How the Lungs Exchange Oxygen and Carbon Dioxide in the Blood

250 Milliliters of Oxygen Transferred Every Minute

At rest, the lungs absorb approximately 250 mL of oxygen from inhaled air and remove approximately 200 mL of carbon dioxide every minute. During vigorous exercise, these rates increase 20-fold or more. This gas exchange occurs across an enormous surface area — the adult human lung contains approximately 300–500 million alveoli (air sacs), creating a total gas exchange surface of 70–100 square meters, roughly the area of a singles tennis court, packed into an organ the size of two rugby balls. The air-blood barrier across which gas diffuses is extraordinarily thin: just 0.2–0.5 micrometers — 200 times thinner than a human hair — allowing rapid diffusion down partial pressure gradients.

The Mechanics of Breathing

Gas exchange requires continuous lung ventilation to maintain alveolar oxygen concentrations and clear accumulating carbon dioxide. Breathing is driven by the diaphragm — a dome-shaped muscle below the lungs — and the external intercostal muscles. During inspiration, diaphragm contraction increases thoracic volume, reducing intrathoracic pressure below atmospheric pressure, and air flows in. Expiration at rest is passive: the elastic recoil of expanded lung tissue expels air without muscular effort.

• Tidal volume (VT): volume of air per normal breath — approximately 500 mL at rest
• Respiratory rate (RR): 12–20 breaths per minute at rest
• Minute ventilation: RR × VT = approximately 6–10 liters/minute at rest, rising to 100–200 L/min during maximal exercise
• Dead space: approximately 150 mL of each breath fills the trachea and bronchi (anatomical dead space) and never reaches alveoli for gas exchange; only 350 mL of each tidal breath participates in gas exchange

The Alveolar Gas Exchange Interface

The respiratory membrane separating alveolar air from pulmonary capillary blood consists of four layers: alveolar epithelial cells (type I and II pneumocytes), their basement membrane, the pulmonary capillary endothelial basement membrane, and capillary endothelial cells. Type II pneumocytes secrete surfactant — a mixture of phospholipids and proteins that reduces surface tension in alveoli, preventing collapse at end-expiration. Surfactant deficiency in premature infants causes respiratory distress syndrome (RDS), the leading cause of neonatal mortality before the introduction of exogenous surfactant therapy in the 1990s.

Gas Diffusion: Driven by Partial Pressure Gradients

Gas molecules diffuse from areas of high partial pressure to low partial pressure. In the lung:

The large O2 gradient (104 → 40 mmHg) drives rapid oxygen diffusion from alveoli into pulmonary capillary blood. CO2 diffuses in the opposite direction driven by a much smaller gradient (46 → 40 mmHg), but CO2 is 20 times more soluble in aqueous solution than O2, allowing rapid equilibration despite the smaller gradient. Complete gas equilibration occurs within 0.25 seconds — one third of the 0.75-second transit time of a red blood cell through the alveolar capillary at rest, providing a three-fold safety margin that is maintained even during exercise-induced increases in cardiac output.

Oxygen Transport in Blood

Only 1.5% of oxygen is dissolved directly in plasma; 98.5% is bound to hemoglobin within red blood cells. Each hemoglobin molecule carries four heme groups, each capable of binding one O2 molecule. The oxygen-hemoglobin dissociation curve is sigmoidal — a shape with critical physiological implications:

• In the lungs, where PO2 is approximately 100 mmHg, hemoglobin is nearly 100% saturated with oxygen
• In peripheral tissues, where PO2 is approximately 40 mmHg, saturation falls to about 75%, releasing roughly 25% of loaded oxygen per circulation — the oxygen extraction fraction
• The Bohr effect: increased CO2, decreased pH (acidosis), and increased temperature shift the curve rightward, reducing hemoglobin's oxygen affinity and promoting oxygen release to metabolically active tissues that generate these conditions
• Carbon monoxide (CO) binds hemoglobin with 200-fold higher affinity than oxygen, displacing O2 and shifting the dissociation curve leftward (Haldane effect) — CO poisoning is rapidly fatal because even at low atmospheric concentrations, CO occupies hemoglobin binding sites and shifts remaining O2 binding so tightly it is not released to tissues

Carbon Dioxide Transport and pH

Carbon dioxide is transported from tissues to lungs via three mechanisms: approximately 7% dissolved in plasma, 23% bound to hemoglobin as carbaminohemoglobin, and 70% as bicarbonate (HCO3−). In red blood cells, carbonic anhydrase rapidly converts CO2 + H2O to H2CO3, which dissociates to H+ + HCO3−. The bicarbonate diffuses into plasma; the H+ is buffered by hemoglobin. In the lungs, the reaction reverses: HCO3− re-enters red blood cells, recombines with H+ to form CO2, and CO2 diffuses into alveoli for exhalation.

Ventilation-Perfusion Matching

Gas exchange efficiency depends on matching ventilation (airflow to alveoli) with perfusion (blood flow through alveolar capillaries). Ideally, each alveolus receives proportional amounts of air and blood. The lung maintains this match through hypoxic pulmonary vasoconstriction (HPV) — when a region of lung receives inadequate ventilation, local hypoxia causes pulmonary arterioles in that region to constrict, diverting blood to better-ventilated areas.

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice.