Homeostasis

Homeostasis

• literally means “same state” • refers to the process of keeping the internal body

environment in a steady state, when the external environment is changed

• ability of organism to maintain a relative stable internal environment, involving continuous monitoring and regulating multiple parameters and coordinating the responses to minimize the disturbance to organism.

• Changes do occur, but the magnitude of the changes must be small and stay within narrow limits.

• Central percepts of animal physiology

Feedback loops• Organisms commonly use negative feedback

loops to homeostatically regulate some factor. – the response opposes or removes the signal to

stabilize the physiological variable back to within its normal range of values (set point).

• There are a few reflexes that are not involved in homeostasis and use positive feedback loops – like the initiation of blood clotting, breaking down

food by digestive enzymes, or the birth process). – Response reinforces or amplifies the stimulus rather

than decreasing it or removing it and may lead to a vicious cycle.

Negative feedback mechanisminvolve four key features:

• System variable– characteristic (e.g., temperature)

that needs to be regulated.• Set point.

– the ideal or most appropriate value of the system variable.

• Detector. – the actual or current value of the

system variable needs to be assessed.

• Correctional mechanism. – to reduce or eliminate the

discrepancy between the actual value and the ideal value.

Variation

• the set level constantly oscillates about the set point.

• a significant time-lag before the corrective hormone-controlled homeostatic mechanism can be activated.

Thermoregulation• the ability of an organism

to keep its body temperature within certain boundaries, even when the surrounding temperature is very different

• helps keep body temperature within optimum range that enables cells to function most effectively.

Importance of thermoregulation

• metabolic activity of animals and plants are affected by temperature– enzyme catalyzed reaction and mobility of atom and

molecule are influenced by temperature– allows enzymes etc to be ‘fine-tuned’ to a particular

set of conditions, and so to operate more efficiently. • Animal physiology are affected via biochemical

sensitivity to temperature– Neuron and muscular activity are generally more

efficient at higher temperatures

Description of temperature sensitivity

Q10= RT/RT-10

• where – RT = rate of a process or reaction at a certain temperature– RT-10= rate of a process or reaction at a temperature 10oC lower

• Most biological Q10 values = 2 or 3….reaction rates double or triple as temperature increases by 10oC.

• An animal’s sensitivity to temperature can change due to acclimatization; the animal’s metabolic rate readjusts to compensate for changes in temperature (metabolic compensation)

Importance of thermoregulation • The lethal temperature for an organism is defined as the

temperature at which 50 % of the individuals die and 50% survive– temperature sensitivities of key enzyme

A →→ B →→ C→→ D ↓ E

– very high temperatures may change the structure of cell membranes and directly cause death.

– extensive freezing • either freeze tolerant or freeze intolerant • Invertebrates in intertidal pools) can have > 90% of their bodies freeze but

will survive as long as the ice crystals are found only outside their cells.• Antifreeze protein to resist ice crystal formation

Heat transfer

Heat balance• Organisms balance heat gain and heat loss from

environment to maintain stable body temperature• Heat gain

– Radiation at atmosphere warmed by Sun– Direct sunlight– Reflected sunlight from Earth– Radiation of heat from warmer environment– Conduction from warmer ground

• Heat loss– Evaporation of water from body surface– Radiation of heat to cooler environment– Conduction to cooler ground– Convection through water or air

Thermoregulation

• Core temperature is homeostatically maintained– Surface temperature

may fluctuate• Behavior -major

mechanism • Second - control the

blood flow to the skin and to other exposed surfaces

Classification• based on response to

temperature– Homeotherm– Poikilotherm– Heterotherm

• based on source of heat that

regulates body temperature– Ectotherms

• external sources such as solar radiation

• use of behavioral means to regulate body temperature

– Endotherms• metabolic heat production• also exhibit thermoregulatory

behavior

Ectotherm• Animals whose

temperature is dependent on environmental temperature

• Low metabolic rate • Regulate temperature

through behavioral means– body temperature

usually 1°C or 2°C lower than environment temperature

Ectotherm• Behavioral thermoregulation

– Burrowing– Sun basking– Seeking shade – Climbing vegetation– Position orientation

