Physiology, Homeostasis, and
Temperature Regulation
Physiology, Homeostasis,
and Temperature Regulation
• Homeostasis: Maintaining the Internal
Environment
• Tissues, Organs, and Organ Systems
• Physiological Regulation and Homeostasis
• Temperature and Life
• Maintaining Optimal Body Temperature
• Thermoregulation in Endotherms
• The Vertebrate Thermostat
Homeostasis:
Maintaining the Internal Environment
• Homeostasis is the maintenance of constant
conditions in the internal environment of an
organism.
• Single-celled organisms and simple multicellular
animals meet all of their needs by direct exchange
of substances with the external environment.
• Simple, multicellular animal lifestyles are quite
limited, however, because no part of their bodies
can be more than a few cell layers thick.
Figure 41.1 Maintaining Internal Stability while on the Go
EXTERNAL
ENVIRONMENT
Skin
Unabsorbed matter
Foods, salts,
and water
Organic
waste products,
salts, and water
Respiratory
system
Cells
CO2
Urinary
system
Blood
(cells + plasma)
Tissues, Organs, and Organ Systems
• Cells grouped together with the same
characteristics or specializations are called
tissues.
• The four basic types of tissue are epithelial,
connective, muscle, and nervous.
• An organ is composed of tissues, usually of
several different types.
Figure 41.2 Four Types of Tissue
Tissues, Organs, and Organ Systems
• A discrete structure that carries out a specific
function in the body is an organ. Examples
include the stomach and the heart.
• Most organs include all four tissue types.
• Most organs are part of an organ system, a
group of organs that function together.
Physiological Regulation and Homeostasis
• Homeostasis depends on the ability to regulate
the functions of organs and organ systems.
• Generally, the regulatory systems are the nervous
system and the endocrine system.
• Maintenance of homeostasis is dependent on
information received, specifically feedback
information that signals any discrepancy
between the set point (the particular desired
condition or level) and the conditions present.
• The difference between the set point and the
feedback information is the error signal.
Figure 41.4 Control, Regulation, and Feedback
Physiological Regulation and Homeostasis
• Cells, tissues, and organs are effectors that
respond to commands from regulatory systems.
Effectors are controlled systems.
• Regulatory systems obtain, process, and integrate
information, then issue commands to controlled
systems, which effect change.
• Regulatory systems receive information as
negative feedback, which causes effectors to
reduce or reverse a process; or positive feedback
which tells a regulatory system to amplify a
response.
• Feedforward information signals the system to
change the setpoint.
Temperature and Life
• Most physiological processes are temperaturesensitive, going faster at higher temperatures.
• The sensitivity of a physiological process to
temperature can be described as a quotient, Q10.
• Q10 is defined as the rate of a reaction at a
particular temperature (RT) divided by the rate of
that reaction at a temperature 10°C lower (RT-10).
Q10 = RT / RT-10
Figure 41.5 Q10 and Reaction Rate
3
2
1
Maintaining Optimal Body Temperature
• Animals may be classified by how they respond to
environmental temperatures:
 Homeotherms maintain a constant body
temperature.
 In poikilotherms, body temperature changes
when environmental temperature changes.
 A third category, heterotherm, fits animals that
regulate body temperature at a constant level
some of the time, such as hibernating
mammals.
Maintaining Optimal Body Temperature
• Animals may also be classified according to the
sources of heat that determine their body
temperature:
 Ectotherms (most animals aside from
mammals and birds) depend on external heat
sources to maintain body temperature.
 Endotherms (all mammals and birds) regulate
body temperature by generating metabolic
heat and/or preventing heat loss.
Figure 41.7 Ectotherms nd Endotherms (Part 1)
Figure 41.7 Ectotherms nd Endotherms (Part 2)
Figure 41.12 “Cold” and “Hot” Fish
Thermoregulation in Endotherms
• Endotherms respond to environmental
temperature change by changing rates of heat
production.
• Within a narrow range of temperatures, the
thermoneutral zone, the metabolic rate of
endotherms is low and independent of
temperature.
• The metabolic rate of a resting animal within the
thermoneutral zone is called the basal metabolic
rate (BMR).
• The BMR of an endotherm is about six times that
of an ectotherm of the same size and at the same
body temperature.
Thermoregulation in Endotherms
• Across all the endotherms, BMR per gram of
tissue increases as animals get smaller.
• The reason for this is unknown.
• It was once thought that larger animals evolved
lower metabolic rates to prevent overheating
because they have low surface area–volume
ratios.
• However, the relationship between metabolic rate
and body mass holds even for very small
organisms and for ectotherms, in which
overheating is not usually a problem.
Figure 41.13 The Mouse-to-Elephant Curve (Part 1)
Figure 41.13 The Mouse-to-Elephant Curve (Part 2)
Thermoregulation in Endotherms
• Most nonshivering heat production occurs in
specialized adipose tissue called brown fat.
• The tissue looks brown because of its abundant
mitochondria and rich blood supply.
• Brown fat cells have the protein thermogenin
which uncouples proton movement from ATP
production, so that no ATP is produced, but heat
is released.
• Brown fat is commonly found in newborn infants
and animals that hibernate.
Figure 41.15 Brown Fat
Thermoregulation in Endotherms
• The coldest environments are almost devoid of
ectotherm reptiles or amphibians.
• Endotherms have many adaptation for reducing
heat loss in cold environments:
 Reduction of surface-to-volume ratios of the
body by short appendages and round body
shapes
 Thermal insulation by thick layers of fur,
feathers, and fat.
 Decreasing blood flow to the skin by
constricting blood vessels, especially in
appendages
Figure 41.16 Adaptations to Hot and Cold Climates (Part 2)
Thermoregulation in Endotherms
• In any climate, getting rid of excess heat may also
be a problem, especially during exercise.
• Reduction or loss of fur or hair allows for easier
loss of heat from the body to the environment.
• Seeking contact with water cools the skin
because water absorbs heat to a greater capacity
than does air.
• Sweating or panting to increase evaporation
provides concomitant cooling (although this
benefit may be offset by water loss).
The Vertebrate Thermostat
• The regulatory system for body temperature in
vertebrates can be thought of as a thermostat.
• This regulator is at the bottom of the brain in a
structure called the hypothalamus.
• The temperature of the hypothalamus itself is the
major source of feedback information in many
species. Cooling it causes fish and reptiles to
seek a warmer environment, and warming it
triggers the reverse behavior.
Figure 41.17 The Hypothalamus Regulates Body Temperature
The Vertebrate Thermostat
• A fever is a rise in body temperature in response
to pyrogens.
• Exogenous pyrogens come from foreign
substances such as invading bacteria or viruses.
• Endogenous pyrogens are produced by cells of
the immune system when they are challenged.
• Pyrogens cause a rise in the hypothalamic set
point, and body temperature rises until it matches
the new set point.