FR2- Energy Balance

Question 1

What is the upper limit of body temperature compatible with life?

Answer 1

Most people suffer convulsions when the internal body temperature reaches about 106°F (41°C); 110°F (43.3°C) is considered the upper limit compatible with life

Question 2

Internal core temperature is homeostatically maintained at what temperature?

Answer 2

100°F (37.8°C)

Question 3

From a thermoregulatory viewpoint, how is the body viewed?

Answer 3

A central core surrounded by an outer shell:

• The temperature within the central core, which consists of the abdominal and thoracic organs, the central nervous system, and the skeletal muscles, generally remains fairly constant. This internal core temperature is subject to precise regulation to maintain its homeostatic constancy. The core tissues function best at a relatively constant temperature of around 100°F (37.8°C)
• The skin and subcutaneous fat constitute the outer shell. In contrast to the constant high temperature in the core, the temperature within the shell is generally cooler and may vary substantially. For example, skin temperature may fluctuate between 68°F and 104°F (20°C and 40°C) without damage

Question 4

What recent device has been developed to measure body temperature?

Answer 4

• A more recent device is the temporal scanner, a computerized instrument that is gently stroked across the forehead to measure the temperature of the blood in the temporal artery, which lies less than 2 mm below the skin surface in this region.
• Temporal temperature is the best determinant of core temperature because it is nearly identical to the temperature of the blood exiting the heart

Question 5

Heat input must balance heat output to maintain a stable core temperature

Answer 5

• Heat input* occurs by way of heat gain from the external environment and internal heat production, the latter being the most important source of heat for the body. Usually, more heat is generated than required to maintain normal body temperature, so the excess heat must be eliminated.
• Heat output* occurs by way of heat loss from exposed body surfaces to the external environmen

Question 6

How does heat move?

Answer 6

Heat always moves down its concentration gradient—that is, down a thermal gradient from a warmer to a cooler region

The temperature of an object is a measure of the concentration of heat within the object

Question 7

Explain radiation

Answer 7

• Radiation is the emission of heat energy from the surface of a warm body in the form of electromagnetic waves, or heat waves, which travel through space.
• When radiant energy strikes an object and is absorbed, the energy of the wave motion is transformed into heat within the object. The human body both emits (source of heat loss) and absorbs (source of heat gain) radiant energy.
• Whether the body loses or gains heat by radiation depends on the difference in temperature between the skin surface and the surfaces of other objects in the body’s environment
• On average, humans lose close to half of their heat energy through radiation

Question 8

Explain conduction

Answer 8

• Conduction is the transfer of heat between objects of differing temperatures that are in direct contact with each other, with heat moving down its thermal gradient from the warmer to the cooler object
• The rate of heat transfer by conduction depends on the temperature difference between the touching objects and the thermal conductivity of the substances involved
• Heat can be lost or gained by conduction when the skin is in contact with a good conductor
• Only a small percentage of total heat exchange between the skin and the environment takes place by conduction alone, however, because air is not a good conductor of heat

Question 9

Explain convection

Answer 9

• The term convection refers to the transfer of heat energy by air (or water) currents.
• As the body loses heat by conduction to the surrounding cooler air, the air in immediate contact with the skin is warmed. Because warm air is lighter (less dense) than cool air, the warmed air rises while cooler air moves in next to the skin to replace the vacating warm air.
• The process is then repeated.
• These air movements, known as convection currents, help carry heat away from the body.
• Without convection currents, no further heat could be dissipated from the skin by conduction once the temperature of the layer of air immediately around the body equilibrated with skin temperature.

Question 10

Explain evaporation

Answer 10

• During evaporation from the skin surface, the heat required to transform water from a liquid to a gaseous state is absorbed from the skin, thereby cooling the body
• Evaporative heat loss occurs continually from the linings of the respiratory airways and from the surface of the skin.
• Heat is continuously lost in expired air as a result of the air being humidified

Question 11

Sweating is a regulated evaporative heat-loss process under what control?

How much sweat is produced at rest, during hot weather and during exercise?