Turtle

• Endotherm / ectotherm– Migration to northern

water

• Problems– Stagnant ocean– Conduction– Convection

Turtle and thermoregulation

• Thick carapace• Blubber • Basking – sea, terrestrial• Gigantothermy• Regional blood flow• Countercurrent exchange• Increase metabolic rate• Brown fat• Red muscle – myoglobin

Countercurrent exchange in turtle

• Matrix form in leatherback

• Radial form in loggerheads and green turtles

Endotherm • Able to regulate their

temperature physiologically through metabolic compensation and maintain a relatively constant body temperature independently of the environmental effects– gain metabolic heat through

cellular respiration and muscular contraction

• Also regulate through behavior– Find shade under vegetation,

reduce activity, panting

Endotherm • Thermoneutral zone: range of

environmental temperatures in which the metabolic rate is low and independent of temperature .

• Basal metabolic rate: metabolic rate of a resting animal at a temperature within the thermoneutral zone

• Below or above the thermoneutral zone , an animal’s metabolic rate increases

Endotherm • Major source of heat - the metabolism of their internal organs.

– the brain contributes 16% of the total — about the same as all skeletal muscle

• Many mammals increase metabolic rate of tissue when surroundings get colder, but it is still uncertain whether humans can.

• During vigorous exercise, heat production by muscles can increase greatly. In the absence of voluntary muscle action, the same effect is achieved by shivering.

• The loss of heat from the extremities can be sharply reduced by diminishing their blood supply. In extreme cold, for example, the blood supply to the fingers can drop to 1% or so of its normal value.

Endotherm• Mechanisms

– Generating and conserving heat• Large number of

mitochondria per cell• Insulation – fat, fur, feather• Countercurrent heat

exchange • Vasoconstriction• Muscle contraction• Huddling

– Avoiding overheating• Panting and vasodilation• Sweating• Flapping ear

Endotherm In cold environments

• using small smooth muscles which are attached to feather or hair shafts;

• increasing body size to more easily maintain core body temperature

• having the ability to store energy as fat • have shortened extremities• have countercurrent blood flow in extremities

In warm environments• living in burrows during the day and being nocturnal• evaporative cooling by perspiration and panting• storing fat reserves in one place (e.g. camel's hump) to avoid

its insulating effect• elongated, often vascularized extremities to conduct body

heat to the air• Birds also avoid overheating by gular fluttering • Dense coats found in desert endotherms

Thermoregulation in human• controlled by the

thermoregulatory centre in the hypothalamus. – Anterior – heat loss– Posterior – heat production

• receives input from two sets of thermoreceptors:– receptors in the hypothalamus

itself monitor the core temperature

– receptors in the skin monitor the external temperature.

• sends impulses to several different effectors to adjust body temperature

Thermoregulation in human• Previously, average oral temperature for healthy adults had

been considered 37.0 °C (98.6 °F), while normal ranges are 36.1 °C (97.0 °F) to 37.8 °C (100.0 °F). In Poland and Russia, the temperature had been measured axillary. 36.6 °C was considered "ideal" temperature, while normal ranges are 36 °C to 36.9 °C.

• Recent studies suggest that the average temperature for healthy adults is 98.2 °F or 36.8 °C (same result in three different studies). Variations (one standard deviation) from three other studies are:

• 36.4 - 37.1 °C• 36.3 - 37.1 °C for males, 36.5 - 37.3 °C for females• 36.6 - 37.3 °C

Thermoregulation in humanIn hot conditions

• Sweat glands under the skin secrete sweat• The hairs on the skin lie flat, preventing heat from being trapped by the layer of

still air between the hairs. • muscle in arteriole walls allowing increased blood flow through the superficial

capillaries in the skin

In cold conditions• Sweat stops being produced.• Piloerection• Arterioles shrink, thereby rerouting blood away from the skin and towards the

warmer core of the body. • Muscles receive messages from the hypothalamus to cause shivering. This

increases heat production as respiration is an exothermic reaction in muscle cells. Shivering is more effective than exercise at producing heat because the animal remains still. This means that less heat is lost to the environment via convection.

