## Temperature Regulation
**Introduction**
Body temperature is a measure of hotness or coldness expressed by the body using standardized arbitrary scales. Each organism has its own body temperature. Invertebrates cannot adjust body temperature, and it depends on the environment. Vertebrates’ body temperature fluctuates over a considerable range due to the rudimentary body temperature regulating mechanism (poikilothermic).
Man and other mammals are homeothermic, and they maintain body temperature constant in spite of wide variations in environmental temperature. Variations in environmental temperature result in heat energy flow (from higher temperature to lower).
Heat flow between 2 bodies is aided by:
* Conduction
* Convection
* Radiation
* Evaporation
Uncontrolled variation in body temperature may result in adverse health effects or death.
## Thermoregulation
Thermoregulation means keeping the core body temperature within a very narrow range. Above the upper limit, enzymes are denatured, and metabolic pathways are blocked. Below the lower limit, metabolism is slowed down, affecting the brain. There must be a balance between producing or gaining heat and losing heat.
## Body Temperature
The actual body temperature results from the balance between the amount of heat produced/gained and the amount lost. The core body temperature ranges between 36 to 38 0C.
**Areas Where Body Temperature Can be Measured**
* Mouth (oral)
* Esophagus
* Ear (aural)
* Armpit (axillary)
* Rectum (rectal)
* Vagina
* Non-contact (infra-red)
Depending on where and when you measure it, body temperature may vary by 0.5 0C.
Tympanic>oral>axillary
## Factors Affecting Temperature
* Ambient temperature
* Diurnal variation
* Age
* Infant – 0.5 0C than normal due to irregular activity, brown fat, immature regulating system
* Old age – subnormal temperature due to low BMR, decreased activity, and weak thermoregulating system
* Ovulation in females – rise in basal temperature by 0.5 0C
* Exercise – as high as 40 0C in temperature has been recorded
* During emotion – temperature rises due to unconscious tensing of the muscles
## Regulation Of Body Temperature
Temperature is mainly regulated in 2 ways:
1. Neural control
2. Behavioural and voluntary control
## Neural Control
The center for the neural regulation of body temperature is located in the hypothalamus. The hypothalamus consists of 2 regulating centers/nuclei:
* Heat loss center – pre-optic and anterior hypothalamic nuclei
* Heat gain center – posterior hypothalamic nuclei
The pre-optic region of the anterior hypothalamus is regarded as the thermostat that maintains the “set-point.” The neural control of body temperature is activated by 2 mechanisms:
1. Thermoregulatory mechanism
* Direct action
* Reflex mechanisms
2. Endocrine mechanism
**Direct Action**
The temperature of the blood that perfuses the brain can directly stimulate the central thermoreceptors located on the hypothalamus and initiate the thermoregulation process. Warm blood (high temperature) flowing via the hypothalamus initiates heat loss responses:
* Vasodilation
* Sweating
* Hyperpnoea
Cool blood (low temperature) causes heat production and conservation responses:
* Vasoconstriction
* Shivering
* Piloerection
**Reflex Mechanisms**
Sensitive thermoreceptors (located under the skin and hypothalamus) provide inputs to the central command. The hypothalamus consists of 2 regulating centers/nuclei:
* Heat loss center (pre-optic and anterior hypothalamic nuclei)
* Heat gain center (posterior hypothalamic nuclei), which include:
* Nerves to skeletal muscle (tone, activity, and shivering)
* Nerves to respirator muscles
**Efferent Fibers/Nerves**
* **Autonomic nervous system**
* Sympathetic adrenergic vasomotor nerves – cutaneous vasoconstriction and vasodilation
* Sympathetic cholinergic nerves to sweat glands
* **Somatic nervous system**
**Endocrine Mechanism**
* **Adrenal medulla** – immediate adrenaline release causes:
* Cutaneous vasoconstriction
* Increased metabolic rate and heat production
* **Adrenal cortex**
* In adrenal cortical insufficiency, there is low BMR
* Patients do not tolerate cold well, and body temperature is subnormal
* **Thyroid hormone** – Calorinergic. In hyperthyroidism, the skin is warm.
## Behavioral and Voluntary Control
* Movement from warm to cold regions or vice versa
* Curling up of the body in cold conditions
* Clothing in thick or thin clothes
* Fans, air conditioners, heaters, etc.
## Clinical Correlates
**Fever**
* Thermostat has been reset to a new point above 37 degrees centigrade
* Actual body temperature is below the new set point, leading to stimulating temperature-raising mechanisms
* Chilly sensation due to cutaneous vasoconstriction and shivering
* Caused by bacterial or viral infection (pyrogen), trauma, or a lesion of the CNS
**Serious heat illnesses – symptoms and signs**
* **Heat exhaustion** – a mild to moderate illness characterized by an inability to sustain cardiac output with moderate (>38.5 0C, 101 0F) to high (>40 0C, 104 0F) body temperatures (hot skin and dehydration)
* **Heat injury** – a moderate to severe illness characterized by organ (e.g., liver, renal) and tissue (e.g., gut, muscle) injury with high body temperatures, usually but not always greater than 40 0C (104 0F)
* **Heat stroke** – a severe illness characterized by central nervous system dysfunction with high body temperatures, usually but not always greater than 41 0C (106 0F)
**Cold injury**
Cold injuries are classified as hypothermia and peripheral cold injuries.
* **Hypothermia** – whole body cooling (35 0C)
* **Peripheral cold injuries** – localized to the extremities and exposed skin;
* Nonfreezing (chilblain, trench foot)
* Freezing (frostbite)
* 1st degree (superficial, “frostnip”)
* 2nd degree (full skin)
* 3rd degree (subcutaneous tissue)
* 4th degree (extensive tissue and bone)
