Smoking is one of the most important preventable causes of disease. Tobacco smoke contains thousands of chemical substances, including nicotine, carbon monoxide, oxidant gases, tar, polycyclic aromatic hydrocarbons, nitrosamines, heavy metals and many irritant particles. These substances damage the organism through several connected mechanisms: oxidative stress, chronic inflammation, endothelial dysfunction, impaired mucociliary clearance, hypoxia, hypercoagulability and carcinogenesis.
The pathophysiological effects of smoking are not limited to the lungs. Smoking also affects the cardiovascular system, blood, immune system, gastrointestinal tract, reproductive system and fetal development. The most clinically important consequences include chronic obstructive pulmonary disease, atherosclerosis, ischemic heart disease, stroke and malignant tumors.
Composition of tobacco smoke
Tobacco smoke contains many harmful substances:
Nicotine – responsible for addiction; stimulates the sympathetic nervous system.
Carbon monoxide – binds to hemoglobin and decreases oxygen transport.
Oxidants and free radicals – cause oxidative stress and tissue damage.
Tar – contains carcinogenic compounds.
Polycyclic aromatic hydrocarbons and nitrosamines – important chemical carcinogens.
Heavy metals such as cadmium – contribute to cellular injury and carcinogenesis.
Irritant particles – activate inflammation in the airways.
General mechanisms of smoking-induced damage
Oxidative stress
Cigarette smoke contains many reactive oxygen species and also stimulates inflammatory cells to produce more free radicals. When the amount of reactive oxygen species exceeds the antioxidant capacity of the tissue, oxidative stress develops.
Oxidative stress causes:
lipid peroxidation of cell membranes,
protein damage,
DNA damage,
activation of inflammatory transcription factors,
endothelial dysfunction,
increased apoptosis and necrosis,
acceleration of aging processes.
This is a central mechanism in chronic obstructive pulmonary disease, atherosclerosis and cancer.
Chronic inflammation
Smoking activates epithelial cells, macrophages and neutrophils. These cells release cytokines, chemokines, proteases and reactive oxygen species.
Important inflammatory mediators include:
TNF-alpha,
IL-1,
IL-6,
IL-8,
leukotrienes,
proteolytic enzymes such as elastase and matrix metalloproteinases.
Chronic inflammation leads to tissue destruction, fibrosis, airway remodeling and systemic inflammatory effects.
Carbon monoxide and tissue hypoxia
Carbon monoxide has a much higher affinity for hemoglobin than oxygen. It forms carboxyhemoglobin, which decreases the oxygen-carrying capacity of blood.
Consequences include:
reduced oxygen delivery to tissues,
tissue hypoxia,
increased cardiac workload,
worsening of ischemic heart disease,
fetal hypoxia during pregnancy.
Nicotine and sympathetic activation
Nicotine stimulates nicotinic acetylcholine receptors and activates the sympathetic nervous system. This increases catecholamine release.
Effects include:
tachycardia,
increased blood pressure,
vasoconstriction,
increased myocardial oxygen demand,
increased platelet activation,
addiction and withdrawal symptoms.
Endothelial dysfunction
Smoking damages the vascular endothelium. It decreases nitric oxide availability and increases oxidative stress, inflammation and platelet adhesion.
This leads to:
vasoconstriction,
impaired vascular relaxation,
increased permeability of the vessel wall,
lipid deposition,
acceleration of atherosclerosis,
thrombosis.
Hypercoagulability
Smoking promotes a prothrombotic state.
Mechanisms include:
increased platelet activation,
increased fibrinogen,
increased blood viscosity,
endothelial injury,
reduced fibrinolysis.
This increases the risk of myocardial infarction, stroke and peripheral arterial disease.
Effects on the respiratory system
The respiratory tract is directly exposed to tobacco smoke. The main consequences are chronic bronchitis, emphysema, COPD, respiratory infections and lung cancer.
Impaired mucociliary clearance
Normally, ciliated epithelial cells remove mucus, dust and microorganisms from the airways. Smoking damages ciliated cells and increases mucus production by goblet cells and submucosal glands.
Consequences include:
mucus retention,
chronic productive cough,
recurrent infections,
airway obstruction.
Chronic bronchitis
Chronic bronchitis is caused by chronic irritation of the bronchi. Smoking causes hypertrophy of mucus glands and goblet cell hyperplasia.
Pathophysiology:
increased mucus production,
narrowing of airways,
impaired ventilation,
chronic cough with sputum,
recurrent bacterial infections,
hypoxemia in advanced disease.
