Temperature homeostasis

Question 1

Define thermoregulation

Answer 1

Balance between heat gain and heat loss. It is controlled by the nervous system

Question 2

How are small alterations in body temperature achieved?

Answer 2

Altering skin blood flow

Question 3

How are large adjustments in body temperature achieved?

Answer 3

Shivering and sweating

Question 4

Where are temperature sensitive receptors found?

Answer 4

Anterior hypothalamus

Question 5

What does activation of heat-sensitive neurons cause?

Answer 5

Skin vasodilation
Sweating

Question 6

What does activation of cold-sensitive neurons cause?

Answer 6

Inhibition of heat-sensitive neurons
Vasoconstriction
Shivering

Question 7

Outside the hypothalamus, where else are thermoreceptors found? How do they have their affect?

Answer 7

Peripheral - connect centrally to cortex via conscious sensation and hypothalamus

Internal surfaces - respiratory and GI tract: Inhalation of cold air –> shivering, consumption of hot food causes sweating and vasodilation

Question 8

What is the most rapid method of temperature control?

Answer 8

Reflex vasodilation and constriction

Afferent = cutaneous nerve
Centre = hypothalamus and spinal cord
Efferent = increased or decreased sympathetic stimulation

Question 9

Describe the difference between the core and peripheral temperature

Answer 9

Core = temperature of thoracic, abdominal. contents and brain
Need to be kept at optimal temperature, measured per rectum
0.5 higher than mouth and axilla
Varies during menstrual cycle, 0.5 higher in latter half
Shows diurnal variation - higher in evening than morning

Peripheral temp less than core as heat is lost from surface to environment

Question 10

How is heat lost from the surface of the body to the surrounding environment?

Answer 10

Conduction and evaporation from skin to air

Convection from ski via air flow

Convection from lungs via tidal air flow

Radiation from naked skin

Question 11

What is the normal core temperature?

Answer 11

36-37.5

Question 12

What is the mechanism of pyrexia?

Answer 12

It is thought that there is a set-point temperature around which the core temperature is regulated by the hypothalamus.

If the set-point temperature is raised then the hypothalamus activates mechanisms leading to increasing core temperature and vice versa.

Set-point temperature is increased; therefore, the body feels cold as the core temperature is lower.

As a result the hypothalamus activates responses to increase body temperature:

■ vasoconstriction
■ piloerection (‘goose flesh’)
■ shivering.

Question 13

Name the causes of pyrexia

Answer 13

• illness
• exercise
• heatstroke
• anterior hypothalamic lesions, e.g. neoplasia, ischaemia, surgery
• hyperthyroidism
• malignant hyperpyrexia (abnormal muscle response to anaesthetics)
• failure of heat-loss mechanism, e.g. dehydration.

Question 14

Define heatstroke

Answer 14

• Occurs with body temperature above 41°C.
• Unacclimatised individuals undertaking exercise in hot, humid conditions.
• Symptoms:
  ■ nausea
  ■ vomiting
  ■ weakness
  ■ headache
  ■ skin feels hot
  ■ circulatory failure
  ■ cerebral oedema
  ■ hepatic failure
  ■ renal failure.

Question 15

What is hypothermia?

Answer 15

Core temperature (rectal) below 35°C.

Question 16

At what temperature is hypothermia fatal? Why?

Answer 16

Usually fatal below 32 degrees

Below 30 degrees temperature regulatory mechanism fails completely

Question 17

What are the symptoms of hypothermia between 32 and 35 degrees?

Answer 17

shivery and feeling cold

Question 18

What are the symptoms of hypothermia below 32 degrees?

Answer 18

bradycardia, hypotension, respiratory depression, muscle stiffness, metabolic abnormalities

Question 19

What is the usual mechanism of death caused by hypothermia?

Answer 19

cardiac arrhythmias, e cially ventricular fibrillation

Question 20

How do anaesthetics affect thermoregulation?

Answer 20

■ depress hypothalamic function

■ vasodilatation with increased heat loss

■ lack of shivering

■ consequently drop in body temperature.

Question 21

How does exercise affect thermoregulation?

Answer 21

■ increases body temperature

■ hypothalamus cannot launch responses that result in loss of heat faster than its production from muscle metabolism.

Question 22

How does circulatory shock influence thermoregulation? Name the exception

Answer 22

■ reduced tissue perfusion

■ reduced cellular metabolism and heat production

■ results in decreased body temperature

■ compensatory mechanisms include vasoconstriction, piloerection and increased secretion of catecholamines

■ skin feels cold

■ exception is septic (endotoxic) shock, where there is vasodilatation and skin feels hot.

Question 23

How do spinal injuries affect thermoregulation?

Answer 23

■ thermoregulatory mechanisms lost below level of injury

■ vasoconstriction lost; therefore, heat loss increased

■ patient unable to shiver

■ sweating in relation to hyperthermia lost below level of lesion

■ quadriplegics tend to assume temperature of environment.

Question 24

How does hyperthyroidism affect thermoregulation?

Answer 24

■ increased basal metabolic rate (BMR)

■ increased O 2 consumption

■ patient hyperactive

■ all of above contribute to increased temperature

■ patient intolerant of heat and feels hot.

Question 25

How does hypothyroidism affect thermoregulation?

Answer 25

■ opposite effects to hyperthyroidism

■ patient feels cold, intolerant of cold weather

■ body temperature low

Question 26

What is the impact of being a neonate/premature baby on temperature homeostasis?

Answer 26

■ large surface area to body weight ratio

■ inability to shiver

■ less insulating fat

■ temperature regulating mechanisms less developed

■ therefore predisposed to increased heat loss.