The document discusses how the hypothalamus regulates body temperature through increasing or decreasing mechanisms in response to heat or cold. In hot conditions, the hypothalamus triggers sweating and vasodilation to cool the body, while in cold conditions it causes piloerection, vasoconstriction, shivering and metabolic heat production to conserve heat. The hypothalamus integrates input from thermoreceptors in the skin and blood to maintain core temperature within a narrow range.

1. Normal body temperatures
Heat production. Heat loss
Sweating and its regulation
Hypothalamus in the body temperature regulation
Temperature -decreasing mechanisms
Temperature -increasing mechanisms

2. Normal body temperatures
 In a healthy individual, the temperature of the core of
the body is regulated by feedback control mechanisms
that maintain it nearly constant around 98.6°F (37°C)
throughout the day, week, month or year. This
thermoregulation is efficiently coordinated by the
central nervous system (CNS)

3. Heat production and Heat loss

4. Sweating and its regulation
 Sweating allows the body to regulate its temperature. Sweating is
controlled from a center in the preoptic and anterior regions of the
brain's hypothalamus,(thermosensitive neurons ).
 The heat-regulatory is also affected by inputs from receptors in the
skin.
 High skin temperature reduces the hypothalamic set point for sweating
and increases the gain of the hypothalamic feedback. However, the
sweating response rised in hypothalamic ('core') temperature is much
larger than the response to the same increase in average skin
temperature. The process of sweating decreases core temperature,
whereas the process of evaporation decreases surface temperature.
 two situations stimulate our sweat glands: during physical heat and
emotional stress. Emotionally induced sweating is restricted to palms,
soles, armpits, and sometimes the forehead, while physical heat-
induced sweating occurs throughout the body.

5. Hypothalamus in the body
temperature regulation
 hypothalamus (Thermostat Center) where there are
neurons sensitive to changes in skin and blood
temperatures.
 The temperature-regulating centers are found in the
Preoptic Area (the anterior portion of the hypothalamus).
This area receives input from Peripheral Thermoreceptors
and Central Thermoreceptors
 The temperature sensory signals from preoptic area and
those form the periphery are combined in the posterior.

8. Hot conditions.Temperature -
decreasing mechanisms
 In hot conditions
 Eccrine sweat glands under the skin secrete sweat (a fluid containing
mostly water with some dissolved ions). This causes heat loss via
evaporative cooling; however, a lot of essential water is lost.
 The hairs on the skin lie flat, preventing heat from being trapped by
the layer of still air between the hairs. This is caused by tiny muscles
under the surface of the skin called Arrector pili muscles relaxing so
that their attached hair follicles are not erect. These flat hairs increase
the flow of air next to the skin increasing heat loss by convection.
When environmental temperature is above core body temperature,
sweating is the only physiological way for humans to lose heat.
 Arterioles Vasodilation occurs, this is the process of relaxation of
smooth muscle in arteriole walls allowing increased blood flow
through the artery. This redirects blood into the superficial capillaries
in the skin increasing heat loss by convection and conduction.

9. Cold conditions. Temperature -
increasing mechanisms
 Sweat stops being produced.
 The minute muscles under the surface of the skin called erector pili muscles (attached to an
individual hair follicle) contract (piloerection), lifting the hair follicle upright. This makes the hairs
stand on end which acts as an insulating layer, trapping heat
 Arterioles carrying blood to superficial capillaries under the surface of the skin can shrink (constrict),
thereby rerouting blood away from the skin and towards the warmer core of the body. This prevents
blood from losing heat to the surroundings and also prevents the core temperature dropping further.
This process is called vasoconstriction. It is impossible to prevent all heat loss from the blood, only to
reduce it. In extremely cold conditions excessive vasoconstriction leads to numbness and pale skin.
Frostbite only occurs when water within the cells begins to freeze, this destroys the cell causing
damage.
 Muscles can also receive messages from the thermo-regulatory center of the brain (the hypothalamus)
to cause shivering. This increases heat production as respiration is an exothermic reaction in muscle
cells. Shivering is more effective than exercise at producing heat because the animal remains still.
This means that less heat is lost to the environment via convection. There are two types of shivering:
low intensity and high intensity. During low intensity shivering animals shiver constantly at a low
level for months during cold conditions. During high intensity shivering animals shiver violently for a
relatively short time. Both processes consume energy although high intensity shivering uses glucose
as a fuel source and low intensity tends to use fats. This is a primary reason why animals store up food
in the winter.
 Mitochondria can convert fat directly into heat energy, increasing the temperature of all cells in the
body. Brown fat is specialized for this purpose, and is abundant in newborns and animals that
hibernate.

10. Fever. Chemicals
called pyrogens
released by white
blood cells raise
the set point of the
thermoregulatory
centre causing the
whole body
temperature to
increase by 2-3 °C.
This helps to kill
bacteria, inhibits
viruses, and
explains why you
shiver even
though you are hot