Physiology, Homeostasis, and Temperature Regulation

Physiology, Homeostasis, and Temperature Regulation

Chapter 39 Key Concepts 39.1 Animals Are Composed of Organs Built from Four Types of Tissues 39.2 Physiological Systems Maintain Homeostasis of the Internal Environment 39.3 Biological Processes Are Temperature-Sensitive

Chapter 39 Key Concepts 39.4 Body Temperature Depends on the Balance between Heat In and Heat Out of the Body 39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss

Investigating Life: Heat Limits Physical Performance
Every year, some athletes suffer heat stroke when body temperature rises above 40C and major organs begin to fail.

Working muscles generate heat that must be dispersed to the environment. In mammals, heat loss portals include non-furred areas such as nose, tongue, and footpads. How can we increase heat loss from the body to protect against heat stress?

Key Concept 39.1 Focus Your Learning
Multicellularity enabled the evolution of organisms that are much larger than single-celled organisms can be. Organs are composed of four tissue types.

39.1 Animals Are Composed of Organs Built from Four Types of Tissues
To survive, animals must extract energy and nutrients from the environment build all internal structures they need eliminate toxins and metabolic waste products sense the environment and respond to it in various ways, including movement

maintain constant conditions in their internal environments reproduce Multicellular organisms have evolved complex body systems to deal with these challenges. Advantages of multicellularity include large size and specialization of cells.

Advantages of bigger size: ability to prey on other organisms, and resist forces in nature such as ocean waves. Size of single-celled organisms is constrained by cell membrane surface area that limits exhange of materials with the environment.

If a multicellular animal consists of only a few layers of cells, those cells can exchange materials directly with the environment. Example: Sponges

Figure 39.1 No Cell Too Far Figure 39.1 No Cell Too Far
The sponge Leucosolenia is a tube about 2 cm. long with an outer and an inner layer of cells and a jellylike matrix. There are pores between the cells of the tube. The inner cells have flagella that create a current of water in through the pores and out through the opening at the top. The individual cells of this animal can obtain nutrients and oxygen directly from the seawater.

Cells of larger animals must be served by an internal environment of extracellular fluid. Some cells are specialized to contribute to the maintenance of the internal environment.

Cell specialization has tremendous adaptive value, but specialized cells may lose other functions. Example: Cells specialized for movement do not have to also capture and process food. The cells in an organism are collaborating to provide services to each other.

Tissues: Groups of similar specialized cells. Four types: Epithelial Muscle Connective Nervous Within each group there are various specializations for different functions.

Epithelial tissues: Sheets of cells that create barriers between different compartments. Includes the outer layers of skin (epidermis) and many types of secretory cells.

Figure 39.2 How to Build an Animal (Part 1)
Tissues are the building blocks.

Organs: Internal structures that carry out specific functions. Most have all 4 tissue types. Example: Digestive tract organs have a lining of epithelial cells, a layer of connective tissue called the mucosa, which includes blood vessels and neurons, and layers of smooth muscle.

Figure 39.3 Tissues Form Organs
The organs of the human digestive system, such as the stomach and small intestine, are made up of all four tissue types.

Individual organs are part of an organ system, a group of organs that work together (e.g., the digestive system). Organizational hierarchy: Cells → Tissues → Organs → Organ systems→ Multicellular organism

Key Concept 39.1 Learning Outcomes
Demonstrate an understanding of the factors that limit cell size. Describe the advantages of multicellularity. Explain structure–function relationships within an organ.

The needs of cells in the multicellular animal are served through exchanges with the internal environment, which consists of the extracellular fluid. Homeostasis of the internal environment is maintained through control and regulation of activities of organs and organ systems.

39.2 Physiological Systems Maintain Homeostasis of the Internal Environment
Functional losses in specialized cells are compensated for by the constancy of the internal environment. Evolution of physiological systems to maintain the internal environment made it possible for multicellular animals to become larger, more complex, and occupy many different environments.

Individual cells get nutrients from the extracellular fluid (ECF) and dump wastes into it. ECF includes blood plasma and the interstitial fluid that bathes every cell. Water and small molecules freely exchange between the interstitial fluid and the blood plasma.

