Animal Respiration: Mechanisms of Gas Exchange and Ventilation

Definitions of Respiration

• Respiration: The biochemical process of breaking down glucose to release energy (ATP).
• Respiratory System: The biological system responsible for gas exchange (O₂ intake, CO₂ output).

Aerobic and Anaerobic Respiration

• Aerobic Respiration: Occurs with oxygen, yielding high energy (38 ATP).
• Anaerobic Respiration: Occurs without oxygen, yielding low energy (2 ATP), and producing lactic acid or ethanol + CO₂.

Principles of Gas Exchange (Diffusion)

Gas exchange occurs by diffusion across moist surfaces. The surfaces must meet specific criteria:

• Must be thin, moist, and highly vascularized.
• Surface area must be large relative to body size.

Note: These three features (thin, moist, vascularized) are critical for effective gas exchange and are repeated often across different organs.

Respiratory Organs in Animals

1. Body Surface (Cutaneous Respiration)

• Examples: Earthworms, amphibians (e.g., frogs).
• Requires moist skin.

2. Tracheal System (Insects)

• Found in insects.
• Air enters via spiracles, travels through tracheae and tracheoles directly to tissues.
• Note: This system does not rely on the circulatory system for gas transport; delivery is direct.

3. Gills (Aquatic Animals)

• Found in aquatic animals (e.g., fish, some amphibians).
• They utilize countercurrent exchange: water and blood flow in opposite directions for maximum efficiency.
• Note: Countercurrent exchange is a key mechanism often tested in biology.

4. Lungs (Terrestrial Vertebrates)

• Found in terrestrial vertebrates (e.g., mammals, birds, reptiles, amphibians).
• Air enters via nostrils, down the trachea, to bronchi, into bronchioles, ending at alveoli (the site of gas exchange).
• Alveoli: Thin, moist, and highly vascularized (meeting the three critical features).

Ventilation Mechanisms in Vertebrates

Ventilation in Amphibians

• Uses positive pressure to push air into the lungs.

Ventilation in Mammals

• Uses negative pressure: The diaphragm contracts, the chest expands, and air is drawn in.

Ventilation in Birds

• Features unidirectional airflow through the lungs via air sacs, making it significantly more efficient than mammalian respiration.

Note: Bird lungs are repeatedly mentioned as highly efficient due to this unidirectional flow.

Respiratory Organ Comparison Table

Key Concepts to Remember

• Thin, moist, vascularized surfaces are essential for gas exchange across all organs.
• Countercurrent exchange in gills maximizes oxygen uptake from water.
• The tracheal system bypasses the circulatory system for direct oxygen delivery to tissues.
• Birds' unidirectional airflow provides superior respiratory efficiency.