Homeostasis

Homeostasis What’s it all about?

Definition • Defined by Claude Bernard (French physiologist) • It is the ability to maintain a constant internal environment despite a continually changing external environment. • “Steady state” – dynamic response; not designed to preempt change but to cope or adapt to it • Examples: • temperature 37◦ C • blood pH 7.4 • blood pressure 120/80 • heart beats 72-80/min

Control Mechanism • 3 components: • Receptor – detects changes in the environment; senses • Control center (integrator) – processes information to direct an appropriate response (2 systems) • Effector – delivers response (motor); muscle and glands

A homeostatic control system has three functional components A receptor, a control center, and an effector Response No heat produced Heater turned off

Room temperature decreases Too hot

Set point

Too cold

Set point

Set point

Control center: thermostat Room temperature increases

Heater turned on Response Heat produced

Feedback Control • 2 types : negative versus positive • Negative – triggers response that counteracts initial change (opposition); fever and the cooling center • Positive – triggers response that amplifies the original change; labor response

Thermoregulation • Keep body temperature in a range to support life and metabolic activities; is the process by which animals maintain an internal temperature within a tolerable range • What is metabolism? (reactions that involve changes in energy) • Role of heat - ^ rate of reaction in physical systems • Biological systems have heat limitations imposed via the fragility of the molecules (protein denaturation)

Temperature limitations • Species are adapted to a temperature range which is optimum to support their life processes. • Generic range – between 0 and 50◦ • Extremes: • Freezing – preserves chemistry; slows down rate of metabolic activity • Heating – increases rate of metabolism within life range (ectotherms)

Methods of Heat Transfer • Conduction – direct transfer of heat between environment and body surface; metal cage, hot rock, cold pool • Diffusion of heat from high to low temperature • Water is the better conductor and insulator

Methods of Heat Transfer Radiation is the emission of electromagnetic waves by all objects warmer than absolute zero. Radiation can transfer heat between objects that are not in direct contact, as when a lizard absorbs heat radiating from the sun.

Convection is the transfer of heat by the movement of air or liquid past a surface, as when a breeze contributes to heat loss from a lizard’s dry skin, or blood moves heat from the body core to the extremities.

Evaporation is the removal of heat from the surface of a liquid that is losing some of its molecules as gas. Evaporation of water from a lizard’s moist surfaces that are exposed to the environment has a strong cooling effect.

Conduction is the direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock.

Convection • Movement of heat by current of air or water over surface of the body • Examples: fan, wing flapping, wind chill factor

Radiation • Emission of electromagnetic waves (heat) produced by all objects warmer than absolute zero (not require contact) • Example: polar bear fur (fiber optic); clear fur directs heat to darker skin below

Evaporation • Loss of heat via evaporation of liquid such as water to a gas (540 cal/gm) • Examples: tepid bath, perspiration, panting, saliva spreading • ***saturation limits

Organism Classification • Definition based upon source of heat absorption (body metabolism versus external environment) • Forms: • Ectotherms – obtain heat from external surroundings • Endotherms – obtain heat as a by-product of living metabolism (60% as by-product of respiration)

Classification • Ectotherms – Include most invertebrates, fishes, amphibians, and non-bird reptiles

• Endotherms – Include birds and mammals ***warm versus cold blood (misnomer)

Ectotherms • Amphibians and reptiles other than birds are ectothermic, meaning that – They gain their heat mostly from external sources – They have lower metabolic rates • ***What about plants?

Characteristics of Ectotherms • Majority do not have an advanced mechanism for thermoregulation • TB ∞ TA (ambient versus body temperature) • Demonstrates van Hoff’s Principle – metabolic activity doubles for every 10◦ C increase in TA • Metabolism is directly related to temperature • ****linear graph

Endotherms • Birds and mammals are mainly endothermic, meaning that – Their bodies are warmed mostly by heat generated by metabolism – They typically have higher metabolic rates than ectotherms

Characteristics of Endotherms • Maintains internal body temperature independent of ambient temperature • Body temperatures typically higher than habitat; efficient at heat conservation • Mammals – 36-39◦ C (96.8-102.2◦F) • Birds – 40-43◦C (104-109.4◦F)

Metabolic Effects • Ectotherms – metabolic rate varies with ambient temperature (life range) • Endotherms – metabolic rate remains fairly high and consistent despite ambient changes

Ectotherms v.Endotherms 40

Body temperature (°C)

River otter (endotherm)

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20 Largemouth bass (ectotherm) 10

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10 20 30 Ambient (environmental) temperature (°C)

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Advantages of Endothermy Endothermy is more energetically expensive than ectothermy (food requirement) – effectively buffers animals’ internal temperatures against external fluctuations – enables the animals to maintain a high level of aerobic metabolism – able to live in a wide variety of habitats – metabolism independent of seasonal changes** – metabolism independent of day/night changes**

An animal’s metabolic rate is the amount of energy an animal uses in per unit of time

This rate can be measured in a variety of ways:

(a) This photograph shows a ghost crab in a respirometer. Temperature is held constant in the chamber, with air of known O2 concentration flowing through. The crab’s metabolic rate is calculated from the difference between the amount of O2 entering and the amount of O2 leaving the respirometer. This crab is on a treadmill, running at a constant speed as measurements are made.

(b) Similarly, the metabolic rate of a man fitted with a breathing apparatus is being monitored while he works out on a stationary bike.

