Homeostasis

• What is homeostasis?

• The maintenance of a constant internal environment within a living organism

• What makes up the feedback circuit?– receptor– coordinator– effector

Homeostasis

• What is negative feedback?

• What is positive feedback?

Temperature and Life

• Life can exist at almost all of the temperatures on Earth

• This is because it is the internal not external temperature that is important to organsisms.

Temperature Change

• Within thermal limits, temp change can cause problems

• Most metabolic reactions are temperature sensitive

• Although enzymes tend to denature above 45º C it is thought that death from excess heat is due to metabolic imbalance as some enzymes work faster than others so allowing intermediate chemicals to build up.

• Q10 is a measure of temp sensitivity

Temperature Maintenance• Homeotherm• Poikilotherm

• Endotherm Maintain a stable core temperature using physiological

and behavioural means examples? Birds and Mammals

• Ectotherm Can control their body temperature only by changing

their behaviour examples? All other animals

Thermoregulation and Behavior

Endotherm thermoregulation

Behavioural regulation

• Wearing more or less clothes

• Building shelters and fires

• Curling up into a ball when cold

• Stretching out when hot

Endotherm Thermoregulation

• Adjust metabolic rate response to environmental temperature changes

• Metabolic reactions product produce heat as a by product

Endotherm Thermoregulation• General body plan modifications

– hair– feathers– shorter appendages– rounder body shapes

Humans and Heat

Humans have a thin covering of insulating fat and body hair is sparse

We are one of only a few animals to be covered in sweat glands

Thermoregulation and Skin Perfusion

How is heat produced and lost?

• Heat can be gained or lost in four ways:

• Conduction

• Convection

• Radiation

• Evaporation

Internal Regulation

• In Humans the basic mechanism that underlies thermoregulation involves the hypothalamus

• This part of the brain monitors the temperature of the blood and uses feedback to control internal body temperatures

• Any increase in heat production is proportional to the temperature the hypothalamus is cooled below the set point

Hypothalamus

Normal body Normal body temperaturetemperature

3737ºCºC

Brain switches Brain switches on coolingon cooling

mechanismsmechanisms

Normal body Normal body temperaturetemperature

3737ºCºC

Brain switches Brain switches on warmingon warmingmechanismsmechanisms

Blood gets Blood gets too hottoo hot

Blood gets Blood gets too coldtoo cold

Blood Blood temperature temperature

fallsfalls

Blood Blood temperature temperature

risesrises

The Skin

• The skin is a large organ responsible for maintaining homeostasis through temperature regulation

• It is also responsible for protection of underlying tissues, retardation of water loss, sensing environmental change, and housing cells of the immune system.

The Skin

• The skin consists of an outer epidermis

• and a dermis, connected to underlying

• tissue by the subcutaneous layer

• (hypodermis).

Physiological Responses To Cold

There are 4 main responses when the hypothalamus detects a drop in temperature:

• Shivering

• Vasoconstriction

• Piloerection

• Increased metabolic rate

Shivering

• Depends on contractile machinery of skeletal muscles to consume ATP

• Tremor results

• Conversion of ATP to ADP results in heat production

Piloerection

• Erection of hairs

• The erection pili muscles in the skin pull our tiny hairs upright – producing goosepimples

Vasoconstriction

• The arterioles that lead to the capillaries in the surface are of the skin constrict and so reduce blood flow to the skin

• A shunt vessel by passes the surface capillaries

Increased metabolic rate

• The body secretes the hormone adrenaline in response to cold.

• This raises metabolic rate and therefore increases heat production.

• People who live in cold conditions for a long time show a more permanent rise in metabolic rate due to the secretion of thyroxine.

When we are too cold

Physiological Responses To Heat

Sweating• Sweat is a salty solution made by sweat

glands, evaporation of sweat from skin’s surface leads to cooling

Vasodilation• Arterioles that lead to skin capillaries dilate

and shunt vessels are closed this leads to an increase in blood flow to the skin moer heat is lost by radiation

When we are too hot

Fig. 49.03(TE Art)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Body temperaturerises

Perturbing factor

Sun

Body temperaturedrops

Response

Perturbing factor

Snow and ice

Effector

Blood vesselsconstrict

Skeletal musclescontract, shiver

To increasebody temperature

Stimulus

Effector

Blood vesselsdilate

Glands releasesweat

Response

Body temperaturedrops

IntegratingcenterSensor

To decreasebody temperature

Negative feedback

Stimulus

Negative feedback

Body temperaturerises

Problems in Temperature Regulation

• Air can hold a limited amount of water vapor when air is saturated with water, sweat may fail to evaporate, and body temperature may remain elevated

• Hypothermia is lowered body temperature, it causes shivering, mental confusion, lethargy, loss of reflexes & consciousness, and eventually major organ failure.

Non shivering Heat Production

• Brown fat

• Thermogenin uncouples proton movement from ATP production, so metabolic fuels are consumed without producing ATP

• Heat is still released