Systems of Gas Exchange

Core Concepts

The physical rules: partial pressure and solubility

Diffusion moves gases from high to low concentration, and for gases the practical measure is partial pressure — the pressure a single gas in a mixture would exert if it alone occupied the volume (Dalton's law). At sea level, atmospheric pressure is about 760 mmHg (commonly taught reference value); air is roughly 21% oxygen, so the partial pressure of oxygen (PO₂) in dry air is about 0.21 × 760 ≈ 160 mmHg. Each gas behaves independently: oxygen moves down its own PO₂ gradient regardless of nitrogen or carbon dioxide. Henry's law adds the second rule: the amount of gas that dissolves in a liquid is proportional to its partial pressure in contact with the liquid, multiplied by the gas's solubility in that liquid. This is why respiratory surfaces must be moist — gas must dissolve in a wet film before it can diffuse through membranes into blood or tissues — and why oxygen and carbon dioxide, with different solubilities, move across membranes at different rates.

The design requirements of any respiratory surface

Regardless of the animal, every effective gas-exchange surface meets the same four requirements: (1) it is large in surface area (more area = more molecules crossing per second); (2) it is thin (diffusion is fast only across short distances); (3) it is moist (gases dissolve before crossing); and (4) it is in contact with the medium carrying the gases — air or water — which is kept fresh by ventilation (moving the medium past the surface) and matched by perfusion (blood or body fluid carrying gases to and from the surface). Diffusion alone works only over tiny distances, which is why single-celled organisms, cnidarians (jellyfish), and flatworms exchange gases directly through their body surfaces — no specialized organs needed. Anything larger needs more surface area folded into a compact organ.

Skin and direct diffusion: small bodies, wet surfaces

The simplest strategy is exchange through the body surface. Single-celled organisms and small animals like cnidarians and flatworms use direct diffusion across the body wall, relying on large surface-to-volume ratios and short diffusion distances. Some larger animals supplement other systems with cutaneous respiration — breathing through the skin. Earthworms exchange a substantial share of their gases through moist skin; amphibians (frogs, salamanders) use their thin, wet, highly vascularized skin as a major respiratory surface in addition to their lungs. The constraint is obvious: the skin must stay moist and thin, which ties these animals to damp environments and limits how large they can grow.

Gills: extracting oxygen from water

Gills are outfoldings of the body surface specialized for gas exchange in water. Fish gills are built of rows of filaments covered with thin lamellae — stacked plates that massively increase surface area. Blood flows through the lamellae in capillaries while water flows over them in the opposite direction: a countercurrent exchange system. Because blood always encounters water slightly richer in oxygen than itself, a favorable gradient is maintained along the entire lamella — letting fish extract far more oxygen than a concurrent (same-direction) flow could (commonly taught as a large share of the dissolved oxygen). Ventilation keeps water moving: bony fish pump water in through the mouth and force it out over the gills, aided by the operculum (gill cover). Gills work beautifully in water, but their delicate filaments collapse and stick together in air — which is why fish cannot breathe out of water. (Note: gills are out-foldings, whereas the mammalian lung's alveoli are in-foldings.)

Tracheal systems: pipes straight to the cells

Insects solved gas exchange without blood transport. Their tracheal system is a branching network of air-filled tubes: air enters through openings called spiracles along the body, travels through progressively finer tracheae and tracheoles, and reaches every cell directly — the finest tubes are fluid-filled and sit next to the tissues that need oxygen. This delivers oxygen straight to cells and removes CO₂ without a circulatory step, which is efficient for small bodies. But it is a diffusion-driven system, and diffusion limits how far gas can travel: beyond a certain body size, the outermost cells would starve for oxygen. That ceiling — plus the constraints of the exoskeleton from the previous chapter — is why insects stay small. (Insects do ventilate by opening and closing spiracles and pumping their abdomen, but the delivery itself is diffusion-based.)

Lungs: in-folded, ventilated surfaces for air

Lungs are in-foldings of the body surface that trap air internally, protecting the delicate exchange surface from drying and damage. Mammalian lungs end in millions of tiny air sacs called alveoli, each wrapped in capillaries — the enormous combined surface area (commonly taught as roughly the size of a tennis court in adult humans) is what makes diffusion fast enough for a large, active animal. Mammals use negative-pressure breathing: expanding the chest cavity lowers pressure inside the lungs, drawing air in. Amphibian lungs are simpler sacs and rely heavily on skin exchange and positive-pressure inflation (buccal pumping). Bird lungs are the most efficient of all: air flows through the lungs in one direction through fine tubes called parabronchi, driven by a system of air sacs, so fresh air and blood flow in the same relative direction continuously and there is almost no "dead" mixed air left behind. This one-way design lets birds extract oxygen more completely and support the intense metabolic demands of flight at high altitude.

Common Confusions