#109 Thermoregulation - The control of body temperature

One of the most important examples of homeostasis is the regulation of body temperature. It involves both coordination  systems - nervous and endocrine. Not all animals can do this physiologically.

Endotherms

- Animals (e.g. birds and mammals) that have a constant body temperature at around 35 - 40°C, also called warm-blooded animals. 

- Use internal corrective mechanisms to maintain body temperature. 

Ectotherms 

- Animals that have a variable body temperature.

- Use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade when hot). Such mechanisms can be very effective, particularly when coupled with internal mechanisms to ensure that the temperature of the blood going to vital organs (brain, heart) is kept constant. 

We use both!

Thermoregulation

All mammals generate heat and have ways to retain it within their bodies. They also have physiological methods to balance heat gain, retention of body heat and heat loss so that they can maintain  a constant body temperature. As a result, they are not dependent on absorbing heat from their surroundings  and can be active at any time of day or night, whatever the external temperature. Most other animals (except birds) rely on external sources of heat and are often relatively inactive when it is cold.

The heat that mammals generate is released during respiration. Much of the heat is produced
by liver cells that have a huge requirement for energy. The heat they produce is absorbed by the blood flowing through the liver and distributed around the rest of the body.

In humans, body temperature is controlled by the thermoregulatory centre in the hypothalamus. It receives input from 2 sets of thermoreceptors:

- Receptors in the hypothalamus monitor the temperature of the blood as it passes through the brain (the core temperature), that remains very close to the set point, which is 37 °C in humans. This temperature fluctuates a little, but is kept within very narrow limits by the hypothalamus.

- Receptors in the skin (especially on the trunk) monitor the external temperature.

Both sets of information are needed so that the body can make appropriate adjustments.

Our first response to encountering hotter or colder condition is voluntary:

- if too hot, we may decide to take some clothes off, or to move into the shade;
- if too cold, we put extra clothes on or turn the heating up!

It is only when these responses are not enough that the thermoregulatory centre is stimulated. This is part of the autonomic nervous system, so the various responses are all involuntary.

When we get too hot, the heat loss centre in the hypothalamus is stimulated; when we get too cold, it is the heat conservation centre of the hypothalamus which is stimulated.

Response to low temperature

If the core temperature decreases, or if the temperature receptors in the skin detect a decrease in the temperature of the surroundings, the hypothalamus sends impulses to several different effectors to adjust body temperature:

• Vasoconstriction - muscles in the walls of arterioles that supply blood to capillaries near the skin surface contract. This narrows the lumens of the arterioles and reduces the supply of blood to the capillaries so that less heat is lost from the blood.
• Shivering - the involuntary contraction  of skeletal muscles generates heat which is absorbed by the blood and carried around the rest of the body.
• Raising body hairs - muscles at the base of hairs in the skin contract to increase the depth of fur so trapping air close to the skin. Air is a poor conductor of heat and therefore a good insulator. This is not much use in humans, but is highly effective for most mammals.
•  Decreasing  the production  of sweat - this reduces the loss of heat by evaporation from the skin surface.
• Increasing  the secretion of adrenaline  - this hormone from the adrenal gland increases the rate of heat production in the liver.

Response to high temperature

When an increase in environmental  temperature is detectedby skin receptors or the central thermoreceptors, thehypothalamus increases the loss of heat from the body andreduces heat production.

• Vasodilation - the muscles in the arterioles in the skin relax, allowing more blood to flow through the capillaries so that heat is lost to the surroundings.
• Lowering body hairs - muscles attached to the hairs relax so they lie flat, reducing the depth of fur and the layer of insulation.
• Increasing sweat production  - sweat glands increase the production of sweat which evaporates on the surface of the skin so removing heat from the body.

Behavioural responses 

The behavioural responses of animals to heat include resting or lying down with the limbs spread out to increase the body surface exposed to the air. We respond by wearing loose fitting clothing, turning on fans or air conditioning and taking cold drinks.

When the environmental temperature decreases gradually:

- The hypothalamus releases a hormone which activates the anterior pituitary gland to release thyroid stimulating hormone (TSH).
- TSH stimulates the thyroid gland to secrete the hormone thyroxine into the blood.
- Thyroxine increases metabolic rate, which increases heat production especially in the liver.

When temperatures start to increase again, the hypothalamus responds by reducing the release of TSH by the anterior pituitary gland so less thyroxine is released from the thyroid gland.

  

Effector	Response to low temperature	Response to high temperature
Smooth muscles in peripheral arterioles in the skin.	- Muscles contract causing vasoconstriction.  - Less heat is carried from the core to the surface of the body, maintaining core temperature.  - Extremities can turn blue and feel cold and can even be damaged (frostbite).	- Muscles relax causing vasodilation.  - More heat is carried from the core   to the surface, where it is lost by radiation.  - Skin turns red.
Sweat glands	No sweat produced.	- Glands secrete sweat onto surface of skin, where it evaporates.  - Water has a high latent heat of evaporation, so it takes heat from the body.
Erector pili muscles in skin (attached to skin hairs)	- Muscles contract, raising skin hairs and trapping an insulating layer of still, warm air next to the skin.  - Not very effective in humans, just causing "goosebumps".	- Muscles relax, lowering the skin hairs and allowing air to circulate over the skin, encouraging convection and evaporation.
Skeletal muscles	Muscles contract and relax repeatedly, generating heat by friction and from metabolic reactions.	No shivering.
Adrenal and thyroid glands	Glands secrete adrenaline and thyroxine respectively, which increase the metabolic rate in different tissues, especially the liver, so generating heat.	Glands stop releasing adrenaline and thyroxine.
Behaviour	Curling up, huddling, finding shelter, putting on more clothes.	Stretching out, finding shade, swimming, removing clothes.

VIDEO 

Controlling body temperature 

Homeostasis

Syllabus 2016-2018  14.1  Homeostasis in mammals Homeostasis in mammals requires complex systems to maintain  internal conditions near  constant. The kidneys  remove wastes from the blood and are the effectors for controlling the water potential of the blood. a)   discuss the importance of homeostasis in mammals and explain the principles  of homeostasis in terms of internal and external stimuli, receptors, central  control,  co-ordination systems, effectors (muscles and glands) b)   define  the term  negative feedback and explain how it is involved in homeostatic mechanisms c)   outline  the roles of the nervous system and endocrine system in co-ordinating homeostatic mechanisms, including thermoregulation, osmoregulation and the control of blood glucose concentration d)   describe the deamination of amino  acids  and outline  the formation of urea  in the urea  cycle (biochemical detail of the urea  cycle is not required) e)   describe the gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs f) describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron g)   describe the roles of the hypothalamus, posterior pituitary, ADH and collecting  ducts in osmoregulation h)   explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon i) outline  the role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon j) describe the three main stages of cell signalling in the control of blood glucose by adrenaline as follows: •   hormone-receptor interaction at the cell surface •   formation of cyclic AMP which binds to kinase  proteins • an enzyme cascade involving activation  of enzymes by phosphorylation to amplify the signal k)   explain the principles  of operation of dip sticks  containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine l) explain how urine analysis  is used in diagnosis with reference to glucose, protein  and ketones