How the Respiratory System Works

Anatomy of the Respiratory Tract

The respiratory system divides into the upper respiratory tract (nose, nasal cavity, pharynx, and larynx) and the lower respiratory tract (trachea, bronchi, bronchioles, and lungs). The upper tract warms, humidifies, and filters incoming air before it reaches the delicate lung tissue. Nasal turbinates, bony shelves covered in mucous membrane, increase the surface area inside the nasal cavity and create turbulent airflow that traps particles. Mucus produced by goblet cells captures dust, pollen, bacteria, and other debris, while cilia on the epithelial surface sweep contaminated mucus toward the throat for swallowing or expulsion.

The pharynx (throat) serves as a shared passageway for air and food, with the epiglottis acting as a flap that covers the larynx during swallowing to prevent food from entering the airway. The larynx, or voice box, contains the vocal cords and also functions as a valve that can close completely during heavy lifting or straining (the Valsalva maneuver). Below the larynx, the trachea extends about 12 centimeters before splitting into the right and left main bronchi at a point called the carina.

The trachea and bronchi are reinforced with C-shaped cartilage rings that prevent collapse while allowing the adjacent esophagus to expand during swallowing. As the bronchi branch into progressively smaller bronchioles, the cartilage gradually disappears and is replaced by smooth muscle. The smallest bronchioles, called terminal bronchioles, have diameters of about 0.5 millimeters and lead to respiratory bronchioles, which open into alveolar ducts and finally into the alveolar sacs where gas exchange occurs.

The Alveoli and Gas Exchange

Alveoli are tiny, grape-like sacs where the actual business of respiration takes place. Each alveolus is surrounded by a dense network of pulmonary capillaries, and the barrier separating air from blood, the respiratory membrane, is only about 0.5 micrometers thick, consisting of the alveolar epithelium, a shared basement membrane, and the capillary endothelium. This extreme thinness, combined with the vast surface area of 480 million alveoli, makes gas exchange remarkably efficient.

Oxygen and carbon dioxide move across the respiratory membrane by simple diffusion, driven entirely by partial pressure gradients. Inhaled air in the alveoli has an oxygen partial pressure (PO2) of about 104 mmHg, while deoxygenated blood arriving in the pulmonary capillaries has a PO2 of about 40 mmHg. This 64 mmHg gradient drives oxygen from the alveoli into the blood. For carbon dioxide, the gradient is reversed: blood PCO2 is about 45 mmHg while alveolar PCO2 is about 40 mmHg, driving CO2 from blood into the alveoli for exhalation. Despite the smaller CO2 gradient, carbon dioxide diffuses about 20 times faster than oxygen because it is much more soluble in the aqueous respiratory membrane.

Type I alveolar cells, thin squamous epithelial cells, make up about 95% of the alveolar surface area and form the gas exchange surface. Type II alveolar cells are cuboidal cells that secrete pulmonary surfactant, a phospholipid mixture that reduces surface tension inside the alveoli and prevents them from collapsing during exhalation. Premature infants born before about 26 weeks of gestation often lack sufficient surfactant, leading to respiratory distress syndrome, a condition that was frequently fatal before the development of synthetic surfactant therapy in the 1980s.

Mechanics of Breathing

Breathing, or ventilation, is the mechanical process of moving air into and out of the lungs. It relies on pressure changes created by the respiratory muscles. Inhalation is an active process. The diaphragm, a dome-shaped muscle separating the thoracic and abdominal cavities, contracts and flattens downward, increasing the vertical dimension of the thoracic cavity. Simultaneously, the external intercostal muscles contract and lift the rib cage upward and outward. These combined movements increase thoracic volume, which decreases intrapulmonary pressure below atmospheric pressure, and air flows in through the airways.

Normal exhalation at rest is largely passive. The diaphragm and external intercostals relax, the elastic recoil of the lung tissue and the chest wall compresses the lungs, intrapulmonary pressure rises above atmospheric pressure, and air flows out. During forced exhalation, such as blowing out candles or coughing, the internal intercostals and abdominal muscles actively contract to compress the thoracic cavity further and push more air out.

Lung volumes and capacities provide measurable indicators of respiratory function. Tidal volume, the amount of air moved in a normal breath, averages about 500 milliliters. Inspiratory reserve volume, the additional air that can be inhaled after a normal inhalation, is about 3,100 milliliters. Expiratory reserve volume, the additional air that can be forcefully exhaled after a normal exhalation, is about 1,200 milliliters. Residual volume, the air remaining in the lungs even after the most forceful exhalation, is about 1,200 milliliters. This residual volume prevents the lungs from completely collapsing and ensures continuous gas exchange. Total lung capacity, the sum of all volumes, is approximately 6,000 milliliters in an average adult male.

Oxygen Transport in the Blood

Once oxygen crosses the respiratory membrane into the blood, about 98.5% binds to hemoglobin inside red blood cells and 1.5% dissolves directly in the plasma. Each hemoglobin molecule contains four heme groups, each with an iron atom that can reversibly bind one oxygen molecule. A single hemoglobin molecule can therefore carry up to four oxygen molecules. When all four binding sites are occupied, hemoglobin is fully saturated. Arterial blood leaving the lungs is typically 95% to 100% saturated.

