BIOL #16: Homeostasis & Bioenergetics

Question 1

Regulator & Conformer

Answer 1

A regulator can use internal mechanisms to control internal changes in the face of external fluctuation
- A river otter (mammal) maintains a body temperature of about 38°C regardless of water temperature

A conformer allows internal conditions to change in accordance with external changes for a particular environmental variable
- The body temperature of a largemouth bass closely matches that of the surrounding water

Question 2

Homeostasis

Answer 2

Homeostasis (steady-state) is the maintenance of relatively constant internal environment.
- Although external conditions may vary as an animal’s environment changes, internal chemical and physical states are kept within a tolerable range.
+ e.g. temperature regulation in sea otters and blood solute concentration regulation in bass

Human homeostasis:

• Maintenance of a fairly constant body temperature of 37°C (98.6°F)
• Maintenance of a blood and interstitial fluid pH within 0.1 pH unit of 7.4
• Maintenance of a blood glucose level in the bloodstream within the range of 70-110 mg glucose/100 ml blood

Temperature, pH, and other physical and chemical conditions have a dramatic effect on the structure and function of enzymes. Most enzymes function best under a fairly narrow range of conditions (i.e. optimal conditions).

Molecules, cells, tissues, organs, and organ systems function at an optimal level when homeostasis occurs.

Question 3

Epithelial Tissue in Homeostasis

Answer 3

Epithelium plays a vital role in homeostasis:

• Creates an internal environment that is dramatically different from the external environment
• Maintains physical and chemical conditions inside an animal that are relatively constant.

One of the most basic functions of epithelial tissue is to control the exchange of materials across its surfaces in a way that is consistent with homeostasis.
- Tight junctions are used to maintain barriers such that controlled exchanges can occur.

Question 4

Mechanisms of Homeostasis

Answer 4

The key to homeostasis is constant maintenance – homeostasis requires monitoring, regulating, and feedback control
- The process is like temperature regulation in a room using a thermostat

For any particular variable (e.g. temperature, pH):
- There is a particular set point (normal or target value)
+ e.g. pH 7.4 in human body fluids
+ Some variables may have a normal range with an upper and lower limit rather than a set point
- Fluctuation around the set point serve as a stimulus, which is detected by a receptor or sensor
- A signal from a sensor triggers a response by a control center – a response is a physiological activity that helps return the variable to the set point

Question 5

Feedback

Answer 5

Homeostatic systems are based on negative feedback, a control mechanism that reduces the stimulus.

• In the thermostat example, the heater produces a response to the stimulus of decreasing temperature by pumping out more heat – this, in effect, reduces the original stimulus of less heat by increasing the temperature.
• In humans, vigorous exercise produces heat, which increases body temperature. The nervous system detects this increase and triggers sweating which helps cool the body, reduces the stimulus, and returns body temperature to the set point.

Unlike negative feedback, positive feedback is a control mechanism that amplifies rather than reduces a stimulus.

Positive feedback loops in animals do not play a major role in homeostasis, but instead help drive physiological processes to completion.
- e.g. pressure during childbirth stimulates contractions of the uterus, which increases pressure, which increases contractions until the process is complete.

Question 6

Regulated Changes

Answer 6

The set points and normal ranges for homeostasis can change under various circumstances – such regulated changes are essential to normal body functions:
- Cyclical changes
+ Variation in hormonal levels responsible for a woman’s menstrual cycle
+ Metabolic changes associated with circadian rhythms that occur in a 24-hour cycle
- Acclimatization (acclimation)
+ Gradual adjustment to changes in external environment – set points may be overshot or undershot until control centers can adjust the response to the environment
* Gradual increase in red blood cell production at high altitudes will ensure enough oxygen is being consumed for cellular respiration

Question 7

Thermoregulation

Answer 7

Heat exchange is critical in animal physiology because individuals that get too hot or too cold may die.
- Overheating can cause proteins to denature and cease functioning and can lead to dehydration.
- Low body temperatures can slow down enzyme function and energy production.
+ For every 10°C (18°F) decrease in temperature, the rates of most enzyme-mediated reactions decrease two- to three-fold

Many animals can control their body temperature through the process of thermoregulation.

