Rete mirabile

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 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.
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.

http://en.wikipedia.org/wiki/Rete_mirabile

Gas bladder (swim bladder)

The gas bladder (also fish maw, less accurately swim bladder or air bladder) is an internal organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. [1]
The gas bladder is a gas-filled sac located in the dorsal portion of the fish. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. In physostomous gas bladders, a connection is retained between the gas bladder and the gut, allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder through the pneumatic duct. In more derived varieties of fish, the bladder has a gas gland that can introduce gases (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills. In order to introduce gas into the bladder, the gas gland excretes lactic acid; the resulting acidity causes the hemoglobin of the blood to lose its oxygen, which then diffuses into the bladder while flowing through a complex structure known as the rete mirabile. Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back.
Physoclist gas bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence. Gas bladders are only found in ray-finned fish, but a few of these fish that do not need to change water depth have lost them. Many cartilaginous fish, including sharks, can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.
In some fish, mainly freshwater species, the gas bladder is connected to the labyrinth of the inner ear by the Weberian apparatus, which provides a precise sense of water pressure (and thus depth), and may also improve hearing.
The combination of gases in the bladder varies; in shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its gas bladder.
Gas bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and into the gas bladders of today's fish. In embryonal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the more derived orders. There are no animals which have both lungs and a gas bladder.

Human uses
In some Asian cultures, fish maw is considered a food delicacy. It is usually served braised or in stews. Fish maws are also used in the food industry as a source of collagen.
Gas bladders in other animals
The Portuguese Man o' War has a special gas bladder that allows its top to float along the surface while its tentacles trail below the water. This organ is unrelated to the one in fish.

http://en.wikipedia.org/wiki/Gas_bladder