Answer 11

• under sympathetic nervous control
• By varying the extent of sweating, which is an important homeostatic mechanism to eliminate excess heat as needed. In fact, when the environmental temperature exceeds the skin temperature, sweating is the only avenue for heat loss because the body is gaining heat by radiation and conduction under these circumstances.
• At normal temperature, an average of 100 mL of sweat is produced per day; this value increases to 1.5 liters during hot weather and climbs to 4 liters during heavy exercise

Question 12

Explain the composition of sweat and how sweat glands are innervated

Answer 12

• Most sweat is an odorless, dilute salt solution actively extruded to the surface of the skin by eccrine sweat glands dispersed all over the body.
• This clear, salty sweat is the kind important in cooling the body. Eccrine sweat glands also produce dermcidin, a newly discovered antimicrobial peptide, a natural antibiotic in sweat that helps defend against potential skin infections.
• Apocrine sweat glands, which are located primarily in the armpits and genital area, produce a thick, milky sweat that is rich in organic constituents, like proteins and lipids.
• Apocrine sweat is initially odorless, but body odor is generated when bacteria in the vicinity break down these organic compounds into substances that have an unpleasant scent
• Apocrine sweat is most abundant during emotional stress and sexual excitement.
• Both eccrine and apocrine sweat glands are stimulated by sympathetic innervation, but the postganglionic sympathetic fibers supplying eccrine glands are unusual in that they release acetylcholine instead of norepinephrine
• ACh binds with muscarinic receptors on the eccrine glands, whereas apocrine glands have the usual adrenergic receptors that bind with catecholamines

Question 13

What is the the most important factor determining the extent of evaporation of sweat

Answer 13

• The relative humidity of the surrounding air (the percentage of H2O vapor actually present in the air compared to the greatest amount that the air can possibly hold at that temperature; for example, a relative humidity of 70% means that the air contains 70% of the H2O vapor it is capable of holding).
• When the relative humidity is high, the air is already almost fully saturated with H2O, so it has limited ability to take up additional moisture from the skin

Question 14

What is the body’s thermoregulatory integrating center?

Answer 14

The hypothalamus

• It receives afferent information about the temperature in various regions of the body and initiates extremely complex, coordinated adjustments in heatgain and heat-loss mechanisms as necessary to correct any deviations in core temperature from normal.
• The hypothalamus is far more sensitive than your home thermostat. It can respond to changes in blood temperature as small as 0.01°C
• the hypothalamus must be apprised continuously of both the core and the skin temperature by specialized temperature-sensitive receptors called thermoreceptors.
• The core temperature is monitored by central thermoreceptors, which are located in the hypothalamus itself, and in the abdominal organs and elsewhere.
• Peripheral thermoreceptors monitor skin temperature throughout the body

Question 15

Where are the two centers for temperature regulation?

Answer 15

Inthe hypothalamus:

• The posterior region, activated by cold, triggers reflexes that mediate heat production and heat conservation.
• The anterior region, activated by warmth, initiates reflexes that mediate heat loss

Question 16

What is the primary involuntary means of increasing heat production?

Answer 16

Shivering

Question 17

Explain why shivering is the primary involuntary means of increasing heat production

Answer 17

• skeletal muscle tone. (Muscle tone is the constant level of tension within the muscles.)
• Soon shivering begins. Shivering consists of rhythmic, oscillating skeletal muscle contractions that occur at a rapid rate of 10 to 20 per second.
• This mechanism is efficient and effective in increasing heat production; all energy liberated during these muscle tremors is converted to heat because no external work is accomplished

Question 18

Explain the process of nonshivering Thermogenesis by Brown Fat

Who is this process especially important to?

Answer 18

• Nonshivering thermogenesis is mediated on cold exposure by the sympathetic nervous system, which increases heat production by stimulating brown adipose tissue, or brown fat, a special type of adipose tissue that is especially capable of converting chemical energy from food into heat
• In humans nonshivering thermogenesis is most important in newborns, who have prominent deposits of brown fat.
• Unlike the ordinary white adipose tissue that stores energy in the form of triglyceride deposits, brown adipose tissue acts like a furnace that burns energy to generate heat.
• Newborns use brown fat to keep warm because they cannot shiver

Question 19

Why is brown fat brown in colour?