Chart showing diurnal variation in body temperature, ranging from about 37.5 °C from 10 a.m. to 6 p.m., and falling to about 36.3 °C from 2 a.m. to 6 a.m.

Thermoregulation in humanHot

• 37°C (98.6°F) - Normal body temperature (which varies between about 36.12-37.5°C (96.8-99.5°F)• 38°C (100.4°F) - Sweating, feeling very uncomfortable, slightly hungry.• 39°C (102.2°F) - Severe sweating, flushed and very red. Fast heart rate and breathlessness. There may be exhaustion accompanying this. Children and

people with epilepsy may be very likely to get convulsions at this point.• 40°C (104°F) - Fainting, dehydration, weakness, vomiting, headache and dizziness may occur as well as profuse sweating.• 41°C (105.8°F) - (Medical emergency) - Fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium and drowsiness can occur.

There may also be palpitations and breathlessness.• 42°C (107.6°F) - Subject may turn pale or remain flushed and red. They may become comatose, be in severe delirium, vomiting, and convulsions can

occur. Blood pressure may be high or low and heart rate will be very fast.• 43°C (109.4°F) - Normally death, or there may be serious brain damage, continuous convulsions and shock. Cardio-respiratory collapse will likely

occur.• 44°C (111.2°F) or more - Almost certainly death will occur; however, patients have been known to survive up to 46.5°C (115.7°F).[20]

Cold• 37°C (98.6°F) - Normal body temperature (which varies between about 36-37.5°C (96.8-99.5°F)• 36°C (96.8°F) - Mild to moderate shivering (it drops this low during sleep). May be a normal body temperature.• 35°C (95.0°F) - (Hypothermia) is less than 35°C (95.0°F) - Intense shivering, numbness and bluish/grayness of the skin. There is the possibility of heart

irritability.• 34°C (93.2°F) - Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioural changes may take place.• 33°C (91.4°F) - Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing.

Shivering may stop. Subject may be unresponsive to certain stimuli.• 32°C (89.6°F) - (Medical emergency) Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose.

Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.• 31°C (87.8°F) - Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm

problems.• 28°C (82.4°F) - Severe heart rhythm disturbances are likely and breathing may stop at any time. Patient may appear to be dead.• 24-26°C (75.2-78.8°F) or less - Death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to

survive with body temperatures as low as 14.2°C (57.5°F).[2

Thermoregulation in human• When fever occurs, the hypothalamus temporarily shifts the

thermostatic setting of the organism to a higher temperature. Fever • may be the result of

– inflammation or– infection – endogenous pyrogen

• Pyrogens released by white blood cells raise the set point of the thermoregulatory centre causing the whole body temperature to increase by 2-3 °C. This helps to kill bacteria, inhibits viruses, and explains why you shiver even though you are hot.

• Prostaglandins during inflammation increases the setting. If the body temperature is not yet there, the body begins shivering violently — causing "chills" — to generate the heat needed. The result is fever when the new set point is reached.

Heat stroke and heat exhaustion• Heat exhaustion

– After performed heavy manual labor or competed in an athletic event on a sweltering hot day chances are you may have experienced

– Symptoms include elevated core body temperature (above 40C), profuse sweating, pale color, muscle cramps, dizzines, and in some extreme circumstances, fainting or loss of consciousness.

– Consequence of disruption of the body's own system of thermoregulation

• Heat stroke– body's temperature rises out of control due to the failure of the

thermoregulating system. – the brain will start to malfunction. Delirium and loss of consciousness set in. – The center of the brain controlling the sweat glands will stop functioningThis

causes the body's temperature to rise even faster. – The metabolic process will speed up causing even more heat in the body. – flushed dry skin

Heterotherm

• strategies in invertebrates and lower vertebrates

• Insects require thoracic temps of 35-40oC for flight

• Shivering thermogenesis - major flight muscles

• Thorax usually insulated with hair-like scales

• Abdomen acts as radiator

Heterotherm

Heterotherm

• Group behavior• Honeybee swarm huddle

to keep the queen warm. • As hive temperature

increase, workers spread out and ventilate the hive with their wings. They may even regurgitate crop fluid to cool the air stream.

