Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 42.6 Breathing Ventilates the Lungs
1. Amphibian Breathing
- Positive pressure breathing: air is pushed into lungs
- Oral cavity floor lowers to draw in air
- Nostrils close and air is forced into lungs
- Exhalation uses elastic recoil and body wall compression
- Puffing behavior may disrupt regular cycles
2. Bird Breathing
- Unidirectional airflow across parabronchi ensures constant exchange
- Air sacs act as bellows; no gas exchange occurs in them
- Each breath cycle takes two inhalations and two exhalations
- No mixing of fresh and stale air → highly efficient system
3. Mammalian Breathing
- Negative pressure breathing: air is pulled into lungs
- Diaphragm and rib muscles expand thoracic cavity → lower pressure → air flows in
- Exhalation is passive unless under exertion
- Lungs are surrounded by a double membrane for smooth expansion/contraction
4. Lung Volumes
- Tidal volume: ~500 mL during resting breathing
- Vital capacity: ~3.4–4.8 L, maximum breath volume
- Residual volume: air remaining after full exhale; increases with age
- Fresh and stale air mix, reducing alveolar PO₂ compared to bird lungs
5. Control of Breathing
- Controlled by the medulla oblongata via pH of cerebrospinal fluid
- More CO₂ → more carbonic acid → more H⁺ → lower pH → increased breathing rate
- CO₂ is the main regulator; O₂ levels only matter when extremely low
- Stretch receptors prevent lung overexpansion
- Ventilation must match cardiac output during activity
In a Nutshell
Breathing is essential for maintaining the pressure gradients that drive gas exchange. Amphibians push air into lungs, birds use unidirectional airflow with air sacs, and mammals rely on negative pressure from diaphragm contraction. Breathing is tightly regulated by CO₂-driven pH changes and synchronized with circulatory demands to ensure efficient oxygen delivery and carbon dioxide removal.