Factors that effect metabolic rate Body size/mass (metabolic rate per gram) – Is inversely related to body size among similar animals – Example tree shrew versus elephant (surface/volume ratio)

Thermoregulation Methods • Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss • Four basic methods: • • • •

Structural Physiological – circulatory paths; thermogenesis Behavioral phenomena Evaporative cooling

Structural Methods • Insulation, which is a major thermoregulatory adaptation in mammals and birds – Reduces the flow of heat between an animal and its environment – May include feathers, fur, blubber,scales,skin – Oils and waxes may serve to insulate

Human Skin Anatomy

Epidermis

Dermis

Hypodermis

Physiological Methods • Many endotherms and some ectotherms – Can alter the amount of blood flowing between the body core and the skin

• In vasodilation – Blood flow in the skin increases, facilitating heat loss

• In vasoconstriction – Blood flow in the skin decreases, lowering heat loss • ***regional vasoconstriction/dilation

Many marine mammals and birds Have arrangements of blood vessels called countercurrent heat exchangers that are important for reducing heat loss 1

Canada goose

Arteries carrying warm blood down the legs of a goose or the flippers of a dolphin are in close contact with veins conveying cool blood in the opposite direction, back toward the trunk of the body. This arrangement facilitates heat transfer from arteries to veins (black arrows) along the entire length of the blood vessels.

2

Artery

Vein

35°C

33°

30º

27º

20º

18º

10º

9º

2

Near the end of the leg or flipper, where arterial blood has been cooled to far below the animal’s core temperature, the artery can still transfer heat to the even colder blood of an adjacent vein. The venous blood continues to absorb heat as it passes warmer and warmer arterial blood traveling in the opposite direction.

Pacific bottlenose dolphin

1

Blood flow Vein Artery

3

3

2 3

As the venous blood approaches the center of the body, it is almost as warm as the body core, minimizing the heat lost In the flippers of a dolphin, each artery is as a result of supplying blood to body parts surrounded by several veins in a countercurrent arrangement, allowing immersed in cold water. efficient heat exchange between arterial and venous blood.

Countercurrent Heat Exchange Bluefin tuna. Unlike most fishes, the bluefin tuna maintains temperatures in its main swimming muscles that are much higher than the surrounding water (colors indicate swimming muscles cut in transverse section). These temperatures were recorded for a tuna in 19°C water.

21º 25º 23º 27º 29º 31º

Body cavity

b) Great white shark. Like the bluefin tuna, the great white shark has a countercurrent heat exchanger in its swimming muscles that reduces the loss of metabolic heat. All bony fishes and sharks lose heat to the surrounding water when their blood passes through the gills. However, endothermic sharks have a small dorsal aorta, and as a result, relatively little cold blood from the gills goes directly to the core of the body. Instead, most of the blood leaving the gills is conveyed via large arteries just under the skin, keeping cool blood away from the body core. As shown in the enlargement, small arteries carrying cool blood inward from the large arteries under the skin are paralleled by small veins carrying warm blood outward from the inner body. This countercurrent flow retains heat in the muscles.

Skin Artery Vein

Blood vessels in gills Heart

Capillary network within muscle Artery and vein under Dorsal aorta the skin

Some endothermic insects have countercurrent heat exchangers that help maintain a high temperature in the thorax

Thermogenesis • Heat production can occur via 2 methods: • Shivering thermogenesis – piloerection; wing flapping • Non-shivering thermogenesis – hormonal; white versus brown fat metabolism

Many species of flying insects Use shivering to warm up before taking flight PREFLIGHT

Temperature (°C)

40

PREFLIGHT WARMUP

FLIGHT Thorax

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30 Abdomen 25

0 2 Time from onset of warmup (min)

4

Behavioral Methods • • • • • • •

Huddling Burrowing Migration (land/water) Hibernation (food versus temperature) Nocturnal life style (predatory) Surface area (sprawl) Basking

Evaporative Cooling Methods Heat of vaporization • • • • • •

Perspiration Transpiration Panting Saliva spreading Bathing Urination

Evaporative Cooling Bathing moistens the skin (heat of vaporization)

Long term temperature control • In a process known as acclimatization – Many animals can adjust to a new range of environmental temperatures over a period of days or weeks

• Acclimatization may involve cellular adjustments (molecular optimums) – Or in the case of birds and mammals, adjustments of insulation and metabolic heat production (fat bodies; degree of saturation – cell membranes)

Torpor – Is an adaptation that enables animals to save energy while avoiding difficult and dangerous conditions – Is a physiological state in which activity is low and metabolism decreases (decrease in heart and respiratory rate, decrease in body temperature) ***conserves food and energy

Torpor (decreased metabolic state) • Estivation, or summer torpor – Enables animals to survive long periods of high temperatures and scarce water supplies

• Daily torpor – Is exhibited by many small mammals and birds and seems to be adapted to their feeding patterns (bats, shrews, hummingbirds)

Hibernation is a form of long-term torpor That is an adaptation to winter cold and food scarcity during which the animal’s body temperature declines Additional metabolism that would be necessary to stay active in winter

Actual metabolism

Anatomy of Heat Control • In humans, a specific part of the brain, the hypothalamus – Contains a group of nerve cells that function as a thermostat – Contains heating and cooling centers

Hypothalmic Regulation Thermostat in hypothalamus activates cooling mechanisms.

Sweat glands secrete sweat that evaporates, cooling the body.

Blood vessels in skin dilate: capillaries fill with warm blood; Increased body Body temperature heat radiates from temperature (such decreases; skin surface. as when exercising thermostat or in hot shuts off cooling surroundings) mechanisms. Homeostasis: Internal body temperature of approximately 36–38°C Body temperature Decreased body increases; temperature thermostat (such as when shuts off warming in cold mechanisms. Blood vesselssurroundings) in skin constrict, diverting blood from skin to deeper tissues and reducing heat loss from skin surface. Thermostat in hypothalamus activates Skeletal muscles rapidly contract, causing shivering, warming mechanisms. which generates heat.