One such example of this phenomenon is the tuna fish. Tunas are regional endotherms, maintaining elevated temperatures in deep red swimming muscles by way of vascular counter-current heat exchangers (retia mirabilia) that trap heat produced as a by-product of tissue metabolism. The close contact of blood vessels in the retia allows heat in the venous blood to be transferred to arterial blood that returns to the tissues, preventing loss of heat to the surrounding water as the venous blood passes through the gills. Some tuna species also warm the viscera (internal organs) and brain through associated vascular networks or retia. These retia keep tissues at temperatures higher than the surrounding water, enhance metabolic capacity, and act as effective insulators, which slow heat loss during the fish's journeys into cooler waters.
One species of tuna that makes such as journey is the yellowfin Thunnus albacares, a tropical-subtropical inhabitant of surface waters, that periodically makes rapid dives into deeper waters in search of prey, often encountering temperature gradients as much as 10°C cooler. Previous studies have found that the Thunnus tuna possess a carotid rete, or vascular network, in the blood supply to the eye and brain that acts as a thermal barrier.
The objective of this study was to measure the ability of the carotid rete to insulate the brain of yellowfin tuna from rapid changes in environmental temperatures. The authors of "The Regulation of Brain Temperature in Yellowfin Tuna: Evidence of Alterations in Blood Flow" are Keith E. Korsmeyer, from the Hawaii Pacific University, Kaneohe, HI, and Richard W. Brill, Honolulu Laboratory, National Marine Fisheries Service, NOAA, Honolulu, HI. They will present their findings at "The Power of Comparative Physiology: Evolution, Integration and Application" an American Physiological Society intersociety meeting being held August 24-28, 2002, at the Town & Country Hotel, San Diego, CA. Find out more at http://www.
Yellowfin tuna were implanted with a device to measure slight changes in temperature through the pineal foramen of the skull to record brain temperature, and then exposed to step changes in ambient, or surrounding, temperature and then back. The effectiveness of the carotid heat exchanger in affecting heat flux was examined by calculating the thermal rate coefficient from the rates of brain temperature change. To test for nervous control of retial efficiency, several tuna were injected with bretylium tosylate to abolish the effects of adrenergic nerve cells, or fibers of the autonomic nervous system that employ norepinephrine as their neurotransmitter.
Excess brain temperatures in the yellowfin were not significant; however, rates of brain cooling during drops in ambient temperature were significantly lower than rates of heating when water temperature was increased. This difference indicates alterations in the effectiveness of the heat-exchanger to reduce heat loss during exposure to colder waters, and enhance heat gain upon return to warm waters. On average, the brain warmed about 50 percent faster than it cooled.
A hypothesis has developed that temperature could be regulated in tissues with retial blood supply by diverting blood through the rete or partially by-passing it via alternate circulatory pathways and thereby altering the efficiency of the heat-exchange systems. Following blockage of adrenergic nervous control of the circulation with bretylium, alterations of heat transfer to the brain were eliminated, suggesting active nervous control of the heat-exchanger efficiency to regulate brain temperature.
Although the researchers did not measure significant excess brain temperature during steady-state conditions, they did find that the carotid rete was effective in minimizing heat loss, which would protect the brain (and perhaps eyes) during rapid dives into colder waters. The heat-exchanger could then be turned "off" so that rapid warming could occur upon return to warm surface waters, with the overall effect of regulating more stable, and warmer, brain temperatures in the face of fluctuating environmental temperatures.
The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions by which animals live, and thus ultimately underlie human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.
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