Physiology of thermoregulation

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

Heat loss and heat gain

The body must balance its heat budgetHeat is gained: by conduction from warm air surrounding the body by the body’s metabolic activity which generates heat e.g.

when muscle moveHeat 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

Maintaining the body temperature

Keeping warm Staying cool

Increased insulation, subcutaneous fat reduces the conduction of heat from the body

Increase blood flow to skin, increases conduction and radiation of heat from the body

Reduced sweating decreases evaporation

Increased sweat secretion, increases evaporation

Increased shivering, increases heat produced by muscle tissue 2 to 5 times

Reduced activity

nerves

Less heat generated

More water covers the skin.

More evaporation

Skin arteries dilateMore blood to the

skin. More radiation & conduction of heat

Muscles of skin arteriole

walls relaxSweat glands

increase secretion

Musclesreduce activity

Core body temperature

>37°C

Hypothalamus

Thermoreceptors

Thermoreceptors

Return to 37°C

Muscles of skin

arteriole walls relax

Core body temperature

>37°CHypothalamus

Sweat glands

increase secretion

nerves

Musclesreduce activity

Thermoreceptors

NEGATIVE FEEDBACK

Blood temperature

Body loses heat

nerves

More heat generated

Less water covers the skin.

Less evaporation

Skin arteries constrict

Less blood to the skin.

Less radiation & conduction of heat

Muscles of skin arteriole

walls constrict

Sweat glands

decrease secretion

Musclesshivering

nerves

Core body temperature

<37°C

Thermoreceptors

Hypothalamus

Thermoreceptors

Return to 37°C

NEGATIVE FEEDBACK

Blood temperature

Body loses less heat

Body gains heat

Muscles of skin

arteriole walls

constrict

Core body temperature

<37°C

Sweat glands

decrease secretion

nerves

Musclesshivering

Thermoreceptors Hypothalamus

nerves

Thermoregulation

Homeothermy—the physiological and behavioral maintenance of a relatively constant internal body temperature (homeothermstypically show circadian fluctuations in temperature)

Poikilothermy—the fluctuation of internal body temperatureclosely related to environmental temperature.

Ectothermy—use only behavioral means to regulate temperature.

Endothermy—physiological and behavioral thermoregulation, but body temperatures may fluctuate widely.

Cold blooded Warm bloodedpoikilothermy ectothermy endothermy homeothermy

Two aspects of thermoregulation

1. Heat gainvolume of thermogenic tissuebasal metabolic rateshivering or other thermogenesis

2. Heat losssurface areainsulation—fat + feathers or hairpanting , sweating, etc

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.

Short- and Long-term Thermoregulation

Short term thermoregulation is predominantly regulated by autonomic and somatic motor activity

Autonomicshivering when cold, sweating when hotperipheral vasodilaton when hot, constriction when coldpiloerection and panting in non-human mammals

Somaticseeking warm (e.g. sunny) or cool (e.g. shade or water) areasminimizing or maximizing surface area

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.

Brown fat accountsfor 5 to 6 percent ofthe body weight of the newborn rabbit.It is concentrated, asshown in sections,around the neck andbetween the shoulder blades.

Human infant atbirth 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.

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).

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⋅

Control of Heat Loss by Radiation and Conduction

For purposes of temperature control, it is convenient to view the body as a central core surrounded by a shell consisting of skin and subcutaneous tissue; we shall refer to this complex outer shell simply as skin.

It is the temperature of the central core that is being regulated at approximately 37°C. As we shall see, the temperature of the outer surface of the skin changes markedly.

Heat Exchange in the Skin

Nonshivering thermogenesis

Muscle contraction is not the only process controlled in temperature-regulating reflexes. In most experimental animals, chronic cold exposure induces an increase in metabolic rate (heat production) that is not due to increased muscle activity and is termed nonshivering thermogenesis.

Its causes are an increased adrenal secretion of epinephrine and increased sympathetic activity to adipose tissue, with some contribution by thyroid hormone as well. However, nonshivering thermogenesis is quite minimal, if present at all, in adult human beings, and there is no increased secretion of thyroid hormone in response to cold. Nonshivering thermogenesis does occur in infants.

Shivering thermogenesis Changes in muscle activity constitute the major control of

heat production for temperature regulation. The first muscle changes in response to a decrease in core body temperature are a gradual and general increase in skeletal-muscle contraction.

This may lead to shivering, which consists of oscillating rhythmical muscle contractions and relaxations occurring at a rapid rate. During shivering, the efferent motor nerves to the skeletal muscles are influenced by descending pathways under the primary control of the hypothalamus. Because almost no external work is performed by shivering, virtually all the energy liberated by the metabolic machinery appears as internal heat and is known as shivering thermogenesis. People also use their muscles for voluntary heat-producing activities such as foot stamping and hand clapping.

Scheme of reflex arc

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 Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm

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.

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.

Thermography in medical practice

Right breast cancer

Summary of Effector Mechanisms in Temperature Regulation

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.

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.

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.

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.

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.

Temperature regulating centres :

The temp. of the body is regulated almost entirely by temp. regulating centers located in the hypothalamus.

1. 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.