The brain is very sensitive to temperature, and its neurones and enzymes can be damaged if it gets much hotter than normal. Some mammals, including gazelles, sheep, goats, cats and dogs, can keep the brain cooler than the rest of the body. This is called selective brain cooling.It works by countercurrent heat exchange at the base of the brain:
• The carotid artery carrying blood to the brain divides into a network of many small arteries called the carotid rete.
• The carotid rete lies in a pool of venous blood, the cavernous sinus.
• This venous blood has come from the nose, where it was cooled by evaporation of water from the moist lining of the nasal passages as the animal breathes or pants.
• Heat passes from the warm arterial blood in the rete to the cooler venous blood, so the blood reaching the brain is cooler than the core.Dividing the artery into many small vessels gives a large surface area and a short distance for heat transfer, so the heat exchange is efficient.
Why it matters. A gazelle chased by a predator produces a lot of heat in its muscles. Its core temperature can rise well above normal, but the rete keeps its brain a few degrees cooler, so it can keep running without brain damage. Desert animals such as oryx can also let their core temperature rise during the hottest part of the day, saving water, because they do not need to sweat to protect the brain.Humans do not have a carotid rete.
• The carotid artery carrying blood to the brain divides into a network of many small arteries called the carotid rete.
• The carotid rete lies in a pool of venous blood, the cavernous sinus.
• This venous blood has come from the nose, where it was cooled by evaporation of water from the moist lining of the nasal passages as the animal breathes or pants.
• Heat passes from the warm arterial blood in the rete to the cooler venous blood, so the blood reaching the brain is cooler than the core.Dividing the artery into many small vessels gives a large surface area and a short distance for heat transfer, so the heat exchange is efficient.
Why it matters. A gazelle chased by a predator produces a lot of heat in its muscles. Its core temperature can rise well above normal, but the rete keeps its brain a few degrees cooler, so it can keep running without brain damage. Desert animals such as oryx can also let their core temperature rise during the hottest part of the day, saving water, because they do not need to sweat to protect the brain.Humans do not have a carotid rete.