The respiratory system is responsible for gas exchange and for keeping oxygen and carbon dioxide levels in the body stable. Oxygen is needed to produce ATP through cellular respiration, while carbon dioxide is a waste product that must be removed. The system also works to keep CO₂ levels steady, since CO₂ directly affects blood pH.
Structure
Upper Respiratory System
Includes the nose, sinuses, and pharynx. Its job is to warm, moisten, and filter air before it reaches the lungs.
Lower Respiratory System
Includes the trachea, which branches into the bronchi, then the bronchioles, and finally the alveoli.
Functional Zones
Conducting Zone
Moves air only — no gas exchange happens here. It’s lined with pseudostratified ciliated columnar epithelium, and the cilia sweep mucus and trapped debris upward and out of the airway.
Respiratory Zone
Contains the alveoli, lined with simple squamous epithelium. This lining is extremely thin, which is what makes gas diffusion possible.
Alveolar Cells
- Type I alveolar cells — the most abundant type, making up about 95% of the alveoli. They form the actual gas-exchange surface.
- Type II alveolar cells — secrete surfactant. The inside of the alveoli is moist, and the hydrogen bonds between water molecules create surface tension that pulls the alveoli toward collapsing. Surfactant disrupts that water attraction, reducing surface tension and keeping the alveoli open.
The Four Steps of Respiration
- Pulmonary ventilation — air moves in and out of the lungs.
- External respiration — gas exchange between the alveoli and the blood.
- Transport of gases — O₂ and CO₂ move through the bloodstream.
- Internal respiration — gas exchange between the blood and body tissues.
Mechanics of Breathing
Breathing is driven by pressure differences, and air always flows from high pressure to low pressure. Atmospheric oxygen sits at roughly 160 mmHg, higher than the pressure inside the lungs, which is what makes inhalation possible.
Inhalation
The medulla signals the thoracic cavity and intercostal muscles to contract and expand. Ribs aren’t directly attached to the lungs — two layers of tissue between the ribs and lungs pull the lungs outward as the thoracic cavity expands. By Boyle’s Law, as volume increases, pressure decreases (at constant temperature). So as the thoracic cavity expands, volume increases and alveolar pressure drops below atmospheric pressure, and air flows in.
Exhalation
This reverses the process: the thoracic cavity shrinks, the alveoli begin to collapse (though never fully), volume decreases, and pressure rises above atmospheric pressure — pushing air back out.
Functional Applications
Dead Space
The portion of inhaled air that never reaches the alveoli to be used. During rapid, shallow breathing, for example, not enough air makes it to the alveoli — it stays in “dead space” structures like the nose, mouth, trachea, and bronchioles.
Surface Area
Less alveolar surface area means less area for gas exchange, so less oxygen diffuses into the blood.
Resistance
Bronchiole constriction and mucus buildup both increase airway resistance, which decreases airflow.
pH
When blood pH drops, CO₂ levels are high. Chemoreceptors detect this, and the brain (medulla and pons) increases breathing rate to blow off the excess CO₂. The key idea: breathing rate is regulated to control blood CO₂ and pH.
Solubility
CO₂ is more soluble than O₂. Both gases have to dissolve in fluid to cross the alveolar membrane, so O₂ crosses from alveoli into blood plasma more slowly than CO₂ crosses out.
Gas Exchange
Gas exchange is driven by partial pressure differences — gases move from high pressure to low pressure. Oxygen’s partial pressure is about 100 mmHg in the alveoli versus 40 mmHg in the blood, driving O₂ into the blood. Carbon dioxide runs the other way: about 45 mmHg in the blood versus 40 mmHg in the alveoli, driving CO₂ out. At altitude, alveolar PO₂ drops to around 60 mmHg while blood PO₂ stays near 40 mmHg — the smaller gradient slows oxygen diffusion compared to sea level.