Table of Contents
Overview: What “Respiration” Means in Animals
In this chapter, “respiration” refers to gas exchange between an animal and its environment and the organs that enable this. (Cellular respiration inside cells is treated elsewhere in metabolism.)
Key ideas:
- Animals need a continuous supply of O₂ for cellular respiration and must eliminate CO₂, a waste product.
- This is achieved by respiratory surfaces and respiratory organs that:
- provide a large surface area,
- are thin and moist,
- are richly supplied with blood or other transport fluids.
We will look at:
- Basic requirements of any respiratory surface
- Different respiratory organs in invertebrates
- Gills
- Lungs in terrestrial vertebrates, especially humans
- How ventilation and gas exchange are organized in different groups
Details of gas transport in blood and tissue-level gas exchange are covered in the later chapter “Heart and Circulatory System”.
General Principles of Animal Respiration
Requirements for an Effective Respiratory Surface
Regardless of the organ type, gas exchange surfaces share key properties:
- Large surface area
More area allows more O₂ to diffuse in and more CO₂ to diffuse out per unit time. - Thin barrier
Gases diffuse efficiently only over small distances; respiratory membranes are often just a few cell layers thick. - Moist surface
O₂ and CO₂ must first dissolve in water or a thin fluid film before diffusing across cell membranes. - Rich perfusion or contact
Either: - strongly perfused by circulatory fluid (blood, hemolymph), or
- in direct contact with tissue fluids (as in insect tracheae).
Adaptations depend strongly on:
- Body size and shape (surface area to volume ratio),
- Habitat (water vs. air),
- Metabolic rate (e.g., active flyers vs. sluggish animals).
Diffusion vs. Ventilation
- Diffusion: random movement of molecules from high to low concentration.
Over short distances (micrometers), diffusion is fast enough for life processes. - Ventilation: bulk flow of air or water to and from the respiratory surface.
- In small or very thin animals, ventilation may be absent; simple diffusion across the body surface is enough.
- Larger, more active animals use muscle-powered ventilation to maintain steep partial pressure gradients of O₂ and CO₂.
Simple Respiration Without Specialized Organs
Some animals lack distinct lungs or gills. Gas exchange occurs:
Entirely Across the Body Surface
- Found in:
- Many unicellular eukaryotes (protists),
- Simple multicellular animals such as flatworms (Platyhelminthes), some cnidarians and sponges,
- Certain small, thin worms and some amphibian larvae (to a degree).
- Features:
- Thin body walls,
- High surface area relative to volume,
- Often live in moist or aquatic environments.
- No dedicated “respiratory organ” or ventilatory muscles; movement of water or body motion may assist exchange.
Body Surface Plus Circulatory System
- In some animals, the entire skin is a respiratory surface, but gas transport to deeper tissues requires a circulatory system.
- Example: many adult amphibians can perform significant cutaneous respiration:
- Skin is thin, moist, well vascularized.
- Lungs exist, but skin may provide a large proportion of O₂ uptake, especially in water.
Respiratory Organs in Invertebrates
Invertebrates show a wide variety of respiratory structures adapted to different lifestyles.
Cutaneous Respiration with Specialized Structures
Some invertebrates enhance skin breathing using extensions:
- External gills or branchiae in aquatic larvae of insects (e.g., mayfly, damselfly larvae) and some worms.
- These are often feathery or finger-like protrusions increasing surface area and bathed by water.
Gills in Aquatic Invertebrates
Aquatic invertebrates (e.g., many crustaceans, some mollusks) often possess:
- Gills (branchiae):
- Delicate, folded or filamentous outgrowths.
- Located in gill chambers or under protective covers (e.g., under the carapace in many crustaceans).
- Heavily supplied with hemolymph (in open circulatory systems).
- Ventilation:
- Achieved by beating appendages (e.g., scaphognathite in crayfish) or body movements.
- Maintains a flow of water with high O₂ and low CO₂.
Tracheal Systems in Insects and Some Other Arthropods
A characteristic adaptation of terrestrial insects (and some other small arthropods) is the tracheal system:
- Structure:
- A network of air-filled tubes (
tracheae) branching throughout the body. - Open to the outside via small openings called spiracles on the body surface.
- Finer branches,
tracheoles, penetrate close to or into cells; gas exchange occurs directly at the cellular level. - Key consequences:
- No direct involvement of the circulatory system in O₂ transport:
- Hemolymph primarily transports nutrients and wastes, not O₂ (exceptions exist).
- Diffusion distances from air in tracheoles to mitochondria are very small.
- Ventilation in insects:
- Small insects: passive diffusion plus small body movements.
