Physiology, Homeostasis, and Temperature Regulation

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Physiology, Homeostasis, andTemperature
Regulation
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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

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HomeostasisMaintaining 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.

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HomeostasisMaintaining the Internal Environment

• Complex, multicellular organisms developed
  specialized cells that help maintain an internal
  environment.
• The internal environment consists of
  extracellular fluid that bathes every cell. Cells
  exchange materials with this environment.
• As multicellular organisms evolved, specialized
  cells formed specialized tissues and organs to
  control various aspects of the internal
  environment.
• Homeostasis is an essential feature of complex
  animals.

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Figure 41.1 Maintaining Internal Stability while
on the Go
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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.

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Figure 41.2 Four Types of Tissue
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Tissues, Organs, and Organ Systems

• Epithelial tissues are sheets of densely packed
  and tightly connected cells that cover inner and
  outer body surfaces.
• Some epithelial tissues have specialized
  functions
• Secretion of hormones, milk, mucus, digestive
  enzymes, sweat
• Some have cilia to move substances.
• Some epithelial cells are modified to be
  chemoreceptors for taste, smell, etc.
• Epithelial cells may have protective, absorptive,
  or transport functions.

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Figure 41.3 Epithelial Tissue
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Tissues, Organs, and Organ Systems

• Epithelial tissues have distinct inner and outer
  surfaces.
• The outer surfaces are the apical ends of the
  epithelial cells. They face the air (skin, lungs)
  or a fluid-filled organ cavity (the lumen of the
  gut).
• Apical ends may have cilia or be highly folded to
  increase surface area.
• The inner surfaces are the basil ends they rest
  on an extracellular matrix called a basal lamina.
• Some epithelial tissue, such as skin, gets much
  wear and tear, and thus has a high rate of cell
  division and replacement.

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Tissues, Organs, and Organ Systems

• Connective tissue consists of cells embedded in
  an extracellular matrix that they secrete.
• An important component of the extracellular
  matrix is protein fibers.
• The most common is collagen, a very strong fiber.
• Collagen is very dense in tough tendons and
  ligaments.
• It also forms a netlike framework for organs, to
  give shape and strength.
• It has low density as loose strands when it fills
  in between organs.

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Tissues, Organs, and Organ Systems

• Other protein fibers include elastin which can be
  stretched to several times its resting length and
  then recoil.
• Tissues that are regularly stretched, such as
  lung walls and artery walls, have abundant
  elastin.

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Tissues, Organs, and Organ Systems

• Cartilage and bone connective tissue provide
  rigid structural support.
• Cartilage is a network of collagen fibers
  embedded in a flexible matrix of proteins and
  carbohydrates. It is found in the external ears,
  nose, and trachea, and lines joints of
  vertebrates.
• The extracellular matrix of bone is hardened by
  the deposition of calcium phosphate.

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Tissues, Organs, and Organ Systems

• Adipose tissue is a connective tissue that forms
  and stores droplets of lipids.
• Adipose tissue serves as a fuel reserve and as a
  cushion to protect internal organs. Layers of
  adipose tissue under the skin help insulate
  against heat loss.
• Blood is also a connective tissue made up of
  cells in a fluid extracellular matrix called
  blood plasma.
• Plasma also contains an abundance of proteins.

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Tissues, Organs, and Organ Systems

• Muscle tissues are made of elongated cells
  capable of contracting and causing movement by a
  sliding of protein filaments past each other.
• They are the most abundant tissues in the body
  and use most of the energy the body produces.

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Tissues, Organs, and Organ Systems

• Nervous tissue is composed of neurons and glial
  cells.
• Neurons are extremely diverse in size and form.
  They function by generating electrochemical
  signals in the form of nerve impulses.
• These impulses are conducted via long extensions
  to other parts of the body where they communicate
  with other neurons, muscle cells, or secretory
  cells to control activities of organ systems.
• Glial cells provide a number of support functions
  for neurons.

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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.

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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.

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Figure 41.4 Control, Regulation, and Feedback
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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.