• Thermoregulation is the process by which animal maintain an internal temperature within a tolerable range
• Each animal species has an optimal temperature range

Question 8

Endotherm vs. Ectotherm

Answer 8

The sources of heat for thermoregulation include the external environment and internal metabolism

An endotherm produces adequate heat to warm its own tissue, mostly by heat generated from metabolism (endothermic)
- E.g. birds and mammals (plus few nonavian reptiles, some fishes, and many insect species)

An ectotherm relies on heat gained from the environment (ectothermic)
- E.g. amphibians, lizards, snakes, turtles, many fishes, and most invertebrates

Endotherms and ectotherms represent two extremes along a continuum of heat sources.
- Endothermy and ectothermy are not mutually exclusive modes of thermoregulation – e.g. a bird is mostly endothermic but may warm itself in the sun on a cold morning, much like an ectothermic lizard does.

Endotherms
- Can warm themselves because their basal metabolic rates are extremely high (metabolism = totality of chemical reactions in an organism)
+ the heat given off by the high rate of chemical reactions is enough to warm the body.
+ Mammals and birds retain this heat because they have elaborate insulating structures such as feathers or fur.
- Can cool themselves with various mechanisms, thus can withstand high temperatures that many ectotherms find intolerable

Ectotherms

• Gain heat directly from the environment.
• Generate only a small amount of heat internally relative to endotherms because ectotherms have low metabolic rates.
• Can tolerate larger fluctuations in their internal temperature
• Can use behavioral mechanism to indirectly adjust body temperature (i.e. seeking shade or basking in the sun)

Question 9

Homotherm vs. Poikilotherm

Answer 9

There is also a continuum describing whether animals hold their body temperature constant:

Homeotherms keep their body temperature constant.
- E.g. river otters with a constant body temperature of 38°C

Poikilotherms can tolerate changes in body temperature that vary with the environment
- E.g. largemouth bass

Many animals lie somewhere between these extremes.

• Some ectothermic fish live in such stable environments that their body temperature does not fluctuate much
• Some endothermic animals, such as bats and hummingbirds may enter inactive periods during which their body temperature decreases substantially but otherwise they are homeotherms

Question 10

Warm- vs Cold-blooded

Answer 10

Ectotherms are commonly referred to as ‘cold-blooded’ and endotherms as ‘warm-blooded’
- These terms represent common misconceptions – ectotherms do not necessarily have low body temperatures
+ When sitting in the sun, many ectothermic lizards have higher body temperatures than mammals

Question 11

Trade-Offs

Answer 11

Endothermy and ectothermy are best understood as contrasting adaptive strategies.
- Like all adaptations, endothermy and ectothermy involve trade-offs.

Endotherms have higher metabolic rates and thus can be more active.

In contrast, ectotherms are able to thrive with much lower intakes of food and can use a greater proportion of their total energy intake to support reproduction because they are not oxidizing food to provide heat.

Question 12

Conduction

Answer 12

Conduction is the direct transfer of heat between two physical bodies that are in contact with each other.

Question 13

Convection

Answer 13

Convection is a special case of conduction in which heat is exchanged between a solid and a liquid (or gas) rather than between two solids.

Question 14

Radiation

Answer 14

Radiation is the transfer of heat between two bodies that are not in direct physical contact.

Question 15

Evaporation

Answer 15

Evaporation is the phase exchange that occurs when liquid water becomes a gas.

Question 16

Insulation

Answer 16

Insulation reduces the flow of heat between an animal and its environment

• Common in mammals and birds
• Sources of insulation include hair, feathers, and layers of fat formed by adipose tissue
• This type of adaptation is particularly important for marine mammals (blubber)

Question 17

Circulatory Adaptations

Answer 17

Circulatory systems provide a major route for heat flow between the interior and exterior body

Adaptations that regulate the extent of blood flow near the body surface or that trap heat within the body core play a significant role in thermoregulation

Question 18

Countercurrent Exchange

Answer 18

Many birds and mammals reduce heat loss via countercurrent exchange
- Countercurrent exchange is the transfer of heat (or solutes) between fluids that are flowing in opposite directions

Such heat-conserving mechanisms are found in the arrangements of arteries and veins in:

• the flippers of whales and dolphins
• the legs of many mammals and birds that live in cold terrestrial environments.