Figure 39.4 The Internal Environment
The extracellular fluid (ECF) is the “internal environment,” and it is about 1.3 of total body water. The ECF is about 20% blood plasma and 80% interstitial fluid. The composition and physiological state of the ECF must remain stable within narrow limits, and maintaining that stability is the job of the body’s organ systems.

Organisms must maintain their internal environment in a state of homeostasis: a narrow range of stable and optimal physical and biochemical conditions. If homeostasis is compromised, cells can be damaged and can die. Maintenance of homeostasis is a central theme of physiology.

Physiological systems are controlled (speeded up or slowed down) by the nervous and endocrine systems. To regulate these systems and maintain homeostasis, information is required. Analogy: A thermostat controls furnace and air conditioner to regulate temperature.

Figure 39.5 A Thermostat Regulates Temperature
A thermostat regulates the temperature of a room by turning the furnace or air conditioner on or off in response to the difference between feedback information (room temperature) and set points that are programmed into the thermostat.

Set point: A reference point (desired temperature). Thermostat acts as a comparator by sensing current temperature and comparing it to the set point. Feedback: Information that is compared to the set point. Error signal: Any difference between the set point and feedback information.

Regulatory systems obtain, integrate, and process information. They issue commands to effectors such as muscles or glands that effect changes in the internal environment. Effectors are controlled systems—they are controlled by neural or hormonal signals from regulatory systems.

Sensors such as light-, temperature-, and pressure-sensitive cells provide feedback information to be compared with internal set points.

Negative feedback is information that corrects an error signal. Whatever force is pushing the system away from its set point must be “negated.”

Responses tend to reach a limit and terminate rapidly.
39.2 Physiological Systems Maintain Homeostasis of the Internal Environment Positive feedback amplifies a response and increases deviation from a set point. Responses tend to reach a limit and terminate rapidly. Examples: Sexual behavior and the birth process

Feedforward information anticipates internal changes and changes the set point. Examples: A timer on a thermostat; hearing the words “on your mark” before a race increases heart rate in anticipation of running

• Explain why homeostasis of the internal environment in a multicellular animal is critical to the animal’s survival. • Use knowledge about fluid compartments in the human body to perform analyses. • Differentiate between negative feedback, positive feedback, and feedforward control mechanisms.

• Cells can survive only within a small range of the temperatures that occur on Earth. • Q10 values are measures of temperature sensitivity of biochemical reactions or physiological processes.

• Temperature acclimatization allows animals to shift their metabolic rates as they experience seasonal changes in external temperatures. • Metabolic acclimatization enables animals to adapt to seasonal temperature changes.

39.3 Biological Processes Are Temperature-Sensitive
Most cells function over a narrow range of temperatures. Below 0C, ice crystals form and damage cell structures. Some animals have antifreeze molecules in their blood that help them resist freezing; others can survive freezing.

Figure 39.6 Frozen Frogs Figure 39.6 Frozen Frogs
Wood frogs (Rana sylvatica) live in the northeastern United States, across most of Canada, and up into northern Alaska. They freeze solid in the winter and survive. During the fall and early winter, urea and glucose accumulate in the frogs’ cells and act as intracellular cryoprotectants (like antifreeze in your car radiator). Also, much intracellular water is redistributed to the extracellular fluid, which can freeze without damaging cells.

Above 40C proteins begin to denature and lose function. Some specialized algae can grow in hot springs at 70C and some archaea live at near 100C. Survival limits for most cells fall between 0 and 40C.

But most organisms have a much narrower tolerance range. To stay within the limits, animals have evolved many thermoregulatory adaptations. These adaptations may determine the geographic range of a species.

Biochemical reactions are temperature- sensitive—the rate increases as temperature increases. Q10 describes temperature-sensitivity: Rate of a reaction at one temperature divided by the rate of the same reaction at a temperature 10 lower.

Figure 39.7 Q10 and Reaction Rate
The larger the Q10 of a reaction or process, the faster its rate rises in response to an increase in temperature.

Change in body temperature can disrupt physiology because not all biochemical reactions have the same Q10. These reactions are linked in complex networks: products of one reaction are reactants for other reactions. Shifting reaction rates can disrupt the overall network.

