4.2 The Respiratory System
Key Takeaways
- Air pathway: nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
- Gas exchange occurs in alveoli through simple diffusion
- Inhalation: diaphragm contracts (moves down), chest expands, pressure decreases
- Most oxygen is carried by hemoglobin in red blood cells
- Rising CO₂ levels are the primary trigger for increased breathing rate
Last updated: January 2026
The respiratory system facilitates gas exchange—bringing oxygen into the body and removing carbon dioxide. This is tightly linked to the cardiovascular system.
Respiratory Anatomy
Upper Respiratory Tract:
| Structure | Function |
|---|---|
| Nose/Nasal cavity | Filters, warms, humidifies air |
| Pharynx (throat) | Passageway for air and food |
| Larynx (voice box) | Contains vocal cords, prevents food entry |
| Epiglottis | Covers trachea during swallowing |
Lower Respiratory Tract:
| Structure | Function |
|---|---|
| Trachea (windpipe) | Main airway to lungs |
| Bronchi | Two main branches into lungs |
| Bronchioles | Smaller airways in lungs |
| Alveoli | Tiny air sacs for gas exchange |
The Pathway of Air
- Air enters through nose or mouth
- Passes through pharynx (throat)
- Through larynx (voice box)
- Down the trachea (windpipe)
- Splits into left and right bronchi
- Into progressively smaller bronchioles
- Reaches alveoli (gas exchange occurs)
Gas Exchange
External respiration: Exchange in the lungs
- Oxygen diffuses from alveoli → blood (capillaries)
- Carbon dioxide diffuses from blood → alveoli
Internal respiration: Exchange at body tissues
- Oxygen diffuses from blood → tissues
- Carbon dioxide diffuses from tissues → blood
The Mechanics of Breathing
| Phase | Diaphragm | Chest | Pressure | Air Movement |
|---|---|---|---|---|
| Inhalation | Contracts (moves down) | Expands | Decreases | Air enters |
| Exhalation | Relaxes (moves up) | Contracts | Increases | Air exits |
Key muscles:
- Diaphragm: Primary breathing muscle
- Intercostal muscles: Between ribs, assist breathing
- Accessory muscles: Neck and shoulder muscles (used in labored breathing)
Lung Volumes and Capacities
| Term | Definition |
|---|---|
| Tidal volume | Normal breath volume (~500 mL) |
| Inspiratory reserve | Extra air you can inhale |
| Expiratory reserve | Extra air you can exhale |
| Residual volume | Air remaining after forced exhale |
| Vital capacity | Maximum air that can be exhaled after maximum inhale |
| Total lung capacity | Vital capacity + residual volume |
Control of Breathing
Medulla oblongata: Primary respiratory control center Pons: Helps regulate breathing rhythm
Chemical factors that affect breathing rate:
- ↑ CO₂ (most important) → ↑ breathing rate
- ↓ O₂ → ↑ breathing rate
- ↓ Blood pH (acidosis) → ↑ breathing rate
Oxygen Transport
- 98.5% bound to hemoglobin in red blood cells
- 1.5% dissolved in plasma
Hemoglobin: Protein in RBCs containing iron; each molecule carries up to 4 oxygen molecules
Carbon Dioxide Transport
| Method | Percentage |
|---|---|
| Bicarbonate ions (HCO₃⁻) | ~70% |
| Bound to hemoglobin | ~23% |
| Dissolved in plasma | ~7% |
Common Respiratory Terms
| Term | Definition |
|---|---|
| Hypoxia | Low oxygen in tissues |
| Hypoxemia | Low oxygen in blood |
| Hypercapnia | High CO₂ in blood |
| Hypocapnia | Low CO₂ in blood |
| Apnea | Absence of breathing |
| Dyspnea | Difficulty breathing |
| Tachypnea | Rapid breathing |
Test Your Knowledge
Where does gas exchange between air and blood occur?
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Test Your Knowledge
What happens to the diaphragm during inhalation?
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Test Your Knowledge
Which factor most strongly influences the body's breathing rate?
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