physiology, homeostasis, and temperature regulation 29

Physiology, Homeostasis, and Temperature Regulation

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Chapter 29 Physiology, Homeostasis, and Temperature Regulation

Key Concepts

• 29.1 Multicellular Animals Require a Stable Internal Environment

• 29.2 Physiological Regulation Achieves Homeostasis of the Internal Environment

• 29.3 Living Systems Are Temperature-Sensitive

Chapter 29 Physiology, Homeostasis, and Temperature Regulation

• 29.4 Animals Control Body Temperature by Altering Rates of Heat Gain and Loss

• 29.5 A Thermostat in the Brain Regulates Mammalian Body Temperature

Chapter 29 Opening Question

What can ground squirrels do to lower the metabolic demands of surviving through the winter?

Concept 29.1 Multicellular Animals Require a Stable Internal Environment

A stable fluid environment makes complex multicellular animals possible.

Most water in an animal’s body is intracellular fluid, within the cells.

The rest is the extracellular fluid, which includes blood plasma and interstitial fluid that bathes each cell.

Figure 29.1 The Internal Environment


Homeostasis is the maintenance of stable conditions in an internal environment.

Cells became specialized for maintaining the internal environment, such as temperature, pH, and ion concentration.

Specialized cells evolved into tissues, organs, and physiological systems that serve specific functions.

Organs are made up of tissues, which are then made up of cells.


Four types of tissue:

• Epithelial

• Connective

• Nervous

• Muscle


Epithelial tissues are sheets of tightly connected epithelial cells that cover inner and outer body surfaces.

Some line blood vessels and hollow organs.

Some secrete substances such as hormones or sweat, or serve transport functions for nutrients.

Others serve sensory functions of smell, taste, and touch.


Connective tissues are dispersed cells in a secreted extracellular matrix.

The composition of the matrix differentiates the types of connective tissues.

Collagen and elastin provide strength and elasticity to cartilage.

Bone matrix is mineralized for strength while the matrix of blood cells—plasma—is liquid.

Adipose tissue, made of fat cells, has little matrix.


Nervous tissues contain two basic cell types—neurons and glial cells.

Neurons generate and conduct electrical signals, or nerve impulses, throughout the body.

They are units of the central and peripheral nervous systems and communicate via chemicals, neurotransmitters.

Glial cells provide support for neuronal function.


Muscle tissues consist of elongated cells that generate force and cause movement.

Three types of muscle tissues:

• Skeletal—responsible for locomotion and movement

• Cardiac—makes up the heart and generates heartbeat and blood flow

• Smooth—involved in movement and generation of forces in internal organs

Figure 29.2 Tissues Form Organs


Organs consist of multiple tissues, and most have all four types.

An organ system is a group of organs that function together.

To maintain homeostasis, each organ and organ system must respond to the demands of the body’s cells.

Concept 29.2 Physiological Regulation Achieves Homeostasis of the Internal Environment

Types of information necessary for physiological systems:

• Set point—a reference point

• Feedback information—what is happening in the system

• Error signal—any difference between the set point and feedback information

Figure 29.3 Control, Regulation, and Feedback


Regulatory systems:

• Obtain, integrate, and process information

• Issue commands to controlled systems

• Contain sensors to provide feedback information that is compared to the set point


Regulatory systems then issue commands to effectors that effect changes in the internal environment.

Effectors are controlled systems because they are controlled by regulatory systems.


Sensory information in regulatory systems includes:

• Negative feedback

• Positive feedback

• Feedforward information


Negative feedback:

• Causes effectors to counteract the influence that creates an error signal

Positive feedback:

• Amplifies a response

• Increases deviation from a set point

Feedforward information:

• Anticipates internal changes and changes the set point.

Concept 29.3 Living Systems Are Temperature-Sensitive

Physiological processes are temperature-sensitive and increase their rate at higher temperatures.

Q10 describes temperature-sensitivity as the quotient of the rate of a reaction at one temperature divided by the rate of the same reaction at a lower temperature.

Q10 = RT/RT–10

Figure 29.4 Q10 and Reaction Rate


Body temperature of some animals is coupled to environmental temperature.

In winter, the body temperature of a fish will acclimatize to colder water.

It may express more or fewer enzymes with different temperature optima.


Thermal classification of animals can be based on source of heat.

Ectotherms such as fish, amphibians, and reptiles get “heat from the outside.”

Endotherms, such as birds and mammals, get “heat from the inside,” producing heat metabolically or by actively losing heat.

Figure 29.5 Ectotherms and Endotherms React Differently to Environmental Temperatures (Part 1)

Figure 29.5 Ectotherms and Endotherms React Differently to Environmental Temperatures (Part 2)


In the thermoneutral zone the metabolic rate is low and independent of temperature.

The basal metabolic rate (BMR) is the metabolic rate of a resting animal at a temperature within the thermoneutral zone.