Certain sharks, fishes, and insects also use countercurrent heat exchange

• Although most sharks and fish are temperature conformers – large, powerful swimmers (great white sharks, bluefin tuna, swordfish) can use this mechanism to keep their swimming muscles warm and sustain powerful movements
• Bumblebees and other flying, endothermic insects use this mechanism to maintain a high temperature in their thorax, where flight muscles are located

Question 18

Vasodilation & Vasoconstriction

Answer 18

Many animals (both endotherms and ectotherms) can respond to changes in external temperature by altering the amount of blood (and hence heat) flowing between the body core and skin.

Vasodilation: widening of blood vessels at the body surfaces, which increases blood flow to the skin, transferring heat to the environment

Vasoconstriction: reduces blood flow and heat transfer by decreasing the diameter of superficial blood vessels

Question 19

Countercurrent Heat Exchanger

Answer 19

In a countercurrent heat exchanger, arteries and veins are located adjacent to each other – as warm blood moves from the body core in the arteries, it transfers heat to the colder blood returning from the extremities in the veins

Because blood flows through the arteries and veins in opposite directions, heat is transferred along the entire length of the exchanger maximizing the rate of heat exchange

Question 20

Evaporative Heat Loss

Answer 20

Evaporative heat loss is used in cooling

• Terrestrial animals lose water by evaporation from their skin and respiratory surfaces
• Water absorbs considerable heat when it evaporates and this heat is carried away from the body surfaces with the water vapor

Some animals have adaptations that can greatly increase the cooling effect of evaporation
- Panting and sweating provides moist surfaces which greatly enhance the effect of evaporative cooling

Question 21

Behavioral Responses

Answer 21

Both endotherms and ectotherms control body temperature through behavioral responses to changes in the environment
- Extreme behaviors performed by endotherms to regulate body temperature include hibernation and migration

Amphibians and most reptiles (other than birds) are ectothermic and used shade- and sun-seeking behavior to thermoregulate
- Many terrestrial insects used similar “orientation” behaviors – e.g. dragonflies angle bodies away or towards the sun

Question 22

Acclimatization

Answer 22

Acclimatization (acclimation) is a phenotypic change that occurs within an individual in response to a short-term change in environmental conditions.

Birds and mammals can acclimate to seasonal temperature changes by adjusting insulation, thus endotherms can keep a constant body temperature year round
- e.g. mammals can grow a thicker coat in winter and shed it in summer

In ectotherms, acclimation often includes adjustments at the cellular level

• Cells may produce variants of enzymes that have the same function but different optimal temperatures
• Proportions of saturated and unsaturated lipids in cell membranes may change (unsaturated lipids keep membranes fluid at lower temperatures)

Question 23

Hypothalmus

Answer 23

Thermoregulation is an important aspect of homeostasis in some animals.
- The regulation of body temperature in humans and other mammals is brought about by a complex system based on negative feedback mechanisms

The sensors for thermoregulation are concentrated in a brain region called the hypothalamus
- A group of nerve cells in the hypothalamus functions as a thermostat, responding to body temperatures outside the normal range by activating mechanisms that promote heat loss or gain

Question 24

Temperature Homeostasis in Endotherms

Answer 24

Warm receptors in the body signal the hypothalamic thermostat when temperatures increase
- The thermostat will inhibit heat retention mechanisms and promote cooling the body (vasodilation, sweating, or panting)

Cold receptors signal when temperatures decrease
- The thermostat will inhibit heat loss mechanisms and activate heat-saving ones (e.g. vasoconstriction, shivering)

Question 25

Bioenergetics

Answer 25

Bioenergetics refers to the overall flow and transformation of energy in an animal
- Food is digested by enzymatic hydrolysis and nutrients are absorbed by body cells. Most nutrient molecules are then used to make ATP.