Body temperature of some animals is coupled to environmental temperature. Example: Fish body temperature is the same as water temperature. In winter, the fish will acclimatize to colder water and will have a higher metabolic rate in order to remain active.

One mechanism for acclimatization is to express isozymes (different forms of an enzyme) with different temperature optima. Another mechanism is change in the composition of cell membranes to maintain optimum fluidity despite changes in temperature.

At high temperatures, our ability to do hard work diminishes. High temperatures can damage cells, so shutting off a muscle’s ability to do work may serve a protective function. Pyruvate kinase inactivates at 40C, shutting down ATP production and muscle function, preventing thermal damage.

Investigating Life: Can the Work Capacity of Muscle be Increased by Extracting Heat from the Palms of the Hands? Hypothesis: Extracting excess heat from the body will increase the capacity of muscles to do work.

Investigating Life: Can the Work Capacity of Muscle be Increased by Extracting Heat from the Palms of the Hands? Method: Divide athletes into 2 groups. Each group completes 5 trials of bench presses, with 3-minute rest periods; experimental goups receives palmar cooling, control group receives none. Repeat the experiment with control and treatment groups switched.

Investigating Life: Can the Work Capacity of Muscle be Increased by Extracting Heat from the Palms of the Hands?, Experiment Investigating Life: Can the Work Capacity of Muscle be Increased by Extracting Heat from the Palms of the Hands?, Experiment Original Paper: Grahn, D. A., V. H. Cao, C. M. Nguyen, M. T. Liu and H. C. Heller Work volume and strength training responses to resistive exercise improve with periodic heat extraction from the palm. Journal of Strength and Conditioning Research 26(9): 2558–2569. At the beginning of the chapter you learned that mammals dissipate excess heat through their non-hairy skin areas. Those skin areas are small, but they have special high-volume blood vessels that transfer heat from the body core to the skin. Humans have these same blood vessel adaptations. In this experiment, heat was extracted from the palm of a hand, and muscle work capacity was measured.

• Explain why animals’ body temperatures must be maintained within narrow ranges. • Plot a given Q10 value for a biochemical process. • Explain how isozymes may be involved in seasonal temperature acclimatization.

• Endotherms can increase metabolic heat production to balance increased heat loss to the environment, whereas ectotherms rely largely on behavior to control their heat exchange with the environment. • The four avenues of heat exchange between an animal and its environment are radiation, convection, conduction, and evaporation.

• A countercurrent heat exchange system in some highly active fish conserves heat generated by muscle activity.

39.4 Body Temperature Depends on the Balance between Heat In and Heat Out of the Body
Homeotherms: Animals that maintain constant body temperature. Poikilotherms: Animals with fluctuating body temperatures.

Ectotherms: Body temperature depends on external sources of heat. Endotherms can vary metabolic heat production (mammals and birds). Heterotherms can behave either as an endotherm or an ectotherm (e.g., hibernating mammals).

With every transfer of energy in biological systems, some energy is lost as heat. Endotherms produce more heat: their cells are less efficient at using energy than the cells of ectotherms. Endotherm cells are “leaky.” Na+ and K+ ions must constantly be pumped (using energy) to maintain ion concentration gradients.

A mutation allowing seemingly faulty ion channels may have allowed enough heat production in small ectotherms to allow them to be active in cooler temperatures, such as night time. This would have been an advantage in exploiting resources at night, with less danger of predation from larger ectotherms.

Two major differences between ectotherms and endotherms:
39.4 Body Temperature Depends on the Balance between Heat In and Heat Out of the Body Two major differences between ectotherms and endotherms: Resting metabolic rate Response to changes in environmental temperatures

Experimentally, when temperature is decreased, an endotherm’s body temperature remains constant, while an ectotherm’s temperature equilibrates to the environmental temperature. The metabolic rates react in opposite ways to cooler temperatures.

Figure 39.8 Ectotherms and Endotherms React Differently to Environmental Temperatures (Part 1)
(A) At the same environmental temperature, an ectotherm and an endotherm of approximately the same body size (here, a lizard and a mouse) have different body temperatures. (B) The metabolic rates of the lizard and mouse react in opposite ways to cooler temperatures.

