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
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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