Changes

Natural drainage of the pouch is throught the slit-like (pharyngeal) openings of the eustachian tube in the lateral wall of the nasopharynx. The connection opens when the horse swallows and grazing normally provides drainage. However, most of the pouch is ventral to his slit, and therefore drainage may be rather ineffective. If blocked, secretions accumulate and the pouch distends producing a palpable swelling.

Natural drainage of the pouch is throught the slit-like (pharyngeal) openings of the eustachian tube in the lateral wall of the nasopharynx. The connection opens when the horse swallows and grazing normally provides drainage. However, most of the pouch is ventral to his slit, and therefore drainage may be rather ineffective. If blocked, secretions accumulate and the pouch distends producing a palpable swelling.

==Function==

== Function ==

The function of guttural pouches is largely unknown, however hypotheses have been put forward:

The function of the guttural pouches has been shown with experimental data to participate in the rapid cooling of arterial blood destined for the brain and surrounding structures. In other words, the horse's guttural pouches are 'brain-cooling devices', cooling blood within the internal carotid arteries during [[hyperthermia]] that occurs during heavy exercise.<ref>{{Cite journal|last=Baptiste|first=Keith E.|last2=Naylor|first2=Jonathan M.|last3=Bailey|first3=Jeremy|last4=Barber|first4=Ernest M.|last5=Post|first5=Klass|last6=Thornhill|first6=Jim|date=2000-02-01|title=Physiology - A function for guttural pouches in the horse|url=https://www.researchgate.net/publication/12647122|journal=Nature|volume=403|issue=6768|pages=382–383|doi=10.1038/35000284|pmid=10667779|issn=0028-0836|bibcode=2000Natur.403..382B}}</ref><ref>{{Cite journal|last=Baptiste|first=Keith E.|title=A preliminary study on the role of the equine guttural pouches in selective brain cooling|journal=The Veterinary Journal|url=https://www.sciencedirect.com/science/article/abs/pii/S1090023398800099?via%3Dihub|doi=10.1016/S1090-0233(98)80009-9}}</ref> However, this proven function has been called into question by another study that neither examined the guttural pouches nor the internal carotid arteries;<ref name=":4">{{Cite journal|last=Mitchell|first=Graham|last2=Fuller|first2=Andrea|last3=Maloney|first3=Shane K|last4=Rump|first4=Nicola|last5=Mitchell|first5=Duncan|date=2006-09-22|title=Guttural pouches, brain temperature and exercise in horses|journal=Biology Letters|doi=10.1098/rsbl.2006.0469|url=https://royalsocietypublishing.org/doi/10.1098/rsbl.2006.0469|pmc=1686210}}</ref> and others have argued that a cooling function would require an unattainable high rate of inspiratory air flow partly diverted into the guttural pouches.<ref>{{Cite journal |author=Shane K. Maloney |author2=Andrea Fuller |author3=Graham Mitchell |author4=Duncan Mitchell |title=On the guttural pouch and selective brain cooling in equids: research letter |date=March 2002 |journal=South African Journal of Science |volume=98 |issue=3 |hdl=10520/EJC97450 |hdl-access=free}}</ref>

 

The issue of necessary guttural pouch air flow rates, to provide rapid cooling of the internal carotid arteries, has been solved by further supporting evidence from microvascular studies of the guttural pouch mucosa.<ref>{{Cite journal |last=Ninomiya |first=Hiroyoshi |last2=Kuwano |first2=Atsutoshi |title=Microvasculature of the Guttural Pouch Mucosa and Relationship With the Internal Carotid Artery in Horses |date=March 2019 |journal=Journal of Equine Veterinary Science |volume=74 |pages=84–89 |doi=10.1016/j.jevs.2018.12.025 |url=https://www.sciencedirect.com/science/article/abs/pii/S0737080618306981?via%3Dihub}}</ref> Many of the guttural pouch mucosal superficial arterioles and capillaries extend outwards, forming two types of vascular plexuses surrounding the internal carotid arteries: one with capillary bundles parallel to the internal carotid arteries in the outer layer of the tunica adventitia (outer peri-arterial plexus) and the other with vein-artery-vein triads within the inner layer of the tunica adventitia (inner peri-arterial plexus). These guttural pouch microvascular plexuses, engulfing the internal carotid arteries, are typical of [[countercurrent exchange|countercurrent]] heat exchangers recognised in other animal species, supporting the data that guttural pouches participate in selective brain cooling, even at lower air flow rates.

 

This completes the triad of internal carotid artery cooling that protects the horse's brain from [[hyperthermia]]. The triad including the function of guttural pouches achieving arterial cooling, via both by utilizing inspiratory air cooling as well as microvascular [[countercurrent exchange|countercurrent]] heat exchangers surrounding internal carotid arteries, and finally afterwards where the internal carotid arteries project upwards passing through the intracranial cavernous venous sinuses accepting cooled venous blood from the nasal sinuses.<ref>{{Cite journal |author=FF McConaghy |author2=JR Hales |author3=RJ Rose |author4=DR Hodgson |date=Dec 1995 |title=Selective brain cooling in the horse during exercise and environmental heat stress |journal=Journal of Applied Physiology |volume=79 |issue=6 |pages=1849-1854 |doi=10.1152/jappl.1995.79.6.1849 |url=https://journals.physiology.org/doi/abs/10.1152/jappl.1995.79.6.1849}}</ref>

==Links==

==Links==