Communication: Homeostasis

Syllabus points

• Outline the need for communication systems within multicellular organisms, with reference to the need to respond to changes in the internal and external environment and to coordinate the activities of different organs.

• State that cells need to communicate with each other by a process called cell signalling.

• State that neuronal and hormonal systems are examples of cell signalling.

• Define the terms negative feedback, positive feedback and homeostasis.

• Explain the principles of homeostasis in terms of receptors, effectors and negative feedback.

• Describe the physiological and behavioural responses that maintain a constant core body temperature in ectotherms and endotherms, with reference to peripheral temperature receptors, the hypothalamus and effectors in skin and muscles.

Need for Communication

• Arctic fox – external environment

Need for Communication

• Internal environment

Major Systems of Communication

• Neuronal system• Endocrine system

Homeostasis

• Ability to maintain a steady state within a constantly changing environment contributes towards the success of living organisms.

“The constancy of the internal environment is the condition of a free life” – Claude Bernard 1857 (French

physiologist concerned with self regulation)

Homeostasis

The maintenance of a constant internal environment:

• Temperature• Concentration of dissolved substances• Blood sugar level• Water level• pH• Carbon dioxide concentrationAll rely on feedback in order to be kept

constant

Alterations to the environment…

Why is it important to control the following?

• Core body temperature

• Plasma water potential

• Plasma Glucose concentration

• Plasma CO2 concentration• Blood pressure

Feedback

All rely on feedback in order to be kept constant

Negative Feedback reversal of a change in the internal environment to return to a steady state or optimum position

Stimulus

Receptor

Communication Pathway

Effector

Response

Pathway for Feedback System

Regulation Negative Feedback

Detected by receptors

Detected by receptors

Change from resting level

Change from resting level

Effectors bring about a

return to resting level

Effectors bring about a

return to resting level

Resting

level

Positive Feedback

• Process that increases any change detected by the receptors and does not lead to homeostasis.

• Very rare in biological systems and leads to unstable and extreme states

Thermoregulation

All animals derive heat from 2 sources: external environment and chemical energy within cells

• Ectothermic: (‘ecto’ = outside) rely more on heat derived from the environment. All animals excepts birds and mammals.

• Endothermic: rely on internal sources of heat

Adaptations to resist changes in temperature leads to thermoregulatory classification of animals: ecto- and endotherms

Endotherms produce significant amounts of heatHow do they do it?

Inefficient metabolism -- leaky cells (uncoupling oxidative phosphorylation.)Production of waste heat -- shivering.Burning fuel without doing work -- brown fat.

Thermal GradientRadiation: Heat transferred by electromagnetic waves. Bodies

unable to absorb much radiated heat but transfer it to other bodies– 50% of total heat loss in man– Main route for controlled heat loss in animals

Convection: heat transferred via air. In endotherms air is warmer than body, therefore a convection current is created bringing in more cool air to body. This can be deterred by fur, feathers, hair or clothing

Conduction: Physical contact between two bodies. This is insignificant for most terrestrial animals

Evaporation: heat lost by body surface as water converts to water vapour– 1 cm3 requires the loss of 2.45 KJ from body – Cannot be controlled

General metabolism

Musclecontraction

Radiation and conduction from the environment

Radiation, conduction and convection to the

environment

Evaporation of sweat

Expiration and excretion

Heat Balance in Endotherms

Endotherms (mammals and birds) maintain a constant body

temperature through mechanisms aimed at balancing their heat gains and heat losses

Consider the ways in which this resting lioness is gaining

and losing heat

Radiation from the sun

Radiation and conduction from warmer parts of the environment

Radiation and conduction to

cooler parts of the environment

Convection of heat by air movements

Evaporation of water from

lungs and body surface

In terms of temperature control, provide an explanation for the relationship between ear size and habitat for the three different foxes

Suggest why the red fox has an ear size that is intermediate betweenthat of the Fennec and Arctic foxes

What is the significance of this behavioural and physiological response for thermoregulation in the mouse?

Endotherms

Advantages: • Constant body temperature

regardless of environmental temperature

• Activity is possible when external temp is cool

Disadvantages:• Significant amount of energy intake

is used to maintain body temperature in the cold

• More food required• Less of energy from food is used for

growth

Thermoregulation in Mammals

• Receptors– Sensory cells detect changes in skin temperature and blood

temperature flowing through hypothalamus – Two different types of receptor (warm and cool)

• Coordinator– Two areas in the hypothalamus:

Heat gain centre – involved in limiting amount of heat

loss from body and with heat generation from metabolism

Heat loss centre – controls activities that will reduce body temperature

• Effectors– Sweat glands, muscles, movement of hair, liver cells, blood

flow in arterioles to capillaries• Negative Feedback System

Negative Feedback in Temperature Control

Increase (usually due to exercise)

Detected by the hypothalamus

Heat loss mechanisms

• Skin

• Blood flow (vasodilation)

• Body hairs

• Liver cells

• Skeletal movement

• Breathing via mouth, nose and

lungsHeat retention/gain mechanisms

• Skin

• Blood flow (vasoconstriction)

• Body hairs

• Liver cells

• Skeletal movement

• Breathing via mouth, nose and

lungs

Detected by the hypothalamus

Decrease (usually due to a fall in environmental temperature)

Heat energy lost from body as

water evaporates.

