Changes

===Sweating===

===Sweating===

Sweating is controlled from a centre in the preoptic and anterior regions of the [[Nervous System - CNS - Anatomy & Physiology|hypothalamus]] where thermosensitive neurons are located. The heat regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the [[Skin - Anatomy & Physiology|skin]].  High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core body temperature. Overall, however, the sweating response to a rise in hypothalamic ('core') temperature is much larger than the response to the same increase in average skin temperature.  As sweat also contains ions in addition to water, sweating requires the intake of salts and water to maintain fluid homeostasis.

Sweating is controlled from a centre in the preoptic and anterior regions of the [[Hypothalamus - Anatomy & Physiology|hypothalamus]] where thermosensitive neurons are located. The heat regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the [[Skin - Anatomy & Physiology|skin]].  High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core body temperature. Overall, however, the sweating response to a rise in hypothalamic ('core') temperature is much larger than the response to the same increase in average skin temperature.  As sweat also contains ions in addition to water, sweating requires the intake of salts and water to maintain fluid homeostasis.

Not all animals can sweat, however, and they must therefore use a different method of cooling when the environmental temperature is very high.  Dogs, for example, pant to increase their heat loss.

Not all animals can sweat, however, and they must therefore use a different method of cooling when the environmental temperature is very high.  Dogs, for example, pant to increase their heat loss.

===Panting===

===Panting===

Dogs, cats, ruminants and birds pant, whereas horses, pigs and most small animals cannot.  Panting requires fast, shallow breathing at the natural resonant frequency of the airways.  Little mechanical energy is required to facilitate panting, therefore, heat production in the respiratory muscles is minimised.  The main mechanism of heat loss during panting is by water evaporation from the moist '''mucous membrane''' of the [[Skull and Facial Muscles - Anatomy & Physiology#Ethmoid bone (os ethmoidale)|nasal cavity]].  The heat of evaporation is removed from the tissues of the nasal cavity, cooling its blood supply.  The cooled blood is collected in large venous sinuses and then passes through the [[Nervous System - CNS - Anatomy & Physiology|rete mirable]] which functions as a heat exchanger, cooling arterial blood to the brain.

Dogs, cats, ruminants and birds pant, whereas horses, pigs and most small animals cannot.  Panting requires fast, shallow breathing at the natural resonant frequency of the airways.  Little mechanical energy is required to facilitate panting, therefore, heat production in the respiratory muscles is minimised.  The main mechanism of heat loss during panting is by water evaporation from the moist '''mucous membrane''' of the [[Skull and Facial Muscles - Anatomy & Physiology#Ethmoid bone (os ethmoidale)|nasal cavity]].  The heat of evaporation is removed from the tissues of the nasal cavity, cooling its blood supply.  The cooled blood is collected in large venous sinuses and then passes through the [[CNS Vasculature - Anatomy & Physiology#Rete Mirable|rete mirable]] which functions as a heat exchanger, cooling arterial blood to the brain.

[[image: Heat exchanger.jpg|thumb|250px|right|Heat exchange in the Rete Mirable. ©Rachael Wallace2008]]

[[image: Heat exchanger.jpg|thumb|250px|right|Heat exchange in the Rete Mirable. ©Rachael Wallace2008]]

Under moderate heat stress, ruminants pant with a closed mouth.  Under significant heat stress, however, all animals pant with open mouths.  This means that air with a high water content cannot condense on the mucosa of the nasal cavity, thus maximising the heat lost through evaporation.  There is also some evaporation from the oral membranes.

Under moderate heat stress, ruminants pant with a closed mouth.  Under significant heat stress, however, all animals pant with open mouths.  This means that air with a high water content cannot condense on the mucosa of the nasal cavity, thus maximising the heat lost through evaporation.  There is also some evaporation from the oral membranes.