If there is no circulatory system the organism is dependant on passive diffusion for conveying oxygen from the respiratory surface to the innermost tissues. In this case its size are limited by the law of diffusion, which states that the rate of diffusion is inversely proportional to the distance over which it has to take place. In practical terms this means that the thicker the tissue the slower the rate at which oxygen reaches the cells furtherst from the surface. In small organisms the distance is short and does not cause a problem. In larger organisms the distance is larger. This has been overcome by either developing a circulatory system with an oxygen carrying pigment, or by reducing the diffusion distance so that none of the cells are far from the surface. Various organisms have done this in various ways:
- Flattened body –e.g. flatworms/leaves on plants – thus reducing the distance between surfaces
- Thin tissues – e.g. Hydra – again reducing the distance of diffusion
- External medium brought into body – e.g. tracheal system in insects – so that in itimate association is obtained with all tissues.
As previously stated amoeba proteus possesses a large surface area to voulme ratio which satisfies its metabolic needs. Many other groups of invertabrate animals such as cnidarians (e.g. jellyfish) and annelids (segmented worms) rely simply on diffusion of gases across their outer surface as a means of respiratory gas exchange. The outer surface is moist and permeable to gases and functions as a gas exchange structure. In addition, their requirement for oxygen is realtively small. As organisms get larger and more complex, their outer surface impermeable to gases, an efficient gas exchange system becomes necessary.
In plants, the leaves have a large surface area to volume ratio. Their leaves have also adapted to their role in respiration by obtaining:
- Large Surface area
- Thin
- Opening and closing of the stomata allows control of transpiration and gaseous exchange.
Access to the respiring cells is by means of the stomata, which are pores in the epidermis of the leaves. Inside the leaf, the large intercellular air spaces in the spongy mesophyll facilitate the diffusion of gases and the cells bordering these air spaces increase the total area available for gas exchange still further.
In mammals, the use of lungs is adapted. Air moves into or out of the lungs as a result of differences in pressure between atmospheric and alveolar air. When the atmospheric pressure is greater than the pressure within the lungs, air tends to flow down the pressure gradient and inspiration occurs. When the pressure in the lungs is greater than atmospheric pressure, air moves out of the lungs and into the atmosphere.
These pressure changes are brought about by changes in the volume of the thorax, which in turn, are produced by contraction or relaxation of the respiratory muscles. The lungs follow these changes passively.
The presence of a respiratory pigment in blood further increases the efficiency of the blood’s oxygen carrying capacity.
Blood that contains any form of respiratory pigment is a more efficient oxygen carrier than one without one. This is because the pigment permits far greater amounts of oxygen to be taken up and transported.
