Rete mirabile

Source: Wikipedia, the free encyclopedia.
Rete mirabile
Rete mirabile of a sheep
Identifiers
TA98A12.0.00.013
TA23928
FMA76728
Anatomical terminology

A rete mirabile (

solutes. This term was coined by Galen.[1][2]

Effectiveness

The effectiveness of retia is primarily determined by how readily the heat, ions, or gases can be exchanged. 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.[citation needed]

The retia can provide for extremely efficient exchanges. In bluefin tuna, for example, nearly all of the metabolic heat in the venous blood is transferred to the arterial blood, thus conserving muscle temperature; that heat exchange approaches 99% efficiency.[3][4]

Birds

In birds with webbed feet, retia mirabilia in the legs and feet transfer heat from the outgoing (hot) blood in the arteries to the incoming (cold) blood in the veins. The effect of this biological heat exchanger is that the internal temperature of the feet is much closer to the ambient temperature, thus reducing heat loss. Penguins also have them in the flippers and nasal passages.

Seabirds distill seawater using countercurrent exchange in a so-called salt gland with a rete mirabile. The gland secretes highly concentrated brine stored near the nostrils above the beak. The bird then "sneezes" the brine out. As freshwater is not usually available in their environments, some seabirds, such as pelicans, petrels, albatrosses, gulls and terns, possess this gland, which allows them to drink the salty water from their environments while they are hundreds of miles away from land.[5][6]

Fish

Fish have evolved retia mirabilia multiple times to raise the temperature[7] (endothermy) or the oxygen concentration of a body part above the ambient level.[8]

In many fish, a rete mirabile helps fill the swim bladder with oxygen, increasing the fish's buoyancy. The rete mirabile is an essential[8] part of the system that pumps dissolved oxygen from a low partial pressure () of 0.2 atmospheres into a gas filled bladder that is at a pressure of hundreds of atmospheres.[9] A rete mirabile called the choroid rete mirabile is found in most living teleosts and raises the of the retina.[8] The higher supply of oxygen allows the teleost retina to be thick and have few blood vessels thereby increasing its sensitivity to light.[10] In addition to raising the , the choroid rete has evolved to raise the temperature of the eye in some teleosts and

sharks.[7]

A

countercurrent exchange system is utilized between the venous and arterial capillaries. Lowering the pH levels in the venous capillaries causes oxygen to unbind from blood hemoglobin because of the Root effect. This causes an increase in venous blood oxygen partial pressure, allowing the oxygen to diffuse through the capillary membrane and into the arterial capillaries, where oxygen is still sequestered to hemoglobin. The cycle of diffusion continues until the partial pressure of oxygen in the arterial capillaries exceeds that in the swim bladder. At this point, the dissolved oxygen in the arterial capillaries diffuses into the swim bladder via the gas gland.[11]

The rete mirabile allows for an increase in muscle temperature in regions where this network of vein and arteries is found. The fish is able to

thermoregulate certain areas of its body. Additionally, this increase in temperature leads to an increase in basal metabolic temperature. The fish is now able to split ATP
at a higher rate and ultimately can swim faster.

The

pectoral muscles of the opah, which generate most of the body heat, are thus able to control the temperature of the rest of the body.[12]

Mammals

In

juxtamedullary glomeruli is important in maintaining the hypertonicity 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. Others reduce the temperature of the

testes increasing their productivity. In the neck of the dog
, a rete mirabile protects the brain when the body overheats during hunting; the venous blood is cooled down by panting before entering the net.

Retia mirabilia also occur frequently in mammals that burrow, dive or have arboreal lifestyles that involve clinging with the limbs for lengthy periods. 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).[13]

The ancient physician

Vesalius
to demonstrate the error.

See also

  • Pampiniform plexus
    , a countercurrent heat-exchanging structure in the spermatic cord

References

  1. ^ Grant, Mark (2000). Galen on Food and Diet. Routledge.
  2. ^ "Rete Mirabile". encyclopedia2.thefreedictionary.com. Retrieved 6 September 2012.
  3. doi:10.1242/jeb.109.1.21
    .
  4. ISBN 978-0-521-24437-4
    .
  5. ISBN 0300076193
    .
  6. ^ Ritchison, Gary. "Avian osmoregulation » Urinary System, Salt Glands, and Osmoregulation". Retrieved 16 April 2011. including images of the gland and its function
  7. ^
    PMID 19181893
    .
  8. ^
    PMID 17449830
    . Retrieved 2021-02-18.
  9. PMID 11719607
    . Retrieved 2021-02-18.
  10. PMID 33166685
    .
  11. ^ Kardong, K. (2008). Vertebrates: Comparative anatomy, function, evolution (5th ed.). Boston: McGraw-Hill.
  12. S2CID 17412022
    .
  13. doi:10.1016/0300-9629(90)90004-C
    .
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