Homeostasis and Thermoregulation

Question 1

What are the costs and benefits of thermoregulation mechanisms in poikilotherms?

Answer 1

• Cold blooded

Benefits:
- Reduced food requirements

Costs:

• Depend on absorbing heat from the environment for function (ECTOTHERMIC)
• inactive in cold
• seek shelter in heat

Question 2

What are the costs and benefits of thermoregulation in homeotherms?

Answer 2

• Warm blooded

Benefits
- Function in wide range of temperatures by producing own heat through metabolism (ENDOTHERMIC)

Costs:
- Maintaining constant temp requires higher energy consumption –> more food

Question 3

Identify areas of the body where temperature gradients exist.

Answer 3

• Rectal = core body temp
• Oral = 0.6C lower
• Underarm and oral are both lower
• Skin temp varies widely up to 20C
• Viscera:
  > Rumen is 2-3C higher because of fermentation
  > Liver is 1-2C higher to filter blood
  > the brain generates the most heat (consumes glucose)
• Testes = lower temp by 4-7C

Question 4

What are the mechanisms for heat gain/ production?

Answer 4

Intrinsic heat gain:

• BMR (thyroid hormones, sympathetic stimulation, temperature)
• Breakdown of nutrients
• By-product of chemical reactions
• Fermentation in rumen/hindgut
• Cellular rate of heat production is increased by hormones T3/T4, A, NA

Extrinsic:

• Radiation (infrared)
• Conduction/Convection
• Muscle-exercise, shivering
• Heat uptake from environment

Question 5

What are the mechanisms for heat loss?

Answer 5

• Radiation (via infrared rays, emitted from warm body)
• Conduction (direct transfer from the body to a cooler object, most sig. in small animals)
• Convection (movement of warm air/water from a body and replacement with cool air/water, reduced by insulation, Piloerection traps air, wetting accelerates conduction & convection)
• Evaporation (increase with sweating, panting/gular flutter, salivation, air current/movement; decreased by humidity)
• Vasodilation
• Blood shunted from deep to superficial vessels
• Counter-current heat exchange occurs between arteries and veins

Question 6

Explain the difference between conduction and convection.

Answer 6

• Conduction (transfer of heat from the body to a cooler object by direct contact, most sig. in small animals e.g. in contact with concrete, water, sink, cement)
• Convection (movement of warm air/water from a body and replacement with cool air/water, reduced by insulation. Air blanket created by fur, thick wool or clothing (piloerection increases thickness of the trapped air, wetting accelerates conduction & convection))

Question 7

What is the “Thermoneutral Zone”?

Answer 7

“Comfort” zone.
Animal does not need to actively lose or gain heat but can maintain temperature at its basal metabolic rate through variations in insulation (vasomotor responses and piloerection).

The upper and lower limits of ambient temperature for each animal’s thermoneutral zone are the upper and lower critical temperatures.

Outside these temps the animal must expend energy to maintain core temperature at the set point.

Question 8

What are the main mechanisms for retaining a stable temperature in tissues?

Answer 8

• Vascular controls (particularly counter-current exchange)
• Vasodilation & Vascoconstriction
• Arterio-venous anastomoses
• BLOOD IS A VERY EFFECTIVE TRANSPORTER OF HEAT

Question 9

What type of feedback control system is body temperature listed under for mammals and birds?

Answer 9

Negative Feedback Control.

Question 10

What is hyperthermia?

Answer 10

Hyperthermia= Elevated body temperature due to failed thermoregulation. No change in set point.

Heat Stroke:

• Production/gain of heat > loss of heat/evaporative cooling fails
• Effects: High temp induces an increase in metabolic rate +/- protein degradation
  (i) 44-45C –> death
  (ii) >42C –> cell damage and CNS lesions

Question 11

What is hypothermia?

Answer 11

Low body temperature causes depression in metabolism and function of key organs, particularly the brain.

