressure, and heart contraction. Nicotine contributes to the development of Coronary Vascular Disease by restricting blood flow to the heart (myocardial ischemia).

Nicotine also affects the shape and functionality of cells that line blood vessels (endothelial cells) and alters secretion of growth factors (basic fibroblast growth factor and transforming growth factor-β1). These changes increase endothelial cell proliferation and the formation of plaques in the blood vessel. Plaque formation can worsen by the formation of new blood vessels induced by nicotine. The plaque formation, in turn, leads to a thickening of the blood vessels, which promote high blood pressure and various cardiovascular conditions.

Nicotine also has a major effect on heart muscle cells. The alkaloid increases proliferation of these cells and plaque formation in new vessels which increase the risk for arterial disorders.

Respiratory System
Nicotine has access to the respiratory system through two routes: inhaled cigarette smoke and indirectly through the central nervous system. Nicotine promotes the development of emphysema by reducing elastin (a protein in tissues that helps organs retain their shape after stretching during contraction or expansion) in the lung and increasing alveolar volume. It also causes bronchoconstriction (a narrowing of the windpipe) by increasing airway resistance through changes in vagal reflex and the parasympathetic ganglia.

Gastrointestinal System
Nicotine has been linked to gastrointestinal disorders such as Gastro Esophageal Reflux Disorder (GERD) and peptic ulcer disease (PUD). Smoking increases gastric acid and secretion of related proteins which activate vasopressin, an antidiuretic. Smoking also increases the risk of developing an infection by the bacteria known as Helicobacter pylori by increasing the harmful effects of the toxins produced by the gastric bacteria.

Immunological System
Nicotine also suppresses the immune system through multiple mechanisms. Tobacco products impair signal activation from the interaction between antigens and their receptors. The T-cell population also decreases due to a block in the cell cycle. Macrophage response is impaired which increases the risk of infection. Fewer immune cells such as inflammatory cells and fibroblasts travel to injured inflamed areas. Wound healing also slows because epithelialization and cell adhesion are impaired, which increases the risk for infection.

Ocular System
Nicotine also has harmful effects on vision and the ocular system by promoting the formation of new blood vessels and branching of original blood vessels. These new vessels, in turn, can induce macular degeneration. In conjunction with diabetes, nicotine also contributes to the development of cataract in animals.
Renal System
The incidence of long-term kidney disease is very high in smokers. Smoking causes changes in the function of the kidney which leads to narrowing of the renal artery (stenosis).

Reproductive
In females, nicotine causes low estrogen by taking metabolites that would normally be used to make estrogen to make androgen. The low estrogen level prevents ovulation and causes irregular menstrual cycles. It can also lead to irregular bleeding. In males, nicotine causes nitrous oxide to be released from nerves. The nitrous oxide causes blood vessels in the penis to dilate.

Animal studies have also found that nicotine can alter the shape of oocytes giving them a non-spherical shape with abnormal surfaces. Oocytes maturation and fertilization are also impaired.

The effect of smoking on pregnancy
Smoking mothers also can have significant effects on the fetus. The effects of nicotine on an unborn fetus range from stunted growth in utero, stillbirth, miscarriage and mental retardation. A study also found that pregnant mothers that smoke has an increased risk of preeclampsia.