Why horse have guttural pouch?

 

Recently, investigators determined that the equine guttural pouches function during selective brain-cooling to maintain blood carried by the internal carotid arteries at a temperature below the core body temperature during hyperthermia, induced by exercise.88 Blood is supplied to the brain principally by the internal carotid arteries, with contributions from the cerebral and occipital arteries. The extracranial portion of the internal carotid artery travels through the medial compartment of the guttural pouch. The temperature of the air within the guttural pouch was fairly constant (37.5 ± 0.05°C) during exercise, and was responsible for cooling the blood within the internal carotid artery by 2°C.88 The heat transfer from the internal carotid artery to the guttural pouch was minimal at rest but became more efficient with exercise.88 Therefore, the function of the guttural pouches in the horse seems to be to cool the brain during periods of hyperthermia.89

Because of the position of the nasopharyngeal openings of the guttural pouches, changes in nasopharyngeal pressures during inspiration and expiration also affect pressures within the guttural pouches.90 When airflow through the nasopharynx is 0 L/s the pressure within the guttural pouches is negative, similar to measurements made in the middle ear of humans.90 After swallowing, the nasopharyngeal aperture opens and pressures within the guttural pouches equilibrate with the nasopharynx. Both at rest and during exercise the guttural pouch static pressures are similar to nasopharyngeal pressures, but slightly out of phase with the respiratory cycle.90 Movement of the head, chewing, snorting, or swallowing causes changes in pressures simultaneously in both guttural pouches. Therefore, the elevated compliance of the guttural pouch makes it susceptible to pressure changes in the nasopharynx associated with airflow but also with head movement.90