- Larger or very active insects (e.g., flying insects): active ventilation:
- Rhythmic contraction of abdominal muscles,
- Opening and closing of spiracles,
- Sometimes air sacs that compress and expand.
- Limitations:
- Diffusion-based system constrains maximum body size in air, though large fossil insects existed under higher atmospheric O₂.
Book Lungs and Book Gills
Some arachnids and horseshoe crabs have lamellar respiratory organs:
- Book lungs (e.g., many spiders, scorpions):
- Located in abdominal cavities.
- Consist of numerous thin, plate-like lamellae stacked like pages in a book.
- Air enters through a slit-like opening; hemolymph flows between lamellae.
- Provide a large moist surface for gas exchange.
- Book gills (horseshoe crabs):
- Externally exposed, leaf-like structures.
- Ventilated by movements of appendages.
Gills in Aquatic Vertebrates
General Structure and Function
In aquatic vertebrates (e.g., most fish), gas exchange occurs through gills:
- Location: often in a pharyngeal region, protected by bony covers (opercula) in bony fish.
- Gills consist of:
- Gill arches: bony or cartilaginous supports,
- Gill filaments: projecting from arches,
- Secondary lamellae: thin, sheet-like structures on each filament, providing huge surface area.
Countercurrent Exchange
Most bony fish use a highly efficient countercurrent exchange system:
- Water flow: enters mouth, passes over gills, exits under operculum.
- Blood flow: in lamellae runs opposite to water flow.
- Result:
- Across the whole length of the lamella, water has a higher O₂ partial pressure than blood.
- O₂ diffuses into blood along the entire surface.
- Efficiency:
- Allows extraction of a large fraction of dissolved O₂ from water, which contains far less O₂ per volume than air.
Ventilation Mechanisms
- Many fish use buccal–opercular pumping:
- Mouth and operculum movements produce a unidirectional flow of water over the gills.
- Some fast-swimming species (e.g., certain sharks, tuna) perform ram ventilation:
- Swim with mouth open, ramming water across gills.
- Reduced or no active pumping; must keep moving to maintain gas exchange.
Lungs in Terrestrial Vertebrates
Transition from water to land required new respiratory solutions:
- Air has much more O₂, is less dense and less viscous than water.
- Respiratory surfaces had to be protected from drying out, yet remain thin and moist.
- The solution in tetrapods: internal lungs.
Basic Lung Types (Overview)
- Amphibians:
- Often simple, sac-like lungs with internal folds.
- Also rely heavily on cutaneous respiration.
- Reptiles:
- More subdivided lungs; some have unicameral (single-chambered) lungs, others highly partitioned.
- Birds:
- Special, highly efficient system with rigid lungs and air sacs.
- Air flows largely unidirectionally through lung parabronchi.
- Mammals:
- Highly branched bronchial tree ending in microscopic alveoli.
Amphibian Ventilation (Buccal Pumping)
Many amphibians ventilate lungs by a buccal pump:
- Air is drawn into the mouth cavity while nostrils open and glottis closed.
- Mouth floor rises; nostrils close; glottis opens.
- Air is pushed from mouth into lungs.
- Expiration may be passive or aided by body wall muscles.
This is distinct from the negative-pressure breathing in mammals.
Mammalian Lungs (Example: Human Lung Anatomy)
Mammals, including humans, have highly developed lungs adapted to high metabolic demands and endothermy.
Respiratory Tract Organization
From outside to gas exchange surface:
- Nasal cavity (and/or mouth):
- Warms, humidifies, and filters inhaled air.
- Pharynx:
- Common passage for air and food.
- Larynx:
- Voice box containing vocal cords; entrance to lower airways.
- Trachea:
- Rigid tube supported by C-shaped cartilaginous rings.
- Lined with ciliated epithelium and mucus-secreting cells.
- Bronchi:
- Trachea divides into left and right main bronchi.
- Further branching into lobar and segmental bronchi.
- Bronchioles:
- Smaller airways without cartilage, with more smooth muscle.
- Lead eventually to terminal and respiratory bronchioles.
- Alveolar ducts, alveolar sacs, alveoli:
- Terminal regions where actual gas exchange occurs.
Alveoli: The Site of Gas Exchange
- Alveoli are tiny air sacs (≈ 200–300 µm in diameter).
- Each lung contains hundreds of millions of alveoli, providing a total area of tens of square meters.
- Wall structure:
- Mostly type I pneumocytes (thin, flat cells forming the gas exchange surface).
- Type II pneumocytes secrete surfactant, reducing surface tension and helping keep alveoli open.
- Respiratory membrane:
- Comprised of:
- Alveolar epithelial cell layer,
- Thin interstitial space (often minimal),
- Capillary endothelial layer.
