Animals can go for days without water and weeks without food, but a continuous supply of oxygen is vital for survival.
This is why vertebrates have evolved a variety of respiratory systems, with organs such as lungs or gills, to extract oxygen from air or water, which have enabled them to occupy diverse habitats.
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What’s the difference between breathing and respiration?
Respiration describes two linked yet distinct processes. The first is aerobic cellular respiration, a series of metabolic reactions within living cells that uses oxygen (O₂) molecules to generate energy while producing carbon dioxide (CO₂). Most animals must constantly absorb O₂ and get rid of excess CO₂. That’s handled by the second process, breathing or physiological respiration.
Breathing is the exchange of gases between the body and external fluid – water or air. It involves delivering fluid (ventilation) to/from an exchange organ, such as a lung or gill, where molecules are prompted to move from regions of high to low concentration (diffusion): O₂ crosses the organ’s exchange surface and into blood for transport around the body, CO₂ is released into air/water.
How do fish breathe through gills?
Fish typically have four or five pairs of gill arches (sometimes open as slits) carrying thousands of plates called lamellae, gas-exchange surfaces where water and blood flow in opposite directions (counter-current) for effective diffusion.
Bony fish force water over their gills by regular movement of cheeks or throat, a method called ‘buccal pumping’ – a valve stops water passing back out via the mouth. Cartilaginous fish such as sharks, by contrast, force fluid through the mouth or special holes (spiracles) by swimming fast, known as ‘ram ventilation’.
And how do mammals use their lungs?
A trachea (windpipe) splits into a pair of branching, air-filled tubes or ‘bronchi’ ending in the sites of gas exchange: hundreds of millions of balloon-like bulbs called alveoli. Ventilation is driven by contracting and relaxing a muscular diaphragm around the ribcage, forcing alveoli to inflate and deflate.
The lining of a mammal lung has complex fractal shapes that increase surface area for diffusion, making it 10 times larger than the relatively flat surfaces in reptiles or amphibians. Human lungs cover roughly the size of a tennis court.
How do birds breathe?
Bird lungs don’t have a tree-like structure with dead ends but interconnected tube networks. The air tubes, or ‘parabronchi’, are the exchange sites where gases flow at 90 degrees (cross-current) to blood. Unlike a mammal lung, where inhaled and exhaled air passes via the same route by tidal breathing, a bird lung has a virtually rigid volume and is ventilated by the surrounding muscles and several air sacs that pump gases through its one-way system.
All this makes avian lungs extremely efficient. Despite using twice as much oxygen as a mammal running, for example, the bar-headed goose can fly over the tallest Himalayan peaks and breathe normally 6km above sea level, without hyperventilating.
Do amphibians breathe through their skin?
Yes, though species do it to differing degrees: on average 20 per cent of oxygen uptake occurs through the skin in frogs, while lungless salamanders obtain all their O₂ via the skin surface. Cutaneous gas exchange, or ‘skin breathing’, requires a thin barrier between the body and surroundings so gases can cross by diffusion. This allows water loss, too, which is why amphibians are limited to aquatic habitats.
Are there other ways to breathe?
Yes! Some gas exchange occurs via the cloaca (orifice) in turtles, and fish such as loaches can survive low-oxygen (hypoxic) conditions using the gut to respire. Meanwhile, scientists have shown that mice and pigs sitting in artificial fluids are capable of ‘enteral ventilation via anus’ – breathing through their bums. While more studies are needed, it provides interesting insight into helping humans suffering from respiratory failure.
Main image: group of Turopolje piglets/Credit: Getty
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