Rete mirabile
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A rete mirabile (Latin for "wonderful net") is a complex of arteries and veins lying very close to each other, found in a number of vertebrates, and serving different purposes. In all cases, however, the function of a rete mirabile depends on the countercurrent flow of blood within it. Anything that can be exchanged across vessel walls and thus between the blood flowing in opposite directions within the rete is prevented to a greater or lesser degree from being exchanged from one end of a rete to the other. Consequently, in different instances retia may be found that are effective in maintaining physiological gradients with respect to temperature, concentration of gases or diffusible solutes. The effectiveness of retia is a function of how readily an affected entity is exchanged; thus for a given length they are most effective with respect to gases or heat, next with respect to small ions, and decreasingly so with respect to other entities.
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, and fishes such as tuna, whose core temperature is higher than that of the cold deep waters they inhabit.
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.
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.