Figure 39.8 Ectotherms and Endotherms React Differently to Environmental Temperatures (Part 2)
(A) At the same environmental temperature, an ectotherm and an endotherm of approximately the same body size (here, a lizard and a mouse) have different body temperatures. (B) The metabolic rates of the lizard and mouse react in opposite ways to cooler temperatures.

But ectotherms such as lizards can regulate body temperature by using behavioral mechanisms. Behaviors include going into a burrow or basking in the sun, seeking shade, and changing body orientation.

Endotherms also use behavior for thermoregulation, such as by moving between sun and shade. More complex behaviors include nest construction and social behavior such as huddling.

Both ectotherms and endotherms can influence body temperature by altering 4 avenues of heat exchange: Radiation: Heat transfer via infrared radiation, from warmer to colder areas. Convection: Heat exchange with a surrounding medium such as air or water.

Conduction: Heat transfer between objects in direct contact.
39.4 Body Temperature Depends on the Balance between Heat In and Heat Out of the Body Conduction: Heat transfer between objects in direct contact. Evaporation: Heat transfer as water evaporates from a surface (e.g., sweating).

Figure 39.9 Animals Exchange Heat with the Environment
An animal’s body temperature is determined by the balance between internal heat production and four avenues of heat exchange with the environment: radiation, convection, conduction, and evaporation.

An energy budget is the balance of heat production and heat exchange. To maintain a constant temperature, heat coming in must equal heat going out.

All the components on the right side of the equation—the heat-loss side—depend on the surface temperature of the animal. If environmental temperature is higher than skin temperature, convection and conduction are avenues of heat gain rather than loss.

Constriction of blood vessels to the skin results in less heat loss.
39.4 Body Temperature Depends on the Balance between Heat In and Heat Out of the Body Surface temperature can be controlled by altering the flow of blood to the skin. Increased blood flow to the skin increases heat loss and lowers body temperature. Constriction of blood vessels to the skin results in less heat loss.

Marine iguanas alternate feeding in the cold ocean waters with basking on hot rocks in the sun. While swimming, they can retain some body heat by changing heart rate and thus rate of blood flow to the skin.

Figure 39.10 Some Ectotherms Regulate Blood Flow to the Skin
Galápagos marine iguanas control blood flow to the skin to alter their heating and cooling rates.

Fur on mammals acts as insulation that retains body heat. But when active, they must lose excess heat. Special blood vessels carry heat to hairless skin surfaces. Heat loss from these areas is tightly controlled by opening and closing of these blood vessels.

Fish produce heat metabolically, but most heat is lost as the blood travels over the gills. In typical “cold” fish, cool, oxygenated blood travels from the gills to the aorta and is distributed to organs and muscles.

Figure 39.11 “Cold” and “Hot” Fish (Part 1)
(A) Blood flowing through fish gills comes into temperature equilibrium with water temperature. In most fish species, this oxygenated but cool blood flows through a large dorsal aorta to the rest of the body. Thus it is an internal cooling source. (B) The anatomy of “hot” fish species circulates most of the cool, oxygenated blood to the body through large peripheral arteries rather than through a central aorta. As this cool arterial blood flows into the swimming muscle, it is warmed by heat being carried out of the swimming muscle by the veins. Thus, the heat generated by the swimming muscles is conserved in the muscle mass. (C) The close, parallel arrangement of the arteries carrying blood into the muscles and the veins carrying blood out of the muscle makes efficient countercurrent heat exchange possible.

Some large fish, such as bluefin tuna, can maintain swimming muscle temperatures 10–15C warmer than the water. These “hot” fish have a smaller aorta and most of the cold oxygenated blood from the gills flows through vessels just under the skin.

These vessels are very close to blood vessels returning warm blood to the gills, and heat flows into the colder blood. This countercurrent heat exchanger keeps the heat within the muscles. The higher temperature increases the fish’s power output, allowing faster swimming.

Some ectotherms can raise body temperature by producing metabolic heat: Insects contract their flight muscles isometrically to warm up for flight. Honeybees regulate temperature as a group, adjusting individual heat production and position in the cluster so that larvae are kept warm.