Emphysema
Emphysema is characterized by destruction of alveolar walls and permanent enlargement of air spaces distal to the terminal bronchioles.
Main mechanism:
smoking activates macrophages and neutrophils,
these cells release proteases,CT of lung with terminal stage emphysema
proteases destroy elastin and alveolar septa,
oxidative stress inhibits antiproteases such as alpha-1 antitrypsin,
alveolar walls are destroyed.
Functional consequences:
decreased elastic recoil,
air trapping,
hyperinflation,
reduced surface area for gas exchange,
dyspnea,
expiratory airflow limitation.
COPD
Chronic obstructive pulmonary disease is usually caused by a combination of chronic bronchitis and emphysema. It is characterized by persistent airflow limitation that is not fully reversible.
Important mechanisms:
chronic airway inflammation,
mucus hypersecretion,
airway narrowing,
bronchial wall remodeling,
destruction of alveoli,
loss of elastic recoil,
ventilation-perfusion mismatch.
Clinical consequences:
dyspnea,
chronic cough,
sputum production,
wheezing,
hypoxemia,
hypercapnia in severe disease,
pulmonary hypertension,
cor pulmonale.
Effects on the cardiovascular system
Smoking is a major risk factor for atherosclerosis, ischemic heart disease, myocardial infarction, stroke and peripheral arterial disease.
Mechanisms include:
endothelial dysfunction,
oxidative modification of LDL,
inflammation in the vascular wall,
increased platelet aggregation,
increased blood viscosity,
vasoconstriction,
decreased oxygen delivery due to carbon monoxide.
Atherosclerosis
Smoking accelerates atherosclerosis by damaging the endothelium and promoting lipid deposition in the intima. Oxidized LDL is taken up by macrophages, forming foam cells. This contributes to fatty streaks and later atherosclerotic plaques.
Complications include:
plaque rupture,
thrombosis,
myocardial infarction,
ischemic stroke,
limb ischemia.
Ischemic heart disease
Smoking increases myocardial oxygen demand through nicotine-induced tachycardia and hypertension. At the same time, carbon monoxide decreases oxygen supply. This imbalance contributes to myocardial ischemia.
Smoking and carcinogenesis
Tobacco smoke contains chemical carcinogens. These substances can damage DNA directly or after metabolic activation.
Main steps of carcinogenesis:
Pie chart of lung cancerscarcinogens enter the body,
they are metabolically activated,
reactive metabolites bind to DNA,
DNA adducts are formed,
if DNA repair fails, mutations accumulate,
mutations affect oncogenes and tumor suppressor genes,
abnormal cells proliferate,
malignant tumor develops.
Smoking is strongly associated with cancers of the lung, oral cavity, larynx, pharynx, esophagus, pancreas, kidney, urinary bladder and cervix.
Important smoking-related lung cancers include:
squamous cell carcinoma,
small cell carcinoma,
adenocarcinoma,
large cell carcinoma.
Effects on blood and oxygen transport
Carbon monoxide from tobacco smoke binds hemoglobin and forms carboxyhemoglobin. This decreases oxygen transport and shifts the oxygen dissociation curve to the left, making oxygen release to tissues more difficult.
Smoking may also increase red blood cell mass as a compensatory response to chronic hypoxia. This can increase blood viscosity and contribute to thrombosis.
Effects on pregnancy and fetal development
Smoking during pregnancy harms the fetus mainly through hypoxia, vasoconstriction and placental dysfunction.
Possible consequences include:
miscarriage,
placental insufficiency,
fetal growth restriction,
premature birth,
low birth weight,
increased risk of sudden infant death syndrome,
impaired lung development.
Passive smoking
Passive smoking means inhalation of tobacco smoke by non-smokers. It can cause similar mechanisms of injury, although usually at lower exposure levels.
It increases the risk of:
respiratory infections in children,
asthma exacerbations,
COPD,
ischemic heart disease,
stroke,
lung cancer.
Summary
Smoking causes disease through multiple interconnected mechanisms. The most important are oxidative stress, chronic inflammation, endothelial dysfunction, carbon monoxide-induced hypoxia, sympathetic activation by nicotine, hypercoagulability and DNA damage. These mechanisms explain why smoking damages many organs, especially the lungs and cardiovascular system. The major clinical consequences are COPD, atherosclerosis, ischemic heart disease, stroke and cancer.
References
GOLD. Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease.
Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease.
Hall JE. Guyton and Hall Textbook of Medical Physiology.
Amboss. Smoking and tobacco-related disease.
Centers for Disease Control and Prevention. Health effects of cigarette smoking.