Bradymetabolism • refers to organisms with a high

active metabolism and a considerably slower resting metabolism.

• often undergo dramatic changes in metabolic speed, according to food availability and temperature.

• in deserts and in areas that experience extreme winters

• capable of "shutting down" their metabolisms to approach near-death states, until favorable conditions return

Gigantothermy • Sometimes called

ectothermic homeothermy)

• Large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their greater volume to surface area ratio.

• Lost heat to environment mere slowly

Countercurrent heat exchange

• Arterial and venous blood vessels that pass very close to each other and have fluid flowing in opposite directions forming special network– rete mirabile.

• Recover heat from blood travelling to extremities using passive processes

• Most endotherms cannot tolerate a rise in body temperature of more than 5°C or so. – Located between carotid arteries and vessels that

distribute blood to brain

Countercurrent heat exchange

Countercurrent heat exchange• Utilized by tuna, great

white sharks and mackerels

• maintains a constant gradient over a longerdistance

• Greater power output.• tuna swimming in the

winter can maintain its active swimming muscles 14°C warmer than the surrounding water.

Countercurrent heat exchange

Countercurrent heat exchange

Countercurrent heat exchange• Sheep have a carotid rete

that uses blood cooled by evaporation in the sinuses to cool the blood flowing to the brain

• Coupled with the wool insulating the head, effectively allows sheep to tolerate a higher core temperature during predator avoidance

“Hot” fish

• The swordfish has special tissue warming the blood going to eyes and brain, allowing prey searching in cool water

• Bluefin tuna has tail muscle generating heat for marathon swim, and rete mirabile at muscle and brain

Sweating • Sweat glands under the

skin secrete sweat– a fluid containing mostly

water with some dissolved ions

• Travels up the sweat duct, through the sweat pore and onto the surface of the skin.

• Causes heat loss by evaporation

• However, a lot of essential water is lost.

Sweating

• Latent heat – Amount of energy released or absorbed

by a chemical substance during phase transition

where:– Q is the amount of energy released or

absorbed during the change of phase of the substance (in joules),

– m is the mass of the substance,– L is the specific latent heat for a

particular substance (J kg-1).• At body temperature, the latent heat

of vaporization of water is 2428 kJ / kg.

Sweating• For short periods, you can sweat up to 4 liters

per hour; for longer periods (up to 6 hours), 1 liter per hour is common. This makes evaporation a major contributor to heat regulation,.

• When the ambient temperature is above body temperature, the only mechanisms left under those conditions are the evaporation of perspiration from the skin and the evaporative cooling from exhaled moisture.

• This heat of vaporization is 540 calories/gm at the boiling point, but is even larger, 580 cal/gm, at the normal skin temperature.

• As part of the physiological regulation of body temperature, the skin will begin to sweat almost precisely at 37°C and the perspiration will increase rapidly with increasing skin temperature.

Piloerection• Elevation of the hair follicles

due to contraction of the little arrectores pilorum muscles

• Started by stimulation of cold to sympathetic nervous system

• Trap the heat between outer skin surface and hair follicle

• cutis anserina• Also due to mental

influences or response to an adrenalin flow.

Adipose tissue• Compose of adipocytes• Storing energy as fat

• White fat / monovacuolar cell– 20% in men and 25% in women– A single large fat droplet with nucleus at periphery, diameter 0.1mm– Secrete resistin, adiponectin and leptin – Store energy (insulin and glucagon)– With receptors for insulin, growth hormones, norepinephrine and glucocorticoid

• Brown fat – In newborn or in shoulder of hibernating mammals, to generate body heat– Numerous smaller droplets and much more mitochondria– More capillaries – greater need for oxygen – Numerous unmyelinated nerves provide sympathetic stimulation to adipocyte– Non-shivering thermogenesis stimulated by increase in circulating noradrenalin– Avoid lethal cold/hypothermia

Brown fat• In neonates, brown fat, 5% of the body mass is located on the back, along the

upper half of the spine and toward the shoulders, is of great importance to avoid lethal cold due to– The higher ratio of body surface to body volume – The higher proportional surface area of the head – The low amount of musculature and the inability or reluctance to shiver– A lack of thermal insulation– The inability to move away from cold areas, air currents or heat-draining materials – The inability to use additional ways of keeping warm (e.g., turning up a heater, drying their

skin, changing clothes or performing physical exercise) – The nervous system is not fully developed and does not respond quickly and/or properly to

cold

• In the upper chest and neck of adults

• Recently it has become clear that brown fat is not closely related to white fat, but to skeletal muscle, instead.