Explain responses to environmental temperature by an ectotherm, an endotherm, and a heterotherm. Describe the paths of heat exchange between an animal and its environment. Explain the principle of countercurrent heat exchange.

• The basal metabolic rate (BMR) of an endotherm is the lowest metabolic rate necessary for biochemical and physiological processes of a resting animal. • Endotherms produce and conserve metabolic heat to offset heat loss in cold environments.

• Mammalian body temperature is controlled by a regulatory center in the hypothalamus that uses hypothalamic temperature as the feedback information.

39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss
Metabolic rate can be measured by measuring consumption of O2 or production of CO2. In the thermoneutral zone, the metabolic rate of endotherms is low and independent of temperature.

Basal metabolic rate (BMR) is the metabolic rate of a resting animal at a temperature within the thermoneutral zone. BMR is the rate at which a resting animal is consuming just enough energy to carry out minimal body functions.

But the BMR per gram of tissue increases as animals get smaller.
39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss BMR is correlated with body size and environmental temperature. Larger animals have higher BMR. But the BMR per gram of tissue increases as animals get smaller. A gram of mouse tissue uses energy at a rate 15 times greater than a gram of elephant tissue.

Figure 39.12 The Mouse-to-Elephant Curve
On a weight-specific basis, the basal metabolic rate of small endotherms is much greater than that of larger endotherms. This classic illustration, originally published in the 1930s, plots O2 consumption per kilogram of body mass (a measure of metabolic rate) against a logarithmic plot of body mass.

Bigger animals have smaller surface area to volume ratio, reducing the capacity to dissipate heat. Large animals may have evolved lower BMR to prevent overheating. Or, larger animals have more support tissue that is not metabolically active. The relationship holds over a broad range of species.

An endotherm’s thermoneutral zone is bounded by upper and lower critical temperatures. In the thermoneutral zone, thermoregulatory responses do not use much energy. Outside this zone, responses require considerable metabolic energy.

Figure 39.13 Environmental Temperature and Mammalian Metabolic Rates
A plot of an endotherm’s metabolic rate versus environmental temperature represents the integrated response of all of its thermoregulatory mechanisms. Outside the thermoneutral zone, maintaining a constant body temperature requires expending energy. Outside extreme limits (0°C and 40°C in this instance), the animal cannot maintain its body temperature and dies.

Endotherms respond to cold by producing heat. Shivering heat production: Shivering muscles contract, ATP is converted to ADP, and heat is released. Increased muscle tone and body movements also contribute to increased heat production.

Birds use only shivering heat production. Mammals shiver and also produce heat in special adipose tissue called brown fat. Brown fat is brown because of abundant mitochondria and blood supply.

A protein called thermogenin in brown fat cells uncouples proton movement from ATP production. Protons leak across the inner mitochondrial membrane rather than passing through ATP synthase. No ATP is produced, but heat is released.

Brown fat is found in newborn mammals and hibernating mammals. Some adult humans have brown fat, and its metabolic activity is stimulated by cold exposure. Obese people tend to have less brown fat than lean people.

Endotherms in cold climates also have adaptations to reduce heat loss:
39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss Endotherms in cold climates also have adaptations to reduce heat loss: Adaptations for smaller surface area-to- volume ratios, such as rounder body shapes and shorter appendages

Figure 39.14 Adaptations to Cold and Hot Climates (Part 1)
(A) The antelope jackrabbit is found in the Sonoran Desert of Arizona. Its large ears serve as heat exchangers, passing heat from the animal’s blood to the surrounding air. (B) The thick fur of the Arctic hare provides insulation, and its rounded body shape lowers its surface area-to-volume ratio. The ears and extremities are smaller than those of its warm-climate relatives, so less heat is lost to the environment.

Figure 39.14 Adaptations to Cold and Hot Climates (Part 2)
(A) The antelope jackrabbit is found in the Sonoran Desert of Arizona. Its large ears serve as heat exchangers, passing heat from the animal’s blood to the surrounding air. (B) The thick fur of the Arctic hare provides insulation, and its rounded body shape lowers its surface area-to-volume ratio. The ears and extremities are smaller than those of its warm-climate relatives, so less heat is lost to the environment.

