The effectiveness of retia is primarily determined by how readily the heat, ions, or gases can be exchanged. And, for a given length they are most effective with respect to gases or heat, then small ions, and decreasingly so with respect to other substances.
In birds with webbed feet, a rete mirabile in the legs and feet transfers heat from the outgoing (hot) blood in the arteries to the incoming (cold) blood in the veins, with the net effect that the internal temperature of the feet is much closer to the ambient temperature, thus reducing heat loss. In this example the rete mirabile functions as a biological heat exchanger. A similar structure is seen in other vertebrate extremities, including the neck of the dog, in order to protect the brain when the body overheats; mammalian testes, which are more productive at lower temperatures; fishes such as tuna, whose core temperature is higher than that of the cold deep waters they inhabit; and penguins who have them in the feet, flippers and nasal passages, to limit body heat lost to the cold environments in which they live.
In some fish, a rete mirabile fills the swim bladder with oxygen, using a countercurrent exchange system where varying pH levels causes oxygen to unbind from blood hemoglobin and then come out of solution when the blood is supersaturated.
In giraffes, a rete mirabile in the neck equalizes blood pressure when the animal bends down to drink.
In mammals, an elegant rete mirabile in the efferent arterioles of juxtamedullary glomeruli is important in maintaining the hypertonicity of the inner zone of the renal medulla. It is the hypertonicity of this zone, resorbing water osmotically from the renal collecting ducts as they exit the kidney, that makes possible the excretion of a hypertonic urine and maximum conservation of body water.
Vascular retia mirabilia are also found in the limbs of a range of mammals. These reduce the temperature in the extremities. Some of these probably function to prevent heat loss in cold conditions by reducing the temperature gradient between the limb and the environment. Retia mirabilia also occur frequently in mammals that burrow, dive or have arboreal lifestyles that involve clinging with the limbs for lengthy periods. In these cases, the rete mirabile may lower limb temperature and therefore the metabolic requirement for oxygen and nutrients in the tissues of the limb. This would be desirable when the blood supply to the limb is limited as a result of a "diving reflex" diverting blood away from the limbs during diving or burrowing or because of postural restrictions on blood supply in the case of clinging, arboreal animals. In the last case, slow-moving arboreal mammals such as sloths, lorises and arboreal anteaters possess retia of the highly developed type known as vascular bundles. The structure and function of these mammalian retia mirabilia are reviewed by O'Dea (1990).
The ancient physician Galen mistakenly thought that humans also have a rete mirabile in the neck, apparently based on dissection of sheep and misidentifying the results with the human carotid sinus, and ascribed important properties to it; it fell to Vesalius to demonstrate the error.