Shivering• Bodily function in response to early hypothermia in warm-

blooded animals. • triggered when the core body temperature drops • Muscle groups around the vital organs begin to shake in

small movements in an attempt to create warmth by expending energy.

• Fever and post-surgical shivering.• Mechanism

– Primary motor center in the dorsomedial portion of the hypothalamus near the wall of the third ventricle.

– This area is normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area but is excited by cold signals from the skin and spinal cord.

– Activated when the body temperature falls below a critical temperature level.

– Increased muscular activity consume ATP without movement. The conversion of ATP to ADP releases heat.

Vasodilation & Vasoconstriction

Vasoconstriction• The reduction in the internal diameter of blood vessels, especially

particularly the large arteries, arterioles, veins or capillaries. • mediated by the action of nerves on the smooth muscle fibres of

the arteriole walls• the flow of blood is decreased, thus, retaining body heat or

increasing vascular resistance. • Skin turn paler• Mechanism by which the body regulates and maintains mean

arterial pressure.• Substances causing vasoconstriction are called vasoconstrictors or

vasopressors. • May also occur in specific tissues causing a localized reduction in

blood flow.

Vasoconstriction • Two common stimuli for eliciting smooth muscle

contraction is circulating epinephrine and activation of the sympathetic nervous system (through release of norepinephrine that directly innervates the muscle

• interact with cell surface adrenergic receptors• result in a signal transduction cascade that leads to

increased intracellular calcium from the sarcoplasmic reticulum

• The rise in intracellular calcium complexes with calmodulin activates myosin light chain kinase responsible for phosphorylating the light chain of myosin to stimulate cross bridge cycling.

Triggering factors of vasoconstriction

• Exogenous – exposure to the severe cold.– medications• in medicine to treat hypotension and as topical

decongestants

• Endogenous– autonomic nervous system, circulating hormones

and intrinsic mechanisms inherent to the vasculature itself

– avoids orthostatic hypotension and heat loss

Vasodilation

• Widening of blood vessels• Result from relaxation of smooth muscle cells

within vessel walls due to vasodilator• May be localized or systemic• Mechanism – Remove stimulus (vasodilator)– Myosin light chain phosphatase cause

dephosphorylation– Calsium ion reuptake by sarcoplasmic reticulum

Heat-shock response• Due to strong heat-shock

protein induction triggered by environmental stress

• Heat shock protein – enhance thermotolerance– facilitate folding under

non-stressful conditions– catalyze the refolding of

partially denatured proteins and thus stabilize protein under stress

Tegula brunnea(subtidal)

Tegula funebralis(intertidal)

Behavioral thermoregulation

• Response to high body temperature– Seek shade – Secrete mucuos (bullfrog)– Aestivation – Light pigmentation

• Response to low body temperature– Basking – Hibernation under mud– Dark pigmentation

Torpor• Adaptative hypothermia

in which metabolism and body temperature are lowered at night to prevent overnight starvation

• Unable to respond quickly to stimuli

• Bats and hummingbirds reduce their set point every day while they are inactive.

Hibernation• Metabolism and body

temperature are lowered for days and months

• Unable to respond quickly to stimuli

• Some mammals release hormones that reduce their set point to around 5°C while they hibernate. This drastically reduces their metabolic rate and so conserves their food reserves e.g. hedgehogs.

Extended sleep

• Only metabolism is lowered for days or months

• Maintain ability to respond to stimuli

Behavioural cooling

Black-footed albatross chickc seeking shade – cooling method

Hippopotamus dipping in water