Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 42.5 Gas Exchange Occurs Across Specialized Respiratory Surfaces
1. Partial Pressure and Respiratory Media
- Gases diffuse from high to low partial pressure
- Air has more O₂ and is easier to move; aquatic animals face higher energy costs due to water’s low O₂ content
2. Respiratory Surfaces
- Must be moist, thin, and large in surface area
- Some animals (e.g., earthworms) use skin for gas exchange
- Most use specialized organs: gills, tracheae, or lungs
- All gas exchange occurs by diffusion
3. Gills and Countercurrent Exchange
- Gills are body surface outfoldings in aquatic animals
- Ventilation keeps water flowing over gills
- Countercurrent exchange: blood flows opposite to water, maintaining O₂ gradient
- Fish can extract over 80% of O₂ in water
4. Tracheal Systems in Insects
- Air tubes deliver O₂ directly to body cells
- Tracheae connect to the outside; tracheoles reach individual cells
- Flight muscle contractions ventilate the system
- Circulatory system is not used for gas transport
5. Lungs
- Lungs are internal sacs that use the circulatory system for gas transport
- Found in land snails, spiders, amphibians, reptiles, birds, and mammals
- Amphibians use skin and small lungs; reptiles and mammals depend on lungs
- Some animals (e.g., turtles) supplement lungs with other moist surfaces
6. Mammalian Respiratory System
- Air path: nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
- Alveoli: tiny sacs (~100 m² total area) where gas exchange occurs
- Lined with surfactant to reduce tension and prevent collapse
- Mucus and cilia filter air; damaged by smoke or pollutants
- Premature infants lacking surfactant may develop respiratory distress syndrome (RDS)
7. Breathing Mechanisms
- Amphibians: use positive pressure to push air into lungs
- Birds: use air sacs and parabronchi for one-way airflow, requiring two cycles per breath
- Bird respiration is highly efficient and supports high metabolic rates
In a Nutshell
Gas exchange in animals depends on specialized surfaces—gills, tracheae, or lungs—adapted to their environment. Efficiency relies on thin, moist structures and partial pressure gradients. Aquatic species use countercurrent flow, insects use air-filled tubes, and mammals rely on alveoli and circulatory transport. The mechanism of breathing varies across species, reflecting their metabolic needs.