Everything about Rete Mirabile totally explained
A
rete mirabile (
Latin for 'wonderful net'; plural
retia mirabilia) is a complex of
arteries and
veins lying very close to each other, found in some
vertebrates. The rete mirabile depends on countercurrent blood flow within the net (blood flowing in opposite directions.) It exchanges
heat,
ions, or
gases between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to
temperature, or concentration of gases or
solutes.
The effectiveness of retia is primarily determined by how readily the heat, ions, or gases can be exchanged. And, for a given length they're 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.
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.
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