Breathing and respiration are essential to life for most animals. But do all animals actually have lungs or use them to breathe? The answer may surprise you.
If you’re short on time, here’s a quick answer to your question: While most vertebrates have lungs, not all animals have lungs or even breathe air. Invertebrates like insects and spiders often have tracheal respiratory systems, while aquatic animals like fish have gills to extract oxygen from water.
In this comprehensive guide, we’ll take an in-depth look at the respiratory systems found across the animal kingdom. We’ll discuss the anatomy and function of lungs in mammals, birds, and reptiles. We’ll also cover how other animal groups like arthropods, mollusks, and fish have evolved different structures and methods to obtain the oxygen they need to survive.
Respiratory Systems in Vertebrates
Mammalian Lungs
Mammals have some of the most complex respiratory systems among vertebrates. Humans and other mammals have lungs that are spongy and filled with small sacs called alveoli. When we breathe in, air travels through the nose, down the trachea and into the lungs.
The alveoli provide a large surface area for gas exchange to happen. Oxygen diffuses from the alveoli into the bloodstream while carbon dioxide makes the opposite journey. The diaphragm and intercostal muscles drive breathing by contracting and relaxing to change the volume of the thorax.
Other adaptations in mammals include a nasal cavity that warms and moistens inhaled air.
Avian Lungs
Birds have a very efficient respiratory system to enable flight. Their lungs are fairly rigid structures with openings called ostia that connect to a system of air sacs throughout the body. Air flows continuously through the lungs in one direction rather than tidal breathing in mammals.
This “flow-through respiration” moves a lot of oxygen-rich air through the avian respiratory system. The air sacs keep the lungs inflated while also serving as bellows to ventilate them. In addition, birds have adaptations like hollow, pneumatic bones connected to the respiratory system to maintain a light body weight suited for flying.
Reptilian Lungs
Reptiles like lizards, snakes and turtles have relatively simple lungs compared to mammals and birds. Their lungs are sac-like structures attached to the inside of the ribcage. Reptiles do not have a diaphragm, so breathing relies on muscles attached to the ribs, spine and pelvis to move air in and out of the lungs.
Many reptiles also utilize buccal pumping by rhythmically opening and closing their mouth to force air into the lungs. Because their lungs are fairly stiff, reptiles cannot take in as much oxygen as mammals. However, their lungs still provide sufficient gas exchange for their metabolic needs.
Amphibian Respiration
Amphibians have the most primitive lungs among terrestrial vertebrates. Frogs, toads and salamanders have simple sac-like lungs similar to reptiles. They breathe by buccal pumping as well as contraction and relaxation of throat muscles.
But amphibians also rely heavily on cutaneous respiration, with oxygen and carbon dioxide diffusing across moist skin. This is why they must keep their skin moist or risk suffocation. Larval amphibians like tadpoles use gills to breathe underwater before developing primitive lungs as adults.
The amphibian respiratory system allows them to transition between aquatic and terrestrial habitats.
Invertebrate Respiratory Structures
Tracheal Systems in Insects and Spiders
Insects and spiders have a respiratory system called the tracheal system which transports oxygen efficiently to tissues through a network of tubes called tracheae. The tracheae connect the outside air to internal organs and muscles. Some key features of insect tracheal systems:
- Tracheae branch throughout the body with smaller diameters as they spread from the exterior to the interior
- Air enters small openings on the insect’s body surface called spiracles
- Spiracles can open and close to regulate oxygen intake
- Tracheae have rigid rings or spirals to prevent collapsing
Gills in Mollusks and Crustaceans
Aquatic invertebrates like mollusks and crustaceans have respiratory structures called gills which are highly vascularized outgrowths that extract oxygen dissolved in water. Some key points on invertebrate gills:
- Gills have an extensive surface area to maximize contact with water
- Water is pumped over the gills, either by cilia or the motion of appendages
- Oxygen diffuses into blood vessels in the gills
- Gills are located on the exterior of the body or inside a protective chamber
- Bivalves like clams and mussels have plate-like gills between their shells
Gas Exchange through the Skin
Some invertebrates perform respiration directly through their skin, which must be kept moist. Examples include:
- Flatworms – Respire through their whole body surface which is only 2 cell layers thick
- Annelids like earthworms – Moist skin allows gas exchange with the air or water
- Nematodes – Have a large surface area to volume ratio optimal for diffusion
For these invertebrates, factors like temperature and moisture levels impact their rate of respiration. Specialized respiratory organs evolved in larger more complex animals.
Respiration in Fish and Other Aquatic Animals
Gills in Bony and Cartilaginous Fish
Fish breathe oxygen from water using specialized organs called gills. Gills are composed of filaments filled with blood capillaries, allowing for an efficient transfer of oxygen into the bloodstream (How Fish Breathe Underwater).
Both bony fish and cartilaginous fish like sharks use gills to obtain oxygen from water.
Bony fish have four pairs of large arch-shaped gills protected by an operculum. As water passes over the gills, oxygen diffuses into the blood capillaries while carbon dioxide diffuses out into the water. Cartilaginous fish have five pairs of less complex, comb-like gills that are exposed to water.
But the oxygen exchange process is the same.
According to biologist J. Z. Young (How Do Fish Breathe), an average fish can extract between 10% to 25% of dissolved oxygen from water during normal respiration.
Gill-like Structures in Larval Amphibians
Amphibian larvae like tadpoles have gill-like external structures that allow them to breathe underwater. Larval salamanders have several pairs of external feathery gills for gas exchange. Tadpoles also have long tail fins with a network of blood capillaries for diffusion of oxygen from water before developing lungs.
According to biologist IJ Gordon (Animal Physiology: The Life Process), around 80% of oxygen uptake in tadpoles occurs through gills, fins and tail skin.
Oxygen Absorption through the Skin
Some aquatic animals can absorb dissolved oxygen through their skins. Larvae, tadpoles, and some adult fish, frogs and salamanders have a thin epidermal membrane that facilitates diffusion of oxygen from water into blood capillaries.
For example, aquatic frogs like mudpuppies and African clawed frogs have mucous glands in their skin to improve oxygen uptake. But the rate of oxygen absorption is typically 10 to 40 times slower than through gills (McNab, Aquatic Oxygen Uptake in an African Clawed Frog).
So skin uptake usually supplements gills or lungs when oxygen demand increases, including for aestivation.
Conclusion
While the vast majority of animals need oxygen to survive, not all use lungs for respiration. Lungs are found in most terrestrial vertebrates, but aquatic species and invertebrates have evolved alternative structures and mechanisms for gas exchange.
Respiratory systems are adapted to an animal’s environment and lifestyle. Gills, tracheal tubes, and even direct absorption through the skin allow animals to obtain the oxygen they need, even without lungs. So while lungs are common, they are not universal across the animal kingdom.