Increased thermal insulation with fur, feathers, or fat
39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss Increased thermal insulation with fur, feathers, or fat Ability to decrease blood flow to the skin by constricting blood vessels Use of countercurrent heat exchange in blood flow to appendages

Adaptations to hot climates:
39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss Adaptations to hot climates: Increased blood flow to the skin, shade- seeking, decreasing activity Increased surface area for heat dissipation (e.g., jackrabbit ears) Large mammals have little or no insulating fur and seek water to wallow in—water has heat absorbing capacity

Evaporation of water by sweating or panting to dissipate heat
39.5 Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss Evaporation of water by sweating or panting to dissipate heat But water loss can be a problem, especially in hot, arid climates. Sweating and panting are active processes that require energy and thus are also generating heat. This is why metabolic rate increases when upper critical temperature is exceeded.

In mammals the hypothalamus in the brain is the main thermoregulatory center. Experiments show that cooling of the hypothalamus causes restriction of skin blood vessels and increases metabolic heat production.

Figure 39.15 The Mammalian Thermostat
Like the home thermostat in Figure 39.5, the mammalian hypothalamus is the regulatory system that controls the body’s heating and cooling mechanisms.

Figure 39.16 The Hypothalamus Regulates Body Temperature
Original Paper: Heller, H. C., Colliver, G. W., and P. Anand CNS regulation of body temperature in euthermic hibernators. American Journal of Physiology. 227:576–582. A mammal’s hypothalamus was subjected directly to temperature manipulation. The body’s responses to the manipulations were as expected if the hypothalamus is the mammalian “thermostat.”

The hypothalamus generates set points for thermoregulation. Temperature of the hypothalamus itself is the major feedback signal. Other information is also integrated, including skin temperature, which is feedforward information that shifts hypothalamic set points.

Hypothalamus set points are higher during wakefulness and during the active part of the daily cycle. Even when an endotherm is kept in constant environmental conditions, its body temperature displays a daily cycle of changes in set point (a circadian rhythm).

Hypothermia: Below-normal body temperature. Unregulated hypothermia results from starvation, extreme cold, illness, etc. Many birds and mammals use regulated hypothermia to survive cold periods and food scarcity.

Daily torpor: Small endotherms such as hummingbirds lower body temperature and metabolic rate during inactive periods to conserve energy. Hibernation lasts for days or weeks; metabolic rate drops and body temperature falls close to ambient (even near freezing) temperatures.

Figure 39.17 Hibernation Patterns in a Ground Squirrel (Part 1)
During most of the year, the ground squirrel regulates its body temperature around 37°C. During winter months, however, these animals hibernate in underground burrows, living off stored fuel (in the form of either fat or cached food). The metabolic demands for these stored fuels are decreased by bouts of torpor, during which body temperature drops close to that of the environment for long periods of time.

Figure 39.17 Hibernation Patterns in a Ground Squirrel (Part 2)
During most of the year, the ground squirrel regulates its body temperature around 37°C. During winter months, however, these animals hibernate in underground burrows, living off stored fuel (in the form of either fat or cached food). The metabolic demands for these stored fuels are decreased by bouts of torpor, during which body temperature drops close to that of the environment for long periods of time.

The ability to reduce thermoregulatory set points so dramatically probably evolved as an extension of the set point decrease that accompanies sleep in all mammals and birds.

Describe the thermoneutral zone and the upper and lower critical temperatures and how they relate to the basal metabolic rate of an endotherm. Interpret a plot of metabolic rate versus environmental temperature for an endotherm.

Explain differences in physical features of endotherms of the same or similar species that live in climates of different temperature extremes. Describe how the mammalian brain thermostat uses feedback and feedforward information to regulate body temperature. Explain the variable features of the mammalian thermoregulatory system.

How can we increase heat loss from the body to protect against heat stress? “Heat portals” in non-hairy mammalian skin include gated shunts that deliver arterial blood directly to veins, bypassing the slower flow of the capillaries. Based on this, biologists have developed a rapid cooling technology.

An area such as the palm of the hand is placed in contact with a cooled surface and a mild vacuum is used to pull more blood into the large, heat-exchanging blood vessels. Because muscle fatigue is partly due to increased temperature, the cooling technology also enhanced athletic performance.

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