Problems of Gas Exchange

What problems are involved in moving oxygen from the external medium to the extracellular fluid surrounding a cell? First, the amount of oxygen available from the environment varies. For instance, there is comparatively little oxygen in an aqueous environment (see table), and the warmer or the saltier the water, the less oxygen it contains. Air, by comparison, is rich in oxygen, and oxygen diffuses half a million times faster in air than in water. It looks, at first glance, as if aquatic animals are at a disadvantage. On the other hand, all molecules must cross cell membranes in solution, and so respiratory surfaces must always be wet. Animals that obtain their oxygen from air lose large quantities of precious water by evaporation from their respiratory surfaces.

The Oxygen Content of Water and Air

Basic Requirements For Gas Exchange Surfaces

There are four basic requirements for a gas exchange surface:

1. a gas exchange surface of adequate dimensions relative to the volume of the organism

Because the exchange of oxygen and carbon dioxide between a living cell and its environment always takes place by diffusion, from an area of high concentration to an area of low concentration of that gas, there must be a sufficient surface area for the respiratory gases to diffuse across the membrane. A small or very thin organism has a large enough surface area compared to its volume that no special exchange surface is necessary. However, as an organism increases in size, the maintenance of a respiratory surface of adequate dimensions relative to the volume becomes a problem. The problem is most acute for the more active animals, whose rapid utilization of energy demands a large amount of oxygen per unit of body volume per unit time. An additional complicating factor is that like terrestrial plants, many animals have evolved relatively impermeable outer body coverings. Coverings such as animal skin with its derivative scales, feathers, and hair, function as protective barriers between the fragile internal tissues and organs and the often hostile outer environment, but their presence, which demands that the gas-exchange surface be confined to a restricted region of the body, makes the problem of adequate exchange area even more critical.

2. a means of keeping the surface moist

All gases must be in solution if they are to move across the cell membrane. In aquatic organisms, such as sponges, hydra, and flatworms, this requirement poses no serious problem, because each cell is either in direct contact with the surrounding water or only a few cells away. For organisms living on land the need to keep the exchange surface moist creates a problem since water is constantly lost by evaporation from the surface. The lost water must be replaced or the animal is in danger of drying out.

3. a means of protecting the fragile gas exchange surface from mechanical injury and desiccation

The need for direct contact between the moist membranes across which gas exchange occurs and the environmental medium (e.g. water or the atmosphere) also poses serious difficulties, especially for terrestrial organisms. The moist membranes must be exposed to the environment to exchange gases, but they must be exposed in such a way as to minimize their chances of drying out. Also, a large, thin, moist surface is often fragile and easily suffers mechanical damage, so the tendency has been toward the evolution of protective devices.

4. a method of transporting gases between the area of exchange with the environment and the more internal cells

Another complication brought on by large size in animals is that many cells are deep within the body of the organism, far from the gas-exchange surface. Diffusion alone is too slow to move gases in adequate concentrations across the immense number of cells that may intervene between these more distant cells and the exchange surface. In general, simple diffusion suffices for movement of substances through aqueous media only when the distances are less than one millimeter. Some other mechanism for conveying gases to and from the individual cells of the organism therefore becomes essential. Very often this mechanism is a blood circulatory system which transports the respiratory gases between the exchange surface and the cells. Typically the exchange surface has a rich supply of blood vessels that are very close to the surface. Oxygen moves by diffusion from the water or air across the surface cells into the bloodstream which then transports the oxygen to the individual cells of the body. Carbon dioxide produced by respiration moves in the opposite direction, from the cells, into the bloodstream, and back to the exchange surface. Both gases must be dissolved in water before they can be absorbed by the blood.