Selective brain cooling in desert animals: the camel (Camelus dromedarius)

1. Animals living in the Arabian desert are subjected to extremely high temperatures during the day in summer and very cold nights in winter. They have developed various adaptive mechanisms in order to cope with these severe heat conditions. 2. The camel (Camelus dromedarius), like many other land animals, resorts to selective brain cooling when it is subjected to heat stress. 3. This mechanism protects the heat-sensitive brain tissue from heat stress and at the same time increases the animals' tolerance to high temperatures. 4. The blood cooled in the nasal cavity by evaporative heat loss is diverted to the brain sinuses via the nasal and angular veins. 5. In the cavernous sinus, the arterial blood in the carotid rete is cooled by the cold venous blood before entering the brain. This will lead to significant cooling of the brain tissue. 6. Active myogenic tone was mainly observed in the facial, nasal and angular oculi veins of the camels head. This tone was found to be sensitive to small changes in temperature in the range 33-45 degrees C. 7. The facial veins constricted, while the nasal and angular oculi veins relaxed to increasing temperatures. This leads to a coursing of cold venous return to the brain's sinuses for selective brain cooling. 8. It is concluded that this myogenic vasoactive mechanism is a major factor in the control of blood flow in the facial area of the camel during heat stress.