Rete mirabile
Encyclopedia
A rete mirabile is a complex of arteries
and vein
s lying very close to each other, found in some vertebrate
s. The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions). It exchanges heat
, ion
s, or gas
es between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature
, or concentration of gases or solutes
.
s 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; mammal
ian 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 penguin
s who have them in the feet, flippers and nasal passages, to limit body heat lost to the cold environments in which they live.
, a rete mirabile fills the swim bladder with oxygen
, increasing the fish's buoyancy
. A countercurrent exchange system is utilized between the venous and arterial capillaries. By lowering the pH
levels in the venous capillaries, oxygen unbinds from blood hemoglobin
. This causes an increase in venous blood oxygen concentration, 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 concentration of oxygen in the arterial capillaries is supersaturated (larger than the concentration of oxygen in the swim bladder). At this point, the free oxygen in the arterial capillaries diffuses into the swim bladder via the gas gland.
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 their 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.
s, an elegant rete mirabile in the efferent arteriole
s of juxtamedullary glomeruli, is important in maintaining the hypertonicity of the renal cortex
. 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).
In giraffe
s, a rete mirabile in the neck equalizes blood pressure when the animal bends down to drink.
The ancient physician Galen
mistakenly thought that human
s 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 Berengario da Carpi first, and then to Vesalius
to demonstrate the error.
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
and vein
Vein
In the circulatory system, veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart...
s lying very close to each other, found in some vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions). It exchanges heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, or gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, or concentration of gases or solutes
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
.
Effectiveness
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.Birds
In birdBird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s 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
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
. A similar structure is seen in other vertebrate extremities, including the neck of the dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
, in order to protect the brain when the body overheats; mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian testes, which are more productive at lower temperatures; fishes such as tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...
, whose core temperature is higher than that of the cold deep waters they inhabit; and penguin
Penguin
Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers...
s who have them in the feet, flippers and nasal passages, to limit body heat lost to the cold environments in which they live.
Fish
In some fishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, a rete mirabile fills the swim bladder with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, increasing the fish's buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
. A countercurrent exchange system is utilized between the venous and arterial capillaries. By lowering the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
levels in the venous capillaries, oxygen unbinds from blood hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
. This causes an increase in venous blood oxygen concentration, 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 concentration of oxygen in the arterial capillaries is supersaturated (larger than the concentration of oxygen in the swim bladder). At this point, the free oxygen in the arterial capillaries diffuses into the swim bladder via the gas gland.
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 their 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.
Mammals
In mammalMammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, an elegant rete mirabile in the efferent arteriole
Efferent arteriole
The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus...
s of juxtamedullary glomeruli, is important in maintaining the hypertonicity of the renal cortex
Renal cortex
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections that extend down between the pyramids. It contains the renal corpuscles and the renal tubules except for parts of...
. It is the hypertonicity of this zone, resorbing water osmotically
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
from the renal collecting ducts as they exit the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, that makes possible the excretion of a hypertonic urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
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).
In giraffe
Giraffe
The giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
s, a rete mirabile in the neck equalizes blood pressure when the animal bends down to drink.
The ancient physician Galen
Galen
Aelius Galenus or Claudius Galenus , better known as Galen of Pergamon , was a prominent Roman physician, surgeon and philosopher...
mistakenly thought that human
Human
Humans are the only living species in the Homo genus...
s also have a rete mirabile in the neck, apparently based on dissection of sheep and misidentifying the results with the human carotid sinus
Carotid sinus
In human anatomy, the carotid sinus is a localized dilation of the internal carotid artery at its origin, the common carotid artery.-Functions:...
, and ascribed important properties to it; it fell to Berengario da Carpi first, and then to Vesalius
Vesalius
Andreas Vesalius was a Flemish anatomist, physician, and author of one of the most influential books on human anatomy, De humani corporis fabrica . Vesalius is often referred to as the founder of modern human anatomy. Vesalius is the Latinized form of Andries van Wesel...
to demonstrate the error.