Animal Respiratory Systems and Gas Exchange

Why animals need gas exchange

Animal cells release energy from food by aerobic respiration, which uses oxygen and produces carbon dioxide.Gas exchange is the process of taking in oxygen from the surroundings and getting rid of carbon dioxide. In very small animals, gases diffuse directly through the body surface. Larger animals have a smaller surface area compared with their volume, so they need specialised respiratory surfaces, and usually a way of moving water or air over them (ventilation) and a circulatory system to carry gases to and from the cells.

Diffusion and Fick’s law

Gases cross respiratory surfaces by diffusion: molecules move randomly, and more of them move from where they are concentrated to where they are scarce. The difference in concentration, or for gases inpartial pressure, is the gradient. Fick’s law says the rate of diffusion is proportional to the surface area and the gradient, and inversely proportional to the thickness of the barrier:

So efficient respiratory surfaces are large, thin and moist (gases must dissolve before they can cross cells), and breathing and blood flow keep the gradient steep by constantly bringing fresh water or air and carrying the oxygen-loaded blood away.

Water versus air

A litre of air contains about 210 mL of oxygen. A litre of water contains only about 5–10 mL, and less when it is warm or salty. Oxygen also diffuses about ten thousand times more slowly in water, and water is about 800 times denser than air and much more viscous, so pumping it over a respiratory surface takes far more energy. Breathing air is easier, but has a different problem: moist respiratory surfaces lose water by evaporation, so air-breathers keep them inside the body.

Fish gills and countercurrent flow

A bony fish has four gill arches on each side, each carrying two rows of filaments covered in thin plates called lamellae, giving a huge surface area. The fish pumps water in through its mouth and out under its gill cover (operculum), so water flows over the gills in one direction. In each lamella, blood flows in theopposite direction to the water. This countercurrent exchange means blood always meets water containing a little more oxygen than itself, so a gradient exists all the way along, and fish can remove about 80% of the oxygen from the water. If both flowed the same way, their oxygen levels would quickly even out and exchange would stop. Gills collapse and stick together out of water, which is why most fish suffocate in air.

Insect tracheae

Insects have no lungs and their blood carries little oxygen. Instead, air enters through spiracles, small openings along the body that can be closed by valves to save water. These lead into tracheae, air-filled tubes reinforced with spiral rings of cuticle, which branch into microscopic tracheoles ending right next to the cells. Oxygen diffuses along the tubes; active insects also pump air by squeezing their abdomen. This system works brilliantly over short distances, and is one reason insects are small. About 300 million years ago, when the air held more oxygen, giant dragonfly relatives had wingspans of around 70 cm.

Mammal lungs

Air enters through the nose and mouth, passes down the trachea and into two bronchi, which branch repeatedly into bronchioles ending in hundreds of millions of tiny air sacs, the alveoli. Their total surface is about 70 m² in an adult human, and the barrier between air and blood is only about half a micrometre thick. Breathing is driven by the diaphragm and rib muscles, which enlarge the chest so air flows in. Air flows in and out the same way (tidal breathing), so some stale air always remains, and the air in the alveoli contains less oxygen than outside air.

Other solutions

• Birds have air sacs that push air through their lungs in one direction, so the lungs are always ventilated with fresh air.
• Frogs and earthworms exchange gases through their moist skin.
• Tadpoles use gills, then switch to lungs and skin as adult frogs.
• Lungfish have both gills and lungs and can gulp air when water is low in oxygen.

Real-world connections

• Fish kills: in hot summers, warm, still water holds less oxygen, and fish can suffocate.
• High altitude: thinner air means a smaller gradient, so climbers breathe harder and their bodies make more red blood cells.
• Lung diseases such as emphysema (less surface area) and pulmonary fibrosis (thicker barrier) reduce gas exchange, especially during exercise.
• Aquaculture farms aerate their ponds to keep oxygen levels high.

Common misconceptions

• “Fish breathe water.” They extract the oxygen dissolved in water; they do not use the oxygen atoms in water molecules.
• “Breathing and respiration are the same.” Breathing moves air or water; respiration releases energy in cells.
• “We breathe out only carbon dioxide.” Exhaled air still contains about 16% oxygen.
• “All animals have lungs.” Gills, tracheae and skin are just as successful in their own environments.

What the model simplifies

The exchange calculations use Fick’s law and a standard oxygen-haemoglobin curve, but the units, flows and transit times are illustrative, not measurements of a real trout, grasshopper or human. The fish model treats one lamella as a pair of opposing streams; the lung model treats alveolar air as perfectly mixed; the insect model reduces the tracheal system to a single diffusion path, with pumping shortening it during flight. The close-up shows a small number of molecules wandering randomly; real numbers are astronomically larger. Carbon dioxide is shown only qualitatively, and the drawings are schematic and not to scale.

Key takeaways

• Oxygen and carbon dioxide cross respiratory surfaces by diffusion down partial-pressure gradients.
• Fick’s law: a large, thin, moist surface and a steep gradient give fast exchange.
• Fish gills use countercurrent flow to extract most of the scarce oxygen in water.
• Insects deliver air directly to their cells through tracheae, which limits their size.
• Mammal lungs have a huge alveolar surface with a very thin barrier, ventilated by tidal breathing.