L1: Homeostasis and Integration: Case of Thermoregulation

Homeostasis purpose

• Maintenance of a constant (normal) internal environment

• Coordinated responses of organs/systems that automatically compensate for environmental changes

Coordinate response=

more than 1 system

Homeostasis (main controls) includes :

• body temperature

• blood pressure

• O2/CO2 levels

How do mammals and birds maintain body

• They are homeotherms

• warm-blooded animals maintain body temperature within narrow range

• low and high temperature NOT tolerated

Core body temperature characteristics

• protects vital organs

• measured by rectal insertion of thermometer

• influenced by environment, exercise, metabolism

*the lower temps. as you get more superficial help to protect the core body. As external temp. increase, the core temp. is maintained while the outer layers increase in temp

Thermoneutral zone

the ambient temperature at which no adjustment is needed

• no additional energy required to maintain body temperature=perfect husbandry conditions

Critical temperatures (hot or cold)

energy required to maintain homeothermic condition (produce [cold stress] or actively dissipate heat [heat stress])

• metabolic rate is used to maintain body temp. during cold stress

Hyperthermia (heat stress) causes:

• generally fatal

• cells cannot function (enzymes and proteins denatured)

• during a fever, immune system increases body temp. to fight bacteria

Hypothermia (cold stress) causes:

• cells can survive freezing so we can sustain a wider ranger of temps. than heat, but blood circulation and respiration fail = death

How does the balance between heat production and heat loss in mammals and birds occur:

• mainly from body production - endothermics (not energy efficient)

• metabolism (main factor) - conversion of nutrients to heat (energy), mainly in heart liver, kidneys and brain

• growth and production (body factory)

• exercise: about 80% of energy produced is heat

How is heat transported in the blood released into the environment

1. radiation

2. conduction

3. convection

4. evaporation/condensation

How is extra-heat produced in the body?

Muscle activity: exposure to cold → shivering

• rhythmic (15/sec) involuntary muscle contraction

• antagonistic muscles contract simultaneously

• nervous input

• no exercise = 100% energy goes to heat production

Non-shivering thermogenesis: ↑ metabolism

• sympathetic nervous system results in secretion of catecholamines (adrenalin) = ↑ metabolism (calorigenic) especially from brown fat

• thyroid hormones → ↑ metabolism

• more sustained prolong response

How is extra-heat lost?

• under normal conditions, metabolic heat is removed from the core by the blood through the skin by heading towards the dermis

• insulation property of the skin inversely proportional to the blood flow

• body can’t generate cold, so heat is lost

Mechanisms of heat transport

• evaporation (most optimal in losing heat)

Gradient physical laws = transfer from high to low

• radiation

• conduction

• convection

Radiation

electromagnetic radiation is emitted and absorbed; relative to the surface and temperature of the object/animal

Conduction

transfer through contact; depends on the conductivity (inverse of insulation). Animal → ground

Convection

transfer via moving air or water

Evaporation

• very efficient, requires energy to break water molecules apart

• through skin and airways

Methods of evaporation

• passive evaporation: normal water loss

• sweating: active process under sympathetic nervous system. Also results in loss of ions

• panting: rapid shallow breathing (not in horses or pigs); as you breath in the air passes the nose and trachea and collects humidity, releasing air with water vapour

• bathing: water absorb energy before evaporating

Thermoregulation: System components

• sensor (sensory input) + integration + effectors (motor output) = reflex = automatic

• thermoreceptors = nerve cells

• involves participation of numerous physiological systems

• voluntary component

Integration center=

Hypothalamus

• information compared to internal reference set points

• contro center

Effectors

• neurons connected to muscle (somatic) for shivering

• sympathetic nervous system for blood flow, sweat gland and metabolism

• hormones: hypothalamopituitary axis

Voluntary component

• hypothalamus connected to the cortex = exchange of information

• change of behavioural response = cognitive behaviour; ex. putting jacket on

Components which respond to heat

Heat receptors:

• In the skin (below epidermis), in the body core (especially hypothalamus - sensitivity of 0.1°C), blood vessels, viscera

• Information transferred through warm sensory fibres

Hypothalamus:

• activation of the heat-loss centre (anterior)

Effectors:

• Sympathetic vasoconstriction fibres slow down = vasodilatation; helps body cool down by sending blood to skin

• Sympathetic activity increase for evaporative heat loss: sweating, panting

• Reduction of physical activity

• Behavioural response: increased exposure area to air, seeking shade, wind

Sensitivity to heat in different animals

• Depends on the animal and the external conditions (humidity level)

• Cattle and sheep very good

• Pigs very bad = little sweat, small mouth for panting (transport at night preferred)

• In birds: air sac (extension to the lung) deep in body cavity = ventilation helps cooling

Components which respond to cold

Cold receptors:

• In the skin (below epidermis). In the body core (hypothalamus, blood vessels, viscera) Information transferred through cold sensory fibres. Much more numerous than warm fibres

Hypothalamus

• activation of heat-producing centre (posterior)

Effectors

• Reduction of heat loss: Sympathetic vasoconstriction fibres fire; Pili muscles from hair follicle contract; “Curl-up” position

• Production of heat: Muscular contraction (shivering) by somatic motor system; Sympathetic and hormonal activity ↑ (catecholamines, thyroid hormones) = metabolism ↑ (especially in brown fat)

Adaptation mechanism to heat

• prolonged exposure to heat = lots of sweat thus loss of ions. Increase in aldosterone (hormone) stimulates reabsorption of Na+ and Cl- from sweat glands

• shedding

Adaptation mechanisms to cold

• change of fur coat, increase in insulation properties (subcutaneous fat)

• hibernation: voluntary decrease in core body temperature (asleep), followed by rapid raise (awake)

Pathology and examples

dysfunction of the thermoregulation system

• fever

• hyperthermia

• hypothermia

Characteristics of a fever

• Occurs by “resetting” hypothalamic thermostat to higher level

• Response is equivalent to a decrease in temperature

• Caused by pyrogens released during infection and tissue damage

Characteristics of hyperthermia

• Happens when temperature exceeds the regulating capability. Heat production exceeds heat loss. Convulsions, nausea, loss of consciousness, death.

• Excessive exercise, exposure to high temperatures

Characteristics of hypothermia

• Heat loss exceeds heat production capability→ Slow nervous activity, muscular failure (cardiac, pulmonary), death

• Critical with newborns