- Extremely thin (often < 1 µm), facilitating rapid diffusion of O₂ and CO₂.
- Dense capillary network:
- Pulmonary capillaries wrap around each alveolus.
- Pulmonary circulation is specifically adapted to maximize gas exchange.
Ventilation: Mechanics of Breathing in Mammals
Mammals use negative-pressure breathing:
- Inspiration (inhalation):
- Diaphragm contracts and moves downward.
- External intercostal muscles contract, lifting and expanding the rib cage.
- Thoracic cavity volume increases; intrapulmonary pressure drops below atmospheric.
- Air flows into lungs.
- Expiration (exhalation):
- Usually passive at rest:
- Diaphragm and intercostals relax.
- Elastic recoil of lungs and chest wall reduces thoracic volume.
- Intrapulmonary pressure rises slightly above atmospheric; air flows out.
- During exercise or forced expiration:
- Internal intercostals and abdominal muscles contract.
- Increases pressure and expels air more forcefully.
Control of breathing (neural regulation, chemoreceptors) is detailed in information-processing chapters; here the focus is on structure and mechanics.
Bird Lungs and Air Sacs: A Special Case
Birds have one of the most efficient respiratory systems among vertebrates, matching the high demands of flight and endothermy.
Structure
- Rigid lungs with numerous tiny parabronchi (small tubes).
- Extensive system of air sacs in the body:
- Thin-walled sacs in the thorax, abdomen, and even within some bones.
- Air sacs themselves are not major sites of gas exchange; instead, they function as bellows.
Unidirectional Airflow
- Air flows through the parabronchi in a largely one-way direction during both inspiration and expiration.
- Requires two breathing cycles for a single packet of air to pass through the system:
- First inspiration: air to posterior air sacs.
- First expiration: air from posterior sacs through lungs (parabronchi).
- Second inspiration: air from lungs to anterior sacs.
- Second expiration: air from anterior sacs out of the body.
- Benefit:
- Maintains a continuous flow of fresh air through lungs, keeping O₂ partial pressure high over the gas exchange surface.
- Increases efficiency compared to tidal (in–out) systems.
Comparative Overview: Water vs. Air Breathers
Physical Differences Between Water and Air
- O₂ content:
- Air: high O₂ content per unit volume.
- Water: much lower O₂ content, influenced by temperature and solutes.
- Density and viscosity:
- Water is denser and more viscous than air.
- Pumping water is energetically costlier than pumping air.
- Diffusion rates:
- Gases diffuse faster in air than in water.
Consequences for Respiratory Design
- Aquatic animals (e.g., fish):
- Need very efficient gills and often countercurrent systems.
- Invest significant energy in ventilating water.
- Terrestrial animals:
- Can develop internal lungs to prevent desiccation.
- Exploit faster diffusion in air and higher O₂ availability.
- Use various pumping mechanisms (buccal pump, negative-pressure thoracic pump, air sac systems).
Protection and Cleaning of Respiratory Organs
Respiratory surfaces are delicate and must be protected:
- Mucus:
- Traps dust and microorganisms.
- Cilia:
- In many airways (e.g., mammalian trachea and bronchi),
- Beat in coordinated waves to move mucus upwards (mucociliary escalator) toward the pharynx to be swallowed or expelled.
- Structural protection:
- Rib cage around lungs in vertebrates.
- Gill covers (opercula) in fish.
- Reflexes:
- Coughing, sneezing, and other reflexes expel foreign particles and irritants.
Adaptations of Respiratory Organs to Lifestyle
Respiratory systems are fine-tuned to an animal’s ecology:
- High-performance flyers and runners:
- Birds and many mammals (e.g., antelopes, bats) have exceptionally large, efficient lungs and high ventilation rates.
- Diving animals (e.g., seals, whales):
- Lungs and chest structures adapted to withstand high pressure.
- Specialized blood and muscle oxygen stores (myoglobin) complement changes in lung use; detailed gas storage is addressed with circulatory topics.
- Desert animals:
- Nasal passages that recapture water from exhaled air using countercurrent cooling and condensation.
- Amphibious animals:
- Flexible use of gills, lungs, and skin across life stages and environmental conditions.
These examples show how the same basic principles (thin, moist surfaces; ventilation; circulation) are modified to suit very different environments and demands.
In summary, respiration in animals is achieved through a remarkable variety of organs—body surfaces, gills, tracheae, lungs—yet all rely on the same physical principles of diffusion and bulk flow to supply O₂ and remove CO₂. The structure and operation of respiratory organs reflect each group’s habitat, metabolic needs, and evolutionary history.