Physiology of thermoregulation

1. Physiology of thermoregulation

2. Core temperature • The core temperature of the human body is 37°C • The core of the human body includes the organs of the thorax, abdomen and the head • This is where the vital organs are located • Their enzyme systems must operate in optimum conditions • The periphery of the body can withstand some deviation from the core temperature

3. Heat loss and heat gain The body must balance its heat budget Heat is gained: • by conduction from warm air surrounding the body • by the body’s metabolic activity which generates heat e.g. when muscle move Heat is lost: • by conduction and radiation to cold air (or water) • by evaporation of sweat from the body surface (c.f. properties of water) • Humans can also affect their body temperature by changing their behavioure.g. wearing different clothes, seeking shade

4. Maintaining the body temperature

5. Core body temperature >37°C Thermoreceptors Hypothalamus nerves Muscles of skin arteriole walls relax Sweat glands increase secretion Muscles reduce activity Skin arteries dilate More blood to the skin. More radiation & conduction of heat More water covers the skin. More evaporation Less heat generated

6. Muscles of skin arteriole walls relax Sweat glands increase secretion Core body temperature >37°C nerves Hypothalamus Thermoreceptors Muscles reduce activity NEGATIVE FEEDBACK Blood temperature Thermoreceptors Body loses heat Return to 37°C

7. Core body temperature <37°C Thermoreceptors nerves Hypothalamus Muscles of skin arteriole walls constrict nerves Sweat glands decrease secretion Skin arteries constrict Less blood to the skin. Less radiation & conduction of heat Muscles shivering Less water covers the skin. Less evaporation More heat generated

8. Muscles of skin arteriole walls constrict nerves Sweat glands decrease secretion Core body temperature <37°C Hypothalamus Thermoreceptors Muscles shivering nerves NEGATIVE FEEDBACK Blood temperature Thermoreceptors Body loses less heat Return to 37°C Body gains heat

9. Thermoregulation Homeothermy—the physiological and behavioral maintenance of a relatively constant internal body temperature (homeotherms typically show circadian fluctuations in temperature) Endothermy—physiological and behavioral thermoregulation, but body temperatures may fluctuate widely. Ectothermy—useonly behavioral means to regulate temperature. Poikilothermy—thefluctuation of internal body temperature closely related to environmental temperature. Cold blooded Warm blooded poikilothermy ectothermy endothermy homeothermy

10. Two aspects of thermoregulation 1. Heat gain volume of thermogenic tissue basal metabolic rate shivering or other thermogenesis 2. Heat loss surface area insulation—fat + feathers or hair panting , sweating, etc

11. Receptors for Temperature Peripheral thermoreceptors are found in the skin Central thermoreceptors are found in the anterior hypothalamus These thermoreceptors are important for behavioral and physiological thermoregulation both in the short term and in the long term.

12. Short- and Long-term Thermoregulation Short term thermoregulation is predominantly regulated by autonomic and somatic motor activity • Autonomic • shivering when cold, sweating when hot • peripheral vasodilaton when hot, constriction when cold • piloerection and panting in non-human mammals • Somatic • seeking warm (e.g. sunny) or cool (e.g. shade or water) areas • minimizing or maximizing surface area

13. Autonomic and hormonal responses to thermal challenges are mediated primarily by the medial preoptic area. Behavioral responses to thermal challenges are mediated principally by the lateral hypothalamus. Maintenance of thermal homeostasis is arguably the greatest factor influencing both energy balance (via calories spent for thermogenesis and the need for fat stores) and water and mineral balance [water lost via evaporation through the skin, metabolic processes necessary for homeothermy and (in many non-humans)], panting.

14. Brown fat accounts for 5 to 6 percent of the body weight of the newborn rabbit. It is concentrated, as shown in sections, around the neck and between the shoulder blades. Human infant at birth has a thin sheet of brown adipose tissue between the shoulder blades and around the neck, and small deposits behind the breastbone and along the spine.

19. Internal Body Temperature • • Humans are homeothermic (i.e. body temperature is • maintained independent of environmental temperature) • • Body temperature often described as either: • – ‘Core’ (typically 36.1-37.8˚C) • – ‘Shell’ (ideally 33˚C but up to 42˚C in contracting muscle) • • Specific Sites of Assessment: • – Brain; Aorta; Oesophagus; Aural Cavity; Rectum; • Intestine (Gant et al. November 2006 MSSE).

20. Heat Balance • • Constant internal temperature requires a balance • between heat gain and heat loss • • Even in a thermoneutral environment, basal • metabolism produces 1 kcal⋅kg-1 • • The specific heat of human tissue only requires • 0.83 kcal⋅kg-1 to raise internal temperature by 1˚C • • Therefore, without heat loss processes, internal • temperature would elevate by 1˚C⋅h -1 even at rest

24. Control of Heat Loss by Radiation and Conduction • For purposes of temperature control, it is convenientto view the body as a central core surrounded by ashell consisting of skin and subcutaneous tissue; weshall refer to this complex outer shell simply as skin. • It is the temperature of the central core that is beingregulated at approximately 37°C. As we shall see, thetemperature of the outer surface of the skin changesmarkedly.