The oxygen-hemoglobin dissociation curve describes the relationship between blood PO2 and hemoglobin saturation. The curve has a sigmoidal (S-shaped) form because of cooperative binding: when one oxygen molecule binds to hemoglobin, it changes the protein's shape and makes it easier for subsequent oxygen molecules to bind. At high PO2 values (in the lungs), hemoglobin binds oxygen readily. At low PO2 values (in metabolically active tissues), hemoglobin releases oxygen. This behavior perfectly matches the body's needs, picking up oxygen where it is abundant and releasing it where it is needed.

Several factors shift the dissociation curve. Increased temperature, increased CO2, increased hydrogen ion concentration (lower pH), and increased 2,3-bisphosphoglycerate (2,3-BPG) all shift the curve rightward, meaning hemoglobin releases oxygen more easily. This is called the Bohr effect, and it is physiologically important because exercising muscles produce heat, CO2, and acid, all of which promote oxygen unloading precisely where demand is highest. Conversely, in the lungs where CO2 is low and pH is slightly higher, the curve shifts leftward, promoting oxygen binding.

Carbon Dioxide Transport and Removal

Carbon dioxide produced by cellular metabolism is transported in the blood by three mechanisms. About 7% dissolves directly in the plasma. About 23% binds to the amino groups of hemoglobin (forming carbaminohemoglobin), not at the same binding site as oxygen. The remaining 70% is converted to bicarbonate ions (HCO3-) by the enzyme carbonic anhydrase inside red blood cells. This enzyme catalyzes the reaction CO2 + H2O -> H2CO3 -> H+ + HCO3-. The bicarbonate ions are transported out of the red blood cells into the plasma in exchange for chloride ions (the chloride shift), and they travel to the lungs in dissolved form.

At the pulmonary capillaries, all three processes reverse. Bicarbonate re-enters the red blood cells, carbonic anhydrase converts it back to CO2, carbaminohemoglobin releases its CO2, and dissolved CO2 leaves the plasma. The CO2 diffuses across the respiratory membrane into the alveoli and is exhaled. An average adult produces about 200 milliliters of CO2 per minute at rest and exhales a roughly equivalent volume.

Control of Breathing

Breathing rhythm is generated by respiratory centers in the brainstem, primarily the medullary respiratory group in the medulla oblongata and the pontine respiratory group in the pons. These centers send rhythmic nerve impulses to the diaphragm via the phrenic nerves and to the intercostal muscles via the intercostal nerves. The basic breathing rhythm is involuntary and automatic, continuing during sleep and unconsciousness, though it can be voluntarily overridden for activities like speaking, singing, or holding your breath.

The primary chemical stimulus for breathing is the level of carbon dioxide in the blood, not oxygen. Central chemoreceptors in the medulla detect changes in cerebrospinal fluid pH, which correlates with blood CO2 levels. When CO2 rises, pH drops, and the chemoreceptors signal the respiratory centers to increase breathing rate and depth. Peripheral chemoreceptors in the carotid bodies and aortic bodies primarily detect blood oxygen levels but also respond to CO2 and pH. They trigger increased ventilation when PO2 drops below about 60 mmHg, a significant decline from the normal arterial PO2 of 95 to 100 mmHg.

This CO2-driven control has important clinical implications. Patients with chronic obstructive pulmonary disease (COPD) who retain CO2 chronically may develop a blunted response to elevated CO2 and become dependent on low oxygen levels (hypoxic drive) to stimulate breathing. Administering high-concentration oxygen to these patients can suppress their respiratory drive, which is why COPD patients typically receive controlled, lower-concentration supplemental oxygen.

Common Respiratory Conditions

Asthma is a chronic inflammatory condition of the airways that affects approximately 262 million people worldwide. During an asthma attack, airway smooth muscle contracts (bronchospasm), the airway lining swells, and excess mucus is produced, all of which narrow the airways and obstruct airflow. Triggers include allergens, cold air, exercise, respiratory infections, and air pollutants. Treatment typically involves inhaled bronchodilators for acute relief and inhaled corticosteroids for long-term inflammation control.

Chronic obstructive pulmonary disease encompasses emphysema and chronic bronchitis, usually caused by long-term exposure to cigarette smoke or air pollution. Emphysema destroys the alveolar walls, reducing surface area for gas exchange and creating enlarged, inefficient air spaces. Chronic bronchitis produces persistent airway inflammation and excessive mucus production. COPD affects over 380 million people globally and is the third leading cause of death worldwide. It is progressive and currently irreversible, though smoking cessation dramatically slows its progression.

Pneumonia is an infection of the lung parenchyma, typically caused by bacteria (most commonly Streptococcus pneumoniae), viruses, or fungi. The alveoli fill with fluid and inflammatory cells, impairing gas exchange and reducing oxygen levels. Community-acquired pneumonia remains one of the leading causes of hospitalization and death, particularly among the elderly and immunocompromised. Vaccines, including pneumococcal conjugate vaccines and annual influenza vaccines, are among the most effective preventive measures.