The lungs are two elastic organs filling most of the thoracic cavity on either side of the heart. Right lung has three lobes, left has two (the left makes room for the heart). Their job is gas exchange — pulling oxygen into blood and pushing carbon dioxide out — and they do it across a surface area of roughly 70 square meters, about the size of a singles tennis court, folded into a few liters of thoracic space.
At a glance
What it does
Gas exchange. Air flows in through the trachea, branches through about 23 generations of bronchi and bronchioles, and reaches the alveoli — tiny thin-walled sacs wrapped in capillaries. Oxygen diffuses across the alveolar-capillary membrane (about 0.5 µm thick) into red blood cells, binds hemoglobin, and gets shipped to tissues. Carbon dioxide, produced by cellular metabolism, diffuses in the opposite direction and gets exhaled.
The system has enormous reserve. At rest, tidal volume is about 500 mL per breath and only a fraction of available alveoli are doing meaningful gas exchange at any instant. During maximal exercise, minute ventilation can rise from about 6 L/min to over 150 L/min in a trained athlete, and nearly all alveoli are recruited. This reserve is why you can smoke for decades and feel fine — until you cannot.
The lungs also filter small clots and debris from the venous return, participate in blood pH regulation (by varying CO2 clearance), produce surfactant to keep alveoli from collapsing at end-expiration, and activate angiotensin I to angiotensin II via pulmonary ACE.
How it works
Breathing is driven by the diaphragm and intercostal muscles. On inspiration, the diaphragm contracts downward, the chest wall expands, intrathoracic pressure drops, and air flows in. Expiration at rest is passive — elastic recoil of the lungs pushes air out. Active expiration uses abdominal muscles and is recruited during exercise or forced breathing.
Ventilation is tightly matched to perfusion. Regions of lung with more airflow get more blood flow; regions with less airflow vasoconstrict to shunt blood elsewhere (hypoxic pulmonary vasoconstriction). This V/Q matching is the key to efficient gas exchange. When it breaks down — pneumonia, pulmonary embolism, ARDS — oxygenation fails even though ventilation might look okay.
Breathing is controlled by the respiratory centers in the medulla and pons, which respond primarily to CO2 (via central chemoreceptors sensing CSF pH) and secondarily to oxygen (via peripheral chemoreceptors in the carotid and aortic bodies). This is why most dyspnea ("air hunger") is really CO2 sensation, not oxygen sensation — and why a healthy person at sea level rarely feels their oxygenation even when it changes modestly.
When it goes wrong
Chronic obstructive pulmonary disease is the big one — nearly always caused by smoking, occasionally by occupational exposure or alpha-1 antitrypsin deficiency. Alveoli are destroyed and airways narrow irreversibly. Symptoms emerge gradually: exertional dyspnea, chronic cough, exacerbations with infection. There is no cure. Smoking cessation is the single most important intervention and it works at any age, even after decades of use.
Asthma is reversible airway inflammation — recurrent wheezing, cough, and dyspnea triggered by allergens, exercise, or infection. Modern inhaled corticosteroid therapy controls it well in most patients. Severe refractory asthma benefits from biologics targeting IL-5, IgE, or Th2 pathways.
Pneumonia — bacterial, viral, fungal — fills alveoli with exudate and impairs gas exchange. Community-acquired pneumonia remains a leading infectious killer of adults, particularly the elderly.
Lung cancer is the deadliest cancer worldwide, dominated by smoking-related adenocarcinoma, squamous cell carcinoma, and small cell carcinoma. Low-dose CT screening in high-risk smokers reduces mortality. Never-smoker lung cancer is a smaller but rising category, often with targetable mutations (EGFR, ALK) and better prognosis than smoking-related disease.
Pulmonary embolism is acute clot migration from venous circulation into the pulmonary arteries. Massive PE kills within minutes. It is under-diagnosed because symptoms can be subtle (dyspnea, pleuritic chest pain, tachycardia) and easy to attribute to something else.
Interactions
Hemoglobin is what actually carries oxygen — anemia drops oxygen delivery even with perfect lungs. Altitude training raises erythropoietin and hemoglobin mass, which is why endurance athletes train high and live high. VO2max (the maximum rate of oxygen consumption during exercise) is highly trainable — a sedentary adult can raise VO2max by 15-25% in a few months of consistent aerobic training, and VO2max is one of the strongest predictors of all-cause mortality.
Smoking does roughly what people think it does, only worse. It causes most COPD, most lung cancer, accelerates atherosclerosis, raises insulin resistance, and reduces fertility. Vaping is probably less bad than smoking, but "less bad than smoking" is not the same as "safe" — the EVALI outbreak and emerging long-term data suggest real harm that is still being characterized.
Honest take
The lung is astonishingly resilient right up to the point it is not. Most lung disease we see in clinics was preventable — the overwhelming driver is tobacco in its various forms, and the returns to quitting remain enormous at any age. If you smoke and are reading this, the single highest-expected-value change you could make for your own lifespan is not a supplement or a diet or a training program: it is quitting, with whatever combination of varenicline, nicotine replacement, and behavioral support works. Aerobic exercise for VO2max is the closest thing the lung has to a supplement with good data, and it is free.
Sources
- GOLD (Global Initiative for Chronic Obstructive Lung Disease) Strategy Report.
- National Lung Screening Trial Research Team, NEJM (2011) — low-dose CT lung cancer screening.
- Kodama et al., JAMA (2009) — meta-analysis of cardiorespiratory fitness and all-cause mortality.