Concept 29.4 Animals Control Body Temperature by Altering Rates of Heat Gain and Loss

The heat budget equation:

Body temperature is the result of thermal energy flowing in from the environment and from metabolism (heatin), and thermal energy leaving the animal (heatout).

If heatin does not equal heatout, body temperature will change.


Gains and losses of thermal energy occur by these mechanisms:

• Metabolism—conversion of ATP to do work produces heat

• Radiation—via infrared radiation

• Convection—through a surrounding medium

• Conduction—by direct contact

• Evaporation—through evaporation of water from a surface


Of central importance to the heat budget equation:

• Surface temperature

• Surface area

These are key factors in heat loss through radiation, conduction, and convection.

Figure 29.6 Animals Exchange Heat with the Environment


Endotherms expend most of their energy pumping ions across membranes.

Cells are “leakier” to ions than cells of ectotherms.

Endotherms spend more energy and release more heat to maintain ion concentration gradients.


If environmental temperature (Ta) falls below an endotherm’s lower critical temperature, animal must produce heat or body temperature (Tb) will fall.

Mammals produce heat in two ways:

Shivering thermogenesis—skeletal muscles contract and release energy from ATP as heat.

Nonshivering heat production—in adipose tissue called brown fat.

Figure 29.7 Brown Fat


The basal metabolic rate (BMR) is correlated with body size and environmental temperature.

The BMR per gram of tissue increases as animals get smaller.

Example: A gram of mouse tissue uses energy at a rate 20 times greater than a gram of elephant tissue.

Figure 29.8 The Mouse-to-Elephant Curve


Reducing heat loss is important in cold climates.

Some cold-climate species have a smaller surface area than warm-climate relatives.

Rounder body shapes and shorter appendages reduce surface area-to-volume ratios.

Figure 29.9 Anatomical Adaptations to Climate (Part 1)

Figure 29.9 Anatomical Adaptations to Climate (Part 2)


Other adaptations to reducing heat loss include:

• 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


Fish produce heat metabolically in their muscles, but most heat is lost as the blood travels over the gills.

In “cold” fish, cold, oxygenated blood travels from the gills to the aorta and is distributed to organs and muscles.


“Hot” fish have a smaller aorta and cold oxygenated blood flows instead in vessels under the skin.

These vessels are close to blood vessels returning warm blood to the gills, and heat flows into the colder blood.

This countercurrent heat exchanger describes the heat exchange between blood vessels carrying blood in opposite directions.

Figure 29.10 “Cold” and “Hot” Fish (Part 1)


A rise in environmental temperature results in increased blood flow to the skin to dissipate heat.

If temperature exceeds the upper critical temperature, overheating is possible.

Evaporation of water through sweating or panting increases heat loss, but is an active process that also generates some heat.


Some ectotherms are able to raise their body temperature by producing heat:

• Insects contract their flight muscles

• Honeybees regulate temperature as a group, adjusting individual heat and position in the cluster so that larvae are kept warm

Figure 29.11 Bees Keep Warm in Winter


Both endotherms and ectotherms may use behavioral regulation to maintain body temperature.

Examples: Lizard moving into sun or shade, or elephant spraying itself with water or dust

Figure 29.12 Ectotherms Can Use Behavior to Regulate Body Temperature (Part 1)

Figure 29.12 Ectotherms Can Use Behavior to Regulate Body Temperature (Part 2)

Concept 29.5 A Thermostat in the Brain Regulates Mammalian Body Temperature

Hormonal and neural mechanisms control thermoregulatory adaptations, such as changes in blood vessels or shivering.

The temperature regulatory system depends on feedback and acts as a thermostat.

In vertebrate brains, the hypothalamus is the major center of the thermostat.

The temperature of the hypothalamus can be the main feedback to the thermostat.


Cooling the hypothalamus can cause body temperature to rise by:

• Constricting blood vessels to the skin

• Increasing metabolic rate

Warming the hypothalamus can lower body temperature by:

• Dilating blood vessels to the skin

• Sweating or panting

Figure 29.13 The Hypothalmus Regulates Body Temperature (Part 1)


The temperature of the hypothalamus is a negative feedback signal—variability from its set point can trigger thermoregulatory responses.

Other factors can change hypothalamic set points:

• Change in skin temperature

• Wakefulness or sleep

• Circadian rhythm—a daily internal cycle


Fever is a an adaptive response to help fight pathogens.

The rise in body temperature is caused by a rise in the set point for metabolic heat production.

Some animals lower their temperature during inactive periods to conserve energy—daily torpor.

Long-lasting regulated hypothermia— hibernation

Answer to Opening Question

Ground squirrels are able to lower their metabolic demands in winter by periodically lowering body temperature.

The squirrel enters its burrow when snow falls and begins a bout of hibernation for about a week.

It then returns to a normal temperature for a day before entering the next bout of hibernation.

Figure 29.14 Hibernation Patterns in a Ground Squirrel

physiology, homeostasis, and temperature regulation 29

Documents

internal environment

stable fluid environment

animals body

temperature sensitive

epithelial cells

glial cells

specialized cells

dispersed cells