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Temperature and Life

• Living cells tolerate only a narrow range of
  temperature. Most cell function is limited to the
  range between 0C and 45C.
• Even within this range, temperature change may
  create problems for animals.
• Heat always moves from a warmer to a cooler
  object, so any environmental temperature change
  will cause change in the temperature of an
  organismunless the organism can regulate its
  temperature.

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Temperature and Life

• Most physiological processes are
  temperature-sensitive, 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 10C lower
  (RT-10).
• Q10 RT / RT-10

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Temperature and Life

• Most biological Q10 values are between 2 and 3,
  meaning that reaction rates double or triple as
  temperature increases by 10C.
• Since not all of the component reactions in an
  animal have the same Q10, temperature change can
  disrupt physiological functioning, throwing off
  the balance and integration that cell processes
  require.
• To maintain homeostasis, organisms must either
  compensate for or prevent temperature change.

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Figure 41.5 Q10 and Reaction Rate
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Temperature and Life

• The body temperature of some animals is closely
  coupled to environmental temperature.
• The animal may adjust its metabolic rate (total
  cell energy turnover, often measured by O2
  consumption) for different seasonal temperatures.
• This process is called acclimatization, or
  metabolic compensation, which is a biochemical
  adjustment of enzyme systems to counter the
  effects of temperature.
• The result is metabolic function that is much
  less sensitive to long-term temperature change
  than to short-term thermal fluctuations.

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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.

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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.

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Maintaining Optimal Body Temperature

• If a lizard (an ectotherm) and a mouse (an
  endotherm) are placed in a closed chamber in
  which the temperature is gradually raised, the
  body temperature of the lizard will equilibriate
  with that of the chamber, whereas the body
  temperature of the mouse will remain constant.
• The metabolic rates also respond differently. In
  the ectotherm, metabolism decreases as air
  temperature decreases.
• In the endotherm, metabolic rate increases as
  temperature decreases, which increases production
  of body heat.

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Figure 41.7 Ectotherms nd Endotherms (Part 1)
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Figure 41.7 Ectotherms nd Endotherms (Part 2)
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Maintaining Optimal Body Temperature

• Ectotherms such as the lizard can use behavior to
  regulate body temperature in the natural
  environment.
• Behaviors include basking in the sun, seeking
  shade, burrowing, or orienting the body with
  respect to the sun.
• Endotherms also use behavioral thermoregulation.
  Most animals select the best thermal environment
  whenever possible, for example by seeking shade,
  breezes, etc.

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Figure 41.8 An Ectotherm Uses Behavior to
Regulate Its Body Temperature
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Figure 41.9 Endotherms Use Behavior to
Thermoregulate
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Maintaining Optimal Body Temperature

• If the body temperature of an animal is to remain
  constant, the heat entering the animal must equal
  the heat leaving the animal. This can be
  expressed as an energy budget.
• Heatin Heatout
• Heatin metabolism solar radiation (Rabs)
• Heatout radiation (Rout) convection
  conduction
• evaporation
• If heat is entering the body through convection
  and/or conduction, the sign of those factors
  changes to negative.

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Figure 41.10 Animals Exchange Heat with the
Environment
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Maintaining Optimal Body Temperature

• Any adaptation that influences the ability of an
  animal to deal with its thermal environment must
  affect one or more components of the budget.
• All of the components on the right (heat-loss)
  side of the equation depend on the surface
  temperature of the animal, which can be
  controlled by altering the blood flow to the skin.

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Maintaining Optimal Body Temperature

• Heat exchange between the internal environment
  and the skin occurs largely through blood flow.
• When blood is close to the surface of the skin,
  heat energy carried by the blood is lost to the
  environment by the four mechanisms listed above.
• When a person is exposed to cold, blood vessels
  of the skin constrict, decreasing blood flow and
  heat transport to the skin and reducing heat
  loss.
• Some ectotherms, such as the marine iguana,
  control blood flow to the skin as an adaptation
  for survival in cold water and hot sun.

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Figure 41.11 Some Ectotherms Regulate Blood Flow
to the Skin (Part 1)
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Figure 41.11 Some Ectotherms Regulate Blood Flow
to the Skin (Part 2)
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Maintaining Optimal Body Temperature

• Some ectotherms raise their body temperature by
  producing heat.
• The flight muscles of insects must be warmed to
  3540C before flight can occur. This is achieved
  by flight muscle contractions, which generate
  heat in a manner similar to shivering in mammals.
  