Question 12

Where are peripheral thermoreceptors found?

Answer 12

In skin & mucous membranes to detect either warm or lack of warmth. Excess temp causes pain.

Question 13

What are the central thermoreceptors (info only, not assessed)

Answer 13

1. Anterior hypothalamus (sensors deteect changes in temp of blood in brain
2. Spinal thermoreceptors (more important in some species e.g. birds

Question 14

What is the main temperature control centre?

Answer 14

Temperature regulating centre is found in the anterior hypothalamus which determines the SET POINT & coordinated responses. (Some temp regulation also occurs in spine).

Question 15

What are the combined neural and humoral regulatory responses?

Answer 15

• Adrenaline
• Noradrenaline
• T3/T4 hormone
  (released in response to hypothalmic cooling. i.e. inccreased sympathetic nervous system (SNS) stimulus - Thyroid Stimulating Hormone release)

Question 16

What are the mechanisms of heat gain?

Answer 16

• Basal Metabolic Rate (thyroid hormones/ sympathetic stimulation)
• Muscle-exercise/shivering
• Heat uptake from environment (radiation and/or conduction and/or convection)

Question 17

What is the most effective method of heat transfer within the body?

Answer 17

Perfusion with blood efficiently transfers heat from muscle and liver (production sites) to the lungs and skin (heat loss sites).

Question 18

What is the “Basal Metabolic Rate”?

Answer 18

Energy is derived from breakdown of nutrients in the body. Heat is the by-product of chemical reactions. e.g. Oxidation of metabolism.

Heat is also produced by fermentation of ingesta by microbes in the rumen and hindgut of some animals.

The cellular rate of heat production is increased by hormones (especially the thyroid hormone T3/T4, adrenaline and noradrenaline).

Question 19

Methods to increase heat loss?

Answer 19

1. CIrculatory changes: cutaneous vasodilation increases blood flow to the skin and increases heat loss by conduction to the environment. Sympathetic vasoconstrictor nerves are inhibited and blood vessels supplying skin dilate leading to increased heat loss.
2. Sweating: Eccrine sweat glands are found in humans and in footpads of animals (assist traction in fright/flight response) and nasal plane. Apocrine glands are most important in animals, especially cattle and horses, less important in pigs, goats, dogs and cats - glands develop at the hair follicles and shed apical cytoplasm during secretion.
3. Panting: Polypneic respiration, often accompanied by salivation and excretion in nasal cavity. –> evaporative cooling of nasal cavity, URT & oral cavity
4. Behaviour: e.g. seek shelter/ seek cooling surface, open body positions etc.

Question 20

Methods to increase heat retention?

Answer 20

1. Physical regulation - behaviour e.g.body position/ skin e.g. piloerection
2. Circulation - vasoconstriction, countercurrent heat exchange from artery to vein across temp gradients in limbs.
3. Chemical regulation - thermogenesis refers to an increase in heat production. Necessary if heat retention mechanisms cannot maintain body temp.
   (i) Shivering
   (ii) nonshivering thermogenesis - many hormones exert calorigenic effect e.g. nordrenalin, adrenalin, thyroxine, glucocorticoids. Brown fat in newborns and hibernators.

Question 21

What is the body water composition?

Answer 21

60-40-20 Rule

60% of total body mass = body water
Intracellular fluid (ICF): makes up 2/3 of total body water & 40% of lean body mass

Extracellular fluid (ECF): makes up 1/3 of total body water & 20% of lean body mass.

Question 22

The ECF is split into 2 major compartments. What are they?

Answer 22

1. Interstitial fluid (ISF) (80% of ECF is between cells)

2. Plasma (20% of ECF is the fluid portion of blood)

Question 23

What are the major ions associated with ECF?

Answer 23

Na+ and Cl-

plasma also has protein anions

Question 24

What are the major ions associated with ICF?

Answer 24

K+ and Phosphates (also protein anions)