What is special about brown fat mitochondria?

Answer 19

• Brown fat is brown in color because it has an abundance of mitochondria that contain iron, which causes the tissue to appear reddish brown.
• The mitochondria of brown fat contain a unique uncoupling protein called thermogenin (“heat producer”) that uncouples the electron transport system from the process of generating ATP (see p. 37) during oxidation of glucose and fatty acids.
• Instead of some of the energy released by the electron transport system being harnessed in ATP by chemiosmosis, all of the energy is dissipated as heat

Question 20

Do Brown fat deposits regress beyond infancy and disappear by 2 years of age?

Answer 20

• However, new radiologic imaging techniques such as PET scans have revealed small, persistent brown fat reserves in adults.
• Brown fat is more abundant in lean people and decreases with advancing age.
• As expected, adult brown fat becomes more active on cold exposure.
• Brown fat not only is a heat-producing, but also a calorieconsuming tissue because all of the calories in nutrients burned by brown fat are turned into heat

Question 21

How many calories does 50g of brown fat burn?

Answer 21

500 calories per day

Question 22

What causes white fat cells to act like brown fat cells?

Answer 22

• Recent studies suggest that exercise may promote “browning” of white adipose tissue.
• Exercising muscles release irisin into the blood.
• This hormone appears to promote synthesis of uncoupling proteins in mitochondria of white fat cells, making these cells act more like brown fat cells

Question 23

What is the amount of heat lost to the environment by radiation and the conduction–convection process largely determined by?

Answer 23

The temperature gradient between the skin and the external environment.

Question 24

How does the skin’s blood vessels diminish the effectiveness of the skin as an insulator and promote heat loss?

Answer 24

By carrying heat to the surface, where it can be lost from the body by radiation and the conduction–convection process. Accordingly, vasodilation of skin arterioles increases heat loss by permitting increased flow of heated blood through the skin

Question 25

What happens to blood vessels in order to decrease heat loss?

Answer 25

Skin arteriolar vasoconstriction, which reduces skin blood flow, decreases heat loss by keeping the warm blood in the central core, where it is insulated from the external environment. This response conserves heat that otherwise would have been lost

Question 26

Vasoconstriction and vasodilation are called vasomotor responses and are coordinated by what structure?

Answer 26

The hypothalamus by means of sympathetic nervous system output

Question 27

Does increased sympathetic activity lead to vasoconstriction?

Answer 27

Yes it does in response to cold exposure whereas decreased sympathetic activity produces heat-losing vasodilation of these vessels in response to heat exposure

Question 28

The cardiovascular control center in the medulla also exerts control over the skin arterioles (and arterioles throughout the body) by means of adjusting sympathetic activity to these vessels for the purpose of blood pressure regulation Does the casoconstrictor response by the medullary cardiovascular control center override the hypothalamic thermoregulatory center?

Answer 28

the skin vasodilator response set in motion by the hypothalamic thermoregulatory center overrides the skin vasoconstrictor response called forth by the medullary cardiovascular control center

Question 29

Coordinated Responses to Cold Exposure: What does the hypothalamus do? Is vasoconstriction the only measure?

Answer 29

* In response to cold exposure, the posterior region of the hypothalamus directs increased heat production, such as by shivering, while simultaneously decreasing heat loss (that is, conserving heat) by skin vasoconstriction and other measures * Puffing up traps a layer of poorly conductive air between the skin surface and the environment, thus increasing the insulating barrier between the core and the cold air and reducing heat loss * After maximum skin vasoconstriction has been achieved, further heat dissipation in humans can be prevented only by behavioral adaptations

Question 30

Do hairs that trap warm air constitute an effective cold response?

