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 Hometherm = consrtant temp warm blooded Poikilotherm = changeable temp cold blooded Hererotherm =

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 Conduction involves the transfer of heat between two objects that are in contact with oine anohter, heat is always conducted from a higher to lower temp e.g. when you sit on a cold chair heat is conducted from your body to the chair Air has a lower thermal conductivity so heat loss to water is much greater Convection is heat transfer due to currents of air or water, our clother reduce heat lost by convection by trapping a layer of air next to the skin Radiation is the loss of inf red heat to the surroundings - most heat loss is due to radiation Evaporation is the change of stae from liquis to gas and it has a cooling effect as energy id used to vapourise the liquid

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

Blood temperature falls Blood temperature rises Controlling body temperature Blood temperature falls Blood gets too hot Brain switches on cooling mechanisms Normal body temperature 37ºC Normal body temperature 37ºC Blood gets too cold Blood temperature rises Brain switches on warming mechanisms

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

Apocrine galds = water based sweat Eccrine glands = oil based sweat – smelly bacteris

Fig. 49.03(TE Art) Perturbing factor Response Body temperature drops Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 49.03(TE Art) Perturbing factor Response Body temperature drops Sun Effector Negative feedback Blood vessels dilate Glands release sweat Stimulus To decrease body temperature Body temperature rises Integrating center Sensor To increase body temperature Stimulus Effector Body temperature drops Negative feedback Blood vessels constrict Skeletal muscles contract, shiver Perturbing factor Response Body temperature rises Snow and ice

Experiment where a naked man sits in a room where the temp around him is gradually raised or lowered. Btween 15 – 30 C the naked man can maintain a constant core temp, a point is reached where his sweating and vasodilation are no longer able to keep him cool he has reached the upper critical temp. After this him metabolic rate increases but the expeiment is always stopped , if it was not then positive feedback would take place and the enzymes would work faster and so increase metabolism and so heat at about 42 c the upper lethal tempo is reached and he would die When temp is lowered the rrom gets so cold the bodt cannot maintain a constant core temp at the lower critical temp metabolic rate increase and generates heat, the exp0eriment is usually stopped at this point if continued a lower letahl temp would be reacheed and death

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