More heat lost by conduction and radiation.

Insulating layer of air reduced. More heat lost by conduction.

Less metabolic heat gained.

Less heat lost by conduction and radiation.

Insulating layer of air trapped. Less heat lost by convection.

Generates metabolic heat.

Energy released as heat rather

than ATP

Hypothermia video

NormalBody

Temperature

Temperature rises

Temperature falls

Detected byThermoregulatory

center

Coordination Of

response

message viahormone

Impulse viaMotor neurones

1. Reduce metabolic rate

LiverCorrective action

1. Hairs lie flat2. More sweat3. More blood flow toSkin (vasodilation)

SkinCorrective action

Less heat produced More heat lost

Temperature falls

Detected byThermoregulatory

center

Coordination Of

response

message viahormone

1. Increase metabolic rate

LiverCorrective action1. Hairs raised

2. Less/no sweat3. Less blood flow toSkin (vasoconstriction)

SkinCorrective action

Impulse via Motor neurones

1. Shivering

MuscleCorrective action

Less heat lost More heat produced

Temperature rises

NormalBody

Temperature

Temperature rises

Temperature falls

Detected byThermoregulatory

center

Coordination Of

response

message viahormone

Impulse viaMotor neurones

1. Reduce metabolic rate

LiverCorrective action

1. Hairs lie flat2. More sweat3. More blood flow toSkin (vasodilation)

SkinCorrective action

Less heat produced More heat lost

Temperature falls

Detected byThermoregulatory

center

Coordination Of

response

message viahormone

1. Increase metabolic rate

LiverCorrective action1. Hairs raised

2. Less/no sweat3. Less blood flow toSkin (vasoconstriction)

SkinCorrective action

Impulse via Motor neurones

1. Shivering

MuscleCorrective action

Less heat lost More heat produced

Temperature rises

Stimulus

Receptor Coordination

Effectors

Response

Stimulus

Receptor

Coordination

Effectors

Response

Ectotherms can use behavior to resist changesin temperature.

But, endotherms also use behavior, and do so preferentially,and ectotherms have anatomical and physiological adaptations.

Anatomical adaptation in certain fast swimming fish make it possible to conserve metabolic heat in core and elevate muscle temp.

In most fish, cold arterial bloodflows centrally to serve muscles.

In “hot” fish, arterial blood flows under skinAnd forms countercurrent heat exchangersWith veins as it enters muscle.

Ectotherms also display variousPhysiological adaptations:Examples are heat production in flying insects, shivering inbrooding pythons, and CVadaptations in marine iguanas.

Water off GalapagosIs cold.Marine iguanas warm bybasking. When they enter water to feed, heartrate and therefore bloodflow to skin decreasesconserving heat in the core.Reverse when they return to land.

Thermoregulation in an Ectothermic Reptile – The Desert Lizard

Desert lizards, like many other reptiles, gain or lose heat by thermoregulatory

mechanisms that involve:• Burrowing behaviour when the ambient behaviour is

low (25oC to 30oC) or high (above 38oC)• Shifting between sun and shade• Postural adjustments that maximise or minimise the

exposure of the body surface to the sun• Physiological adjustments: vasodilation of skin

capillaries during basking behaviour and vasoconstriction during periods of low temperature; dispersion or contraction of black pigment cells in the skin with the lizards becoming lighter as the temperature increases

Postural adjustments and seeking out shade are behavioural mechanisms used by desert lizards to regulate their body temperature

1. Early morning; lizard emerges from its cool burrow (body temperature around 20°C)

2. Lizard basks in the morning sun and body temperature rises to around 38°C

3. Lizard maintains a relatively constant body temperature(38-40°C) by moving between the sun and shade as ambient temperatures rise towards mid-day

4. Lizard spends much time in the shade during the heat of the afternoon

5. Lizard spends mostof its time in the sun in the early evening and is very active before returning to its burrow for the night

alternating betweensun and shade

sheltering alternating betweensun and shade

basking

retreat toburrow for the night

EctothermsAdvantages:• Body temperature fluctuates with external temperature. • They are unable to increase respiration rates to generate

heat internally and therefore rely on external sources of heat to keep warm.

• Behavioural and structural mechanisms important in limiting variation in body temperature

• Need less food than endotherms to supply their metabolic needs

Disadvantages: • Less active in cooler temperatures, warm up in morning

before active• Not capable of activity during winter so must have sufficient

stores of energy to survive over winter• Found in limited range of environments.

Temperature Regulation

Behavioural changes:• To warm up: Bask in sun or lie on warm surface• To cool down: Stay underground or lie in the

shade

Physiological or anatomical adaptations:• Horned lizard: can alter surface area by

expanding or contracting its rib cage• Frilled lizard uses its frill to help absorb heat• Locusts increase their abdominal breathing

movements to increase evaporation of water and aid cooling.