Answer 30

Even though the hair-shaft muscles contract in humans in response to cold exposure, this heat-retention mechanism is ineffective because of the low density and fine texture of most human body hair. The result instead is useless goose bumps

Question 31

After maximum skin vasoconstriction has been achieved, further heat dissipation in humans can be prevented only by behavioral adaptations, such as:

Answer 31

* Postural changes that reduce as much as possible the exposed surface area from which heat can escape. * Hunching over or clasping the arms in front of the chest. * Putting on warmer clothing further insulates the body from too much heat loss. Clothing entraps layers of poorly conductive air between the skin surface and the environment, thereby diminishing loss of heat by conduction from the skin to the cold external air and curtailing the flow of convection currents.

Question 32

Summarise coordinated adjustments in Response to cold exposure

Answer 32

Question 33

Coordinated Responses to Heat Exposure: What is the main response? What happens if the air temperature rises above the temperature of maximally vasodilated skin?

Answer 33

* During heat exposure, the anterior part of the hypothalamus reduces heat production by decreasing skeletal muscle activity and promotes increased heat loss by inducing skin vasodilation * When even maximal skin vasodilation is inadequate to rid the body of excess heat, sweating is brought into play to accomplish further heat loss through evaporation * If the air temperature rises above the temperature of maximally vasodilated skin, the temperature gradient reverses itself so that heat is gained from the environment. Sweating is the only means of heat loss under these conditions

Question 34

What voluntary measures can be taken to respond to heat exposure? Is wearing light-colored, loose clothing is cooler than being nude?

Answer 34

* Humans also employ voluntary measures, such as using fans, wetting the body, drinking cold beverages, and wearing cool clothing, to further enhance heat loss. * Contrary to popular belief, wearing light-colored, loose clothing is cooler than being nude. Naked skin absorbs almost all radiant energy that strikes it, whereas light-colored clothing reflects almost all radiant energy that falls on it. * Thus, if light-colored clothing is loose and thin enough to permit convection currents and evaporative heat loss to occur, wearing it is actually cooler than going without any clothes

Question 35

What is the thermoneutral zone?

Answer 35

* Skin vasomotor activity is highly effective in controlling heat loss in environmental temperatures between the upper 60s and mid-80s. * This range, within which core temperature can be kept constant by vasomotor responses without calling supplementary heat-production mechanisms (shivering) or heat-loss mechanisms (sweating) into play, is called the thermoneutral zone

Question 36

During a fever, the hypothalamic thermostat is “reset” at an elevated temperature, how and why does this oocur? Why do you sometimes feel cold when you have a fever?

Answer 36

* The term fever refers to an elevation in body temperature as a result of infection or inflammation. * In response to microbial invasion, macrophages release endogenous pyrogen, which acts on the hypothalamic thermoregulatory center to raise the thermostat setting * The hypothalamus now maintains the temperature at the new set level instead of maintaining normal body temperature. * If, for example, endogenous pyrogen raises the set point to 102°F (38.9°C), the hypothalamus senses that the normal pre-fever temperature is too cold, so it initiates coldresponse mechanisms to raise the temperature to 102°F. * It promotes skin vasoconstriction to rapidly reduce heat loss and initiates shivering to rapidly increase heat production, both of which drive the temperature upward * These events account for the sudden cold chills often experienced at the onset of a fever * blankets to help raise body temperature by conserving body heat. Once the new temperature is achieved, body temperature is regulated as normal in response to cold and heat

Question 37

Why do many medical experts believe that a rise in body temperature has a beneficial role in fighting infection? How can Aspirin be helpful?

Answer 37

* A fever augments the inflammatory response and may interfere with bacterial multiplication. * During fever production, endogenous pyrogen raises the set point of the hypothalamic thermostat by triggering the local release of prostaglandins, which are local chemical mediators that act directly on the hypothalamus * . Aspirin reduces a fever by inhibiting synthesis of prostaglandins. * Aspirin does not lower the temperature in a nonfebrile person because in the absence of endogenous pyrogen, prostaglandins are not present in the hypothalamus in appreciable quantities