Body temperature is controlled within a fairly narrow range in mammals including humans. Regulation of body temperature is acheived by balancing heat generation within the body and heat loss mainly through the skin. These functions of the hypothalamus require
connections with autonomic pathways that regulate the loss of heat by their action on skin blood
vessels and sweat glands.
its influence over the skeletal musculature in the process of shivering,
which increases metabolism and therefore body temperature.
and the production of releasing factors that regulate anterior pituitary
hormones that influence metabolism in the longer term.
Temperature -sensitive neurons in the anterior hypothalamus determine whether the body retains heat or dissipates it.
Fever occurs during infections because of the release of endogenous pyrogens that act on pre-optic areas of the anterior hypothalamus. They appear to raise the 'set-point' (normally ~37 degrees C)around which body termperature is regulated, and result in the generation of heat (e.g.by shivering) and its retention by constriction in skin blood vessels.
Hypothermia occurs when the body cools excessively due to reduced metabolism or exposure to cold (e.g. as in immersion in cold water).
Image source: www.allometric.com
The effector organs by which body temperature is controlled by the hypothalamus
Effectors used in the regulation of body temperature by the hypothalamus
The hypothalamus senses the temperature of blood flowing through it and controls it within a narrow range by balancing heat production and heat loss.
Heat production depends on metabolic activity. The basal level of metabolism depends on the production of thyroid hormones regulated by neurosecretory mechanisms and the anterior pituitary. Metabolism can be increased rapidly by the secretion of catecholamines from the adrenal medulla and by muscular activity, either voluntary (i.e. exercise) or involuntary (i.e. shivering).
Heat loss can be increased by vasodilatation of skin blood vessels and sweating. Sweating causes heat loss using the latent heat of evaporation of the water in sweat; some heat loss occurs also by hyperventilation (panting) when respiratory water loss contributes to cooling, and is an important mechanism in hairy mammals.
Heat loss through the skin depends on the delivery of blood to skin capillaries, and heat loss is minimised by cutaneous vasoconstriction, mediated largely by the noradrenergic vasoconstrictor neurones of the sympathetic system.
Insulation is another mechanism involved in regulating heat loss. Voluntary activity that increases insulation is provided by adjusting the amount and nature of clothing; the amount of fat in the skin is also a major determinant of body insulation. Air trapping between hairs during piloerection is another rapid means of improving insulation, particularly in furry animals.
Image source: what-when-how.com
This diagram shows areas of the hypothalamus involved in heat loss (anterior) and heat production (posterior)
Thermoreceptor in the hypothalamus and heat balance
The anterior hypothalamus contains neurones that are extremely sensitive to changes in the temperature of the blood. Any rise in body temperature causes these neurones to discharge and initiate heat loss from the body using sympathetic pathways to cutaneous vessels and sweat glands, and panting in animals with few sweat glands.
Heat conservation and increased heat production is mediated by another posterior group of neurones within the hypothalamus. These have effects on the autonomic and endocrine systems that result in changes in the secretion of adrenaline and thyroid hormones.
Body temperature control is a fine balance between hear gain and heat loss, and a negative feedback system in involved in its control. The normal 'set point' in the control system is ~37 degrees C, and any deviation from this narrow range is accompanied by physiological changes that result in heat gain or heatloss, to compensate for the initial shift in body temperature. The 'set point' can change in circumstances such as fever.
Endogenous pyrogens are a group of molecules classed as cytokines and are produced by activated immune cells; exogenous pyrogens include lipopolysaccharides released from the walls of bacteria that infect the body.
Endogenous pyrogens released during infections act on neurones in the anterior hypothalamus and cause fever - a raised body temperature. They alter the set point of the feedback control system, and prostaglandins are involved in the process, which is why antipyretic drugs such as aspirin lower body temperature during a fever.