Homeostatic Mechanisms

Question 1

What does skin temperature rise and fall with?

Answer 1

The environmental temperature. (as an aside not rectal thermometers are more accurate and read typically .6C warmer or 1.0F warmer than oral)

Question 2

Core temperatures vary with? What effect does the variable have on the temperature (ie. raises it or lowers it?)

Answer 2

Time of day - lowest between 3am and 6am with the highest between 3pm and 6pm
Stage of menstrual cycle - increasing 1C during the post-ovulatory phase (progesterone)
Level of activity - increasing with exercise and emotional states
Age - being higher in active children and lower in aged adults.

Question 3

Maintenance of a stable body temperature involves ______ feedback control with a ______ gain

Answer 3

Negative. Very high gain (~25-30). For example the core body temperature of humans changes 1C for every 25-30C change in environmental temperature.

Question 4

A body immersed in water exchanges most heat by?

Answer 4

Convection

Question 5

Which heat removal mechanism (ie. radiation, conduction, convection, evaporation) dissipates the most heat?

Answer 5

Evaporation by sweating normally dissipates nearly all of the heat produced during exercise.

Question 6

Most body heat is generated how and where?

Answer 6

By cellular metabolism in the deep organs (liver, brain, heart, active skeletal muscle)

Question 7

The rate of heat loss is determines largely by?

Answer 7

1 how rapidly heat is carried from the core to the skin and (2) transferred from the skin to the surroundings. (mostly by convection through the blood.

Question 8

What is passive or unregulated heat transfer?

Answer 8

In the steady state, the rate of heat production by the body core must be matched by the flow of heat from the core to the skin, and from skin to environment. *Note various homeostatic controls-systems not directly involved in temperature regulation can also affect heat flow.

Question 9

The skin is a highly effective controlled heat radiator system, how does it work?

Answer 9

At relatively low environmental temperatures the arterioles and arteriovenous anastomosis that supply blood to the venous plexus of the skin are constricted. As the environmental temperature increases, vasodilation suserves heat conductance through the skin. The Sympathetic nervous system controls this.

Question 10

What happens during acclimatization to hot weather?

Answer 10

There is a change in sweat glands allowing a change from 1L/hr in sweat to up to 2-3L/hr loss in sweat. Aldosterone secretion from the adrenal cortex also leads to a decrease in loss of NaCl in sweat.

Question 11

How are sweat glands innervated?

Answer 11

By an acetylcholine-secreting sympathetic nerve.

Question 12

Where do free nerve endings that function as thermal sensors located? How do they respond to changes in local temperature?

Answer 12

The skin and in the hypothalamus. They alter their frequency of firing of action potentials. They anticipate changes in core temp.

Question 13

Does the skin have anatomically distinct receptors for warmth and cold?

Answer 13

Yes. There are 10-fold as many cold receptors in many parts of the skin. The skin receptors work with the distinct deep body receptors which are sensitive primarily to cold in the body core to prevent hypothermia. They project to a control center in the hypothalamus.

Question 14

When do the cold and the warmth receptors fire with the same frequency?

Answer 14

When the skin temperature is at 37C

Question 15

There are both heat and cold-sensitive neurons in the hypothalamus which has proportionately more neurons?

Answer 15

There are more heat-sensitive neurons.

Question 16

What is “anticipatory feedback?

Answer 16

Changes in skin temperature reflect the environment and the resulting reflexes prevent corresponding changes in body core temperature.

Question 17

Where does negative feedback control happen? What is it used for?

Answer 17

Changes in the core temperature such as during exercise. Serves to minimize the change in core temperature.

Question 18

How does the body react to an increase in body temperature?

Answer 18

Skin vasodilation, sweating, and decreased heat production to reduce body heat.

Question 19

How does the body react to a decrease in body temperature?

Answer 19

Engages skin vasoconstriction, piloerection (not as important in humans), thermogenesis. Sympathetic/ chemical excitation involving epinephrine and norepi may be relevant in infants.

Question 20

What is the most common cause of lethal hypothermia?

Answer 20

Immersion in cold water for an extended period of time

Question 21

Why is the rise in core temperature an essential feature of homeostatic thermoregulation?

Answer 21

It provides the error signal that sustains the sweating response during exercise.

Question 22

What effect does pyrogens have on the set point of the hypothalamic temperature-regulating center?

Answer 22

They increase the set point thereby enhancing heat conservation and heat production.

Question 23

How do aspirin and acetaminophen inhibit the raising of the set point?

Answer 23

They inhibit prostaglandin synthesis and reduce fever.

Question 24

What effect does raising the set point have on the hypothalamus and the body?

Answer 24

The hypothalamus ‘thinks’ that the body is too cold. The person develops chills leading to heat conservation mechanisms such as; vasoconstriction, piloerection, eli secretion, and shivering.

Question 25

How does heat stress and cold stress affect the body?

Answer 25

Heat stress and cold stress both compromise the cardiovascular system. Heat stress compromises the fluid and electrolyte balance while cold stress can compromise neuronal function.

Question 26

Describe heat exhaustion (heat collapse)

Answer 26

A common heat disorder due to a failure in cardiovascular homeostasis in a hot environment. Decrease in circulation blood volume caused by skin vasodilation and a sweating-induced decrease in central venous pressure. Patient typically has dilated pupils and sweats profusely. Rest in a cool environment plus fluid/electrolyte replacement. Core temperature may be normal or mildly elevated.

Question 27

Describe heatstroke

Answer 27

Elevated core body temperature in conduction with sever neurological disturbances including loss of consciousness and confusions. Cell/tissue damage occurs throughout the body. The weather conditions can affect heatstroke (dry air versus humid due to differences in convective currents and evaporation). Treatment requires rapid lowering of core body temperature, vigorous hydration, airway maintenance, avoidance of aspiration.

Question 28

What is the difference between classical heatstroke and exertional.

Answer 28

Classical is where environmental stress overwhelms and impaired thermoregulatory system (most patients have preexisting chronic disease). Exertional is where the primary factor is high metabolic heat production. Patients are generally soldiers or athletes.

Question 29

Describe malignant hyperthermia

Answer 29

A massive increase in metabolic rate, oxygen consumption, and heat production in skeletal muscle that can be lethal. Many of the afflicted have a mutation in the ryanodine receptor which disrupts calcium homeostasis in skeletal muscles. Treatment involves discontinuation of the triggering agent (if chemical), use of ryanodine receptor antagonists, and cooling of the body.

Question 30

Describe hypothermia

Answer 30

Core temperature below 35C. Heat production can not increase to compensate for heat loss. Hypothermia leads to drowsiness, slurred speech, braycardia, and hypoventilation associated with cold-induced decreases in metabolic rate. Severe hypothermia can lead to coma, hypotenstion, and fatal cardiac arrhythmias (V-fib).

Question 31

Describe frostbite

Answer 31

Exposure to extremely low temperatures casuses freezing of surface areas (vulnerable - earlobes, hands, feet.) Permanent necrotic damage occurs when extensive ice crystals form in the cells of the skin and subcutaneous areas. Gangrene often follows and surgical removal is necessary. Sudden cold induced vasodilation can occur but is more developed in lower animals