An animal’s bioenergetics determines nutritional needs and is related to the animal’s size, activity, and environment

Question 26

Metabolic Rates

Answer 26

Types of questions asked by physiologists:

• How much of the total energy an animal obtains from food does it need just to stay alive?
• How much energy must be expended to walk, run, swim, or fly from one place to another?
• What fraction of the energy intake is used for reproduction?

Physiologists can answer such questions by measuring the rate at which an animal uses chemical energy and how this rate changes in different circumstances

Metabolism is the totality of chemical reactions in an organism

Metabolic rate is the amount of energy an animal uses per unit of time

• Put another way, this is the sum of all of the energy used in biochemical reactions over a given time interval
• This energy is measured in joules (J) or in calories (cal) or kilocalories (kcal)

Question 26

Measuring Metabolic Rates

Answer 26

Measuring heat loss

• Metabolic energy can be measured by monitoring an animal’s rate of heat loss because nearly all of the chemical energy used in cellular respiration is eventually transformed into heat
• A calorimeter is a closed, insulated chamber that can measure an animal’s heat loss

Measuring O2 consumption or CO2 production

Long-term measurements
- Uses records of food consumption rates, energy content of the food (4-5 kcal/g for protein or carbohydrates and 9 kcal/g for fat), and the chemical energy lost in waste products (feces and nitrogenous waste)

Question 26

Minimum Metabolic Rates

Answer 26

Animals must maintain a minimum metabolic rate for basic functions (e.g. cell maintenance, breathing, and heartbeat)

Researchers measure minimum metabolic rate for basic functioning differently for endotherms and ectotherms

• Basal metabolic rate (BMR) is the minimum metabolic rate of a nongrowing, nonstressed endotherm at rest, with an empty stomach, under a “comfortable” temperature range
• Standard metabolic rate (SMR) is the minimum metabolic rate of a fasting, nonstressed ectotherm at rest at a particular temperature

Question 27

Effects of Size on Metabolic Rates

Answer 27

Large animals have more body mass and therefore require more chemical energy

Metabolic rate is roughly proportional to body mass to the three-quarters power (m3/4) for a wide range of organisms, spanning from bacteria to blue whales

Metabolic rates follow this relationship for both endotherms and ectotherms

The relationship of metabolic rate to size profoundly affects energy consumption by body cells and tissues

The energy it takes to maintain each gram of body mass is inversely related to body size

Each gram of a mouse requires about 20Xs as many calories as a gram of elephant, even though the whole elephant uses far more calories overall compared to the whole mouse

The mouse’s higher metabolic rate per gram demands a higher O2 consumption rate, as well as a higher breathing rate, blood volume (relative to size), and heart rate. Plus, a mouse must eat more food per unit of body mass

Why does a mouse have a higher relative metabolic rate than an elephant?
- Put another way, why does a mouse have a higher metabolic rate per gram of body tissue compared to an elephant?

In the case of endotherms, smaller body sizes translate into greater surface area-to-volume ratios (SA:V), resulting in greater relative heat loss to the environment per unit time
- To maintain a constant high body temperature despite rapid heat loss, a small animal must oxidize food at a high rate (eat a lot!)
+ Because of this, there is a lower limit on the size of endotherms – the smallest mammals are shrews (4 grams) which must eat nearly their own body weight in food every day and can starve to death in only a few hours if food deprived

The relationship between body size and metabolic rate is similar for ectotherms but the reasons for higher metabolic rates at smaller sizes is still not well understood.

The relationship of metabolic rate to size demonstrates how trade-offs shape the evolution of body plans
- As body size become smaller, each gram of tissue increases in energy costs
- As body size increases, energy costs per gram of tissue decreases
+ BUT an even larger fraction of body tissue is required for exchange, support, and locomotion in larger organisms

Question 28

Maximum Metabolic Rate

Answer 28

Maximum metabolic rate refers to the highest rate of ATP use for an organism
- Occurs during peak activity, e.g. sprinting, lifting weights

For most terrestrial animals, the average daily rate of energy consumption is 2 to 4 times BMR (for endotherms) or SMR (for ectotherms).

Humans in most developed countries have an unusually low average daily metabolic rate of ~1.5 times BMR – an indication of relatively sedentary lifestyles.