• Honeybees regulate temperature in a hive by group
  clustering to produce metabolic heat so the brood
  temperature stays at about 34C even as
  temperatures outside of the hive drop well below
  freezing.

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Maintaining Optimal Body Temperature

• In most fish, blood passing through the gills
  comes in close contact with water, so the
  temperature of the blood tends to be about the
  same temperature as the water.
• Some large fish, such as bluefin tuna and great
  white shark, can raise body temperature 1015C
  above the water temperature.
• In the large swimming muscles, heat is exchanged
  through a countercurrent heat exchanger, a
  structural plan that allows cool blood returning
  from the gills to be warmed by warm blood from
  the muscles.

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Figure 41.12 Cold and Hot Fish
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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.

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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 areavolume 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.

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Figure 41.13 The Mouse-to-Elephant Curve (Part 1)
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Figure 41.13 The Mouse-to-Elephant Curve (Part 2)
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Thermoregulation in Endotherms

• The thermoneutral zone is bounded by a lower
  critical and upper critical temperature.
• When environmental temperature falls below the
  lower critical temperature, mammals
  thermoregulate by generating heat (thermogenesis)
  through shivering and nonshivering heat
  production.
• Birds use only the shivering mechanism.
• In shivering, skeletal muscles use ATP to release
  only heat. Active body movement also generates
  heat.

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Figure 41.14 Environmental Temperature and
Mammalian Metabolic Rates
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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.

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Figure 41.15 Brown Fat
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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

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Figure 41.16 Adaptations to Hot and Cold
Climates (Part 1)
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Figure 41.16 Adaptations to Hot and Cold
Climates (Part 2)
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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).

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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.

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The Vertebrate Thermostat

• In endotherms, cooling the hypothalamus causes
  the body temperature to rise.
• Warming the hypothalamus causes dilation of blood
  vessels in the skin and/or sweating or panting in
  attempts to lower body temperature.

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Figure 41.17 The Hypothalamus Regulates Body
Temperature
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The Vertebrate Thermostat

• The hypothalamus generates a set point like a
  setting on a thermostat. Hypothalamic temperature
  is a negative feedback system.
• Vertebrates also integrate other sources of data,
  such as information from temperature sensors in
  the skin.
• Mammals also have the ability to shift the
  hypothalamic set points.
• The temperature of the skin can be considered
  feedforward information that adjusts the
  hypothalamic set point.
• Set points are also higher during wakefulness and
  the active part of the daily cycle.

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Figure 41.18 Adjustable Set Points
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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.

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The Vertebrate Thermostat

• Immune system cells called macrophages attack
  pyrogens and release interleukins, chemicals that
  signal other cells and trigger other responses,
  including release of prostaglandins.
• Interleukins also raise the hypothalamic set
  point.
• Aspirin is an inhibitor of prostaglandin
  synthesis, so it lowers the set point and makes
  the body more comfortable.
• Evidence suggests that moderate fevers help the
  body fight infections, but extreme fevers can be
  dangerous.

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The Vertebrate Thermostat

• Animals can save energy by turning down the
  thermostat to below normal (hypothermia).
• Many animals use regulated hypothermia as a means
  of surviving periods of cold and food scarcity.
• An adaptive hypothermia called daily torpor can
  drop body temperature 1020C and save
  considerable metabolic energy.
• Regulated hypothermia lasting days or weeks with
  drops to very low temperatures is called
  hibernation. The reduction in metabolic rate
  results in enormous energy savings.

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Figure 41.19 A Ground Squirrel Enters Repeated
Bouts of Hibernation during Winter
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The Vertebrate Thermostat

• Arousal from hibernation occurs when the
  hypothalamic set point returns to normal.
• Many species of mammals hibernate, but only one
  bird, the poorwill, has been found to do so.
• This drastic decrease of the set point probably
  came about as an evolutionary extension of the
  set point decrease that accompanies sleep in
  nonhibernators.