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| Thermoregulating System - The Guttural Pouch Component |
The guttural pouch is a “thin-walled, fist-sized” sac that is located in the horse’s skull, near the base of each Eustachian tube – the part of the auditory canal that connects the middle ear to the back of the throat. This pouch has been a mystery in the equine world for some time. Over the centuries, there have been many theories as to its purpose: an aid for hearing, swallowing, or sneezing; whinny modifier or resonator; an air pressure regulator for the eardrum; and even a flotation aid when swimming. Another common belief has been that its use has long since been lost as a result of evolution. Similar to the human appendix, it too has been seen more as a source of disease, with such illnesses as guttural pouch empyema, guttural pouch mycosis, and tympany. One recent discovery, however, seems to suggest that it is most likely a component of the horse's thermoregulating system, cooling the blood before it reaches the brain.
Most large mammals have an advanced cooling system called a carotid rete, which consists of the carotid artery splitting up into a fine mesh of blood vessels that “are cooled by venous blood returning from the skin surface.” This system performs in much the same way as a car’s radiator does, in that it cools the “hot” blood coming from the heated muscle groups by dispersing it through this massive network of arteries before it reaches the brain. The horse, however, only has a rudimentary version of this carotid rete, in which the carotid artery splits up, but not into the mesh of vessels. So the question remains, how does the horse’s brain not “burn up” from the incredible amount of heat the body generates?
The answer, according to results of studies conducted by Dr. Jonathan Naylor of the University of Saskatchewan in Canada and his former graduate student, Dr. Keith Baptiste, is the guttural pouch. Baptiste had developed the theory that the sac cooled the blood as it pumps through the internal carotid artery, “which runs for about 10 cm through the thin mucous membrane of the guttural pouch on its way to the brain.” He had already proven his theory through his experimentation on horse cadavers. He found that the saline solution that he ran through the carotid arteries was indeed cooled by the air pumped into the guttural pouch using a vacuum cleaner. Unfortunately, as Baptiste related, “most people laughed at [him] because it was about dead horses and vacuum cleaners.” He needed to prove his theory with a live horse. It was at this point that he called upon the assistance of Dr. Naylor, as well as several other specialists of varying fields at Saskatchewan, to perform the experiment with a living horse.
Using the same medical procedure to “tie off dangerous hemorrhages,” the team of scientists threaded a catheter through the internal carotid artery. In order to measure the temperature along the segment of artery within the guttural pouch, they inserted thermocouples via the catheter at three different points: at the entrance from the carotid to the pouch; midway in the blood vessel inside the pouch; and, at the exit point of the pouch where the artery heads toward the brain. These thermocouples would provide constant feedback of the temperatures of the blood flowing through the internal carotid artery, especially as it passed through the guttural pouch. A thermocouple was also carefully inserted into the guttural pouch itself in order to monitor the temperature of the sac. For comparison purposes in their study, they monitored the rectal temperature, as well.
They “hooked up” four horses in this manner, and put each one on a treadmill in a “temperature controlled room.” At rest, the temperature in the guttural pouch was actually slightly warmer than the blood. Once the horse started moving and generating heat, the core temperature began to rise. When the horse began cantering, the first thermocouple recorded an average temperature of 103.1o F, while the one at the exit site showed a lower measurement of approximately 100.04o F. The temperature within the guttural pouch itself, remained a cool, even 99.5o F. Since the scientists suspected that the horse’s rudimentary carotid rete may have helped to cool the blood, they focused more attention on the second thermocouple – the one placed midway along the length of the vessel segment inside the pouch – than the one at the entrance. On average, they measured a difference in temperature of 1.8o F, suggesting that the guttural pouch distinctly contributed to cooling the blood.
Despite these telling results, however, there is still much to be studied on how the guttural pouch works to cool the blood. For instance, these scientists are still left with the questions of where the heat from the blood goes, how the guttural pouch remains cool despite the hot blood moving through it, and how the pouch is ventilated. But for now, at least one question has seemingly been answered – the guttural pouch is not some useless remnant of evolution, but rather a significant factor in the horse’s thermoregulatory system. |
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