25. Heat Exchange in the Skin

26. Nonshiveringthermogenesis • Muscle contraction is not the only process controlledin temperature-regulating reflexes. In most experimentalanimals, chronic cold exposure induces anincrease in metabolic rate (heat production) that is notdue to increased muscle activity and is termed nonshiveringthermogenesis. • Its causes are an increasedadrenal secretion of epinephrine and increased sympatheticactivity to adipose tissue, with some contributionby thyroid hormone as well. However, nonshiveringthermogenesis is quite minimal, if present atall, in adult human beings, and there is no increasedsecretion of thyroid hormone in response to cold. Nonshiveringthermogenesis does occur in infants.

27. Shivering thermogenesis • Changes in muscle activityconstitute the major control of heat production fortemperature regulation. The first muscle changes in responseto a decrease in core body temperature are agradual and general increase in skeletal-muscle contraction. • This may lead to shivering, which consists ofoscillating rhythmical muscle contractions and relaxationsoccurring at a rapid rate. During shivering, theefferent motor nerves to the skeletal muscles are influencedby descending pathways under the primary controlof the hypothalamus. Because almost no externalwork is performed by shivering, virtually all the energyliberated by the metabolic machinery appears as internalheat and is known as shivering thermogenesis. Peoplealso use their muscles for voluntary heat-producingactivities such as foot stamping and hand clapping.

28. Scheme of reflex arc

30. Head Thermogram • Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latininfra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750nm and 1 mm

31. Infrared thermography • Infrared thermography is a non-contact, non-destructive test method that utilizes a thermal imager to detect, display and record thermal patterns and temperatures across the surface of an object.

32. Thermology • Thermology is the medical science that derives diagnostic indications from highly detailed and sensitive infrared images of the human body. Thermology is sometimes referred to as medical infrared imaging or tele-thermology and utilizes highly resolute and sensitive infrared (thermographic) cameras. Thermology is completely non-contact and involves no form of energy imparted onto or into the body. Thermology has recognized applications in breast oncology, chiropractic, dentistry, neurology, orthopedics, occupational medicine, pain management, vascular medicine/cardiology and veterinary medicine.

33. Thermography in medical practice • Right breast cancer

35. Summary of Effector Mechanisms in Temperature Regulation

36. Role of brown fat • In newborns very important role in thermo regulative processes has brown fat. It’s present under the skin of neck, between scapulars. That gives condition for blood supply of brain, where the cells are very sensate to disbalance of temperature homeostasis. Brown fat is well innervated by sympathetic nerves and well provided with blood. • In the cells of brown fat small drops of fat are present. In a white cells there is only one drop of fat. Quantity of mitochondria, cytochroms is greater in brown fat. Speed of fat acids oxidation 20 times higher, but absent synthesis and hydrolysis of ATP, that is why the heat produced immediately. That is caused by presents of special membrane polypeptide – termogenine. When it is necessary increase of brown fat oxygenation may be added to increase the heat production in 2-3 times. Children, especially of first year life, do not so sensitive as adult to change of temperature homeostasis. That's why they don't cry when they lost heat.

37. B.) Heat loss from the skin surface 1. Radiation : It means transfer of heat as infrared electromagnetic rays from one object to another at a different temp. with which it is not contact.Human body radiates heat rays in all directions but is also exposed to heat rays radiated from the surrounding. Therefore, heat is lost by this method when the temp. of the surrounding is less than the body temp. At normal temp. about 60% of the total heat loss from a nude person occurs by radiation.

38. 2. Conduction : • It means heat exchange between objects at different temp. that are in contact with one another. Only minute amount of heat are normally lost from the body by direct conduction to other objects such as a chair or bed (3%). On the other hand large amounts of heat are lost by conduction to air.

39. 3. Convection : • It is the removal of heat from the body by convection air currents.Heat must first be conducted to the air and then carried away by the convection current.About 12% of heat loss from the body occurs by conduction to the air and then by convection.When the body is exposed to wind heat loss by convection is greatly increased.

40. 4. Evaporation : • Evaporation of 1g water removes about 0.6 Kcal. Of heat. Even when a person is not sweating a certain amount of water still evaporates from the skin and lungs at a rate of about 600 ml/day. This is known as the insensible water loss. • Sweating provides a very important way of heat loss from the body which can be regulated. As long as skin temp. is greater than the temp. of surrounding, heat can be lost by radiation & conduction, but when the temp. of the body gains heat by radiation& conduction.

41. Temperature regulating centres : • The temp. of the body is regulated almost entirely by temp. regulating centers located in the hypothalamus. • The anterior hypothalamus-preoptic area : The preoptic & anterior hypothalamic nuclei contain two types of neurons : A) Heat – Sensitive neurons(receptors) which are present in large numbers.They increase their rate of firing as the temp. rises.