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Homeostasis and the
Organization of the Animal Body
Chapter 31
Dynamic Constancy
• The cells of the animal body cannot survive if
the internal environment deviates from a narrow
range of acceptable states
• Homeostasis refers to the constancy of the
body’s internal environment
• The internal environment is actually in a state of
dynamic constancy
• The body actively adjusts to ongoing internal
and external changes to maintain constant
conditions
Dynamic Constancy
• Homeostatic mechanisms regulate a
variety of conditions in the fluids
surrounding cells
– Temperature
– Water and salt levels
– Glucose
– pH
– Oxygen and carbon dioxide
Body Temperature Regulation
• Animals are grouped according to source
of body warmth
– Endotherms
– Ectotherms
• Endotherms generate body heat through
metabolic reactions
• Maintain constant body temperature
• Birds, mammals
Body Temperature Regulation
• Ectotherms derive body heat from
environment
• Maintain heat by occupying a constant
environment or behaviorally, e.g. basking
in sun
• Reptiles, amphibians, fishes, invertebrates
Feedback Systems
• Homeostatic mechanisms that maintain
internal constancy are collectively known
as feedback systems
• There are three parts to feedback
systems
– Control center (with set point)
– Sensor
– Effector
Feedback Systems
• There are two types of feedback systems
– Negative feedback: more common;
counteracts the effects of changes in the
internal environment to maintain homeostasis
– Positive feedback: rarer; drive rapid, selflimiting changes, e.g. the birth process
Negative Feedback
• In negative feedback the response to
change is to counteract the change
resulting in a return to the original
condition
– “Negative” because it negates the initial
change
Negative Feedback
• Negative feedback maintains a set point
by detecting a deviation from the set point
(stimulus) using a receptor
• The receptor signals a control center that
activates an effector mechanism that
counteracts the stimulus
Positive Feedback
• Positive feedback intensifies the original
change
– Tends to be self-limiting
– Occurs during labor
• Labor contractions force baby’s head against
the cervix causing, causing it to dilate
• Stretch-receptor neurons in the cervix signal
the hypothalamus
• Hypothalamus releases oxytocin, which
stimulates stronger uterine contractions
• Delivery relieves pressure on the cervix,
halting the + feedback cycle
Systems Act in Concert
• Homeostasis is maintained by body
systems working together as a team
• Numerous mechanisms constantly
respond to various (chemical) stimuli that
change as a result of an animal’s activities
and changes in environment
Hierarchy of Animal Organization
Cells  Tissues  Organs  Organ Systems
Animal Tissues
• An animal tissue is composed of cells that
are similar in structure and perform a
specialized function
• There are four major categories of animal
tissues
– Epithelial tissue
– Connective tissue
– Muscle tissue
– Nervous tissue
Epithelial Tissue
• Continuous sheets (membranes) that
cover the outside body and line its cavities
– Adapted to various functions, e.g. protection,
absorption, gas exchange, etc…
– Continually lost and replaced by cell division
• Some epithelial tissues form glands
(clusters of cells that are specialized to
release substances)
– Exocrine glands
– Endocrine glands
Epithelial Tissue
• Exocrine glands release secretions using
ducts
– Examples: sweat glands and sebaceous glands
• Endocrine glands typically release
secretions into blood
– Examples: adrenal glands and thyroid gland
Connective Tissue
• Connective tissues exist mostly to support
and bind other body tissues
• They secrete large quantities of extracellular
substances, often including tough collagen
protein fibers, among living cells
• Three main categories of connective tissue
– Loose connective tissue
– Fibrous connective tissue
– Specialized connective tissue
Loose Connective Tissue
• Attaches to epithelium to form membranes
• Contains protein fibers and syrup-like
extracellular fluid
• Surrounds, cushions, and supports most
organs
– Example: the dermis
Fibrous Connective Tissue
• Contains densely packed collagen fibers
that provide strength
• Examples
– Tendons: connect bones to muscles
– Ligaments: connect bones to bones
Specialized Connective Tissue
• Have diverse functions and structures
• Examples
– Cartilage - function in support, friction-reduction, and
shock absorption
– Bone
– Fat (adipose tissue) - used for energy storage and
insulation
– Blood - tissue composed of cells suspended in
extracellular fluid
– Lymph - Composed of fluid leaked out of blood at
capillary blood vessels
Muscle Tissue
• Contract (shorten) when stimulated
• Three types
–
–
–
–
–
Cardiac muscle - Located in heart
Spontaneously active and not under conscious control
Smooth muscle - Located in tubular organs
Produces slow, sustained, involuntary contractions
Skeletal muscle - Produces voluntary contractions,
usually to move skeleton
– Cells are cylindrical muscle fibers
Nerve Tissue
• Nerve tissue enables animals to sense
and respond to the world
• Makes up the brain, spinal cord, and
nerves that travel to all parts of the body
• Composed of two cell types
– Neurons, that transmit electric signals
– Glial cells, that surround, support, and
electrically insulate neurons
Organs
• Organs are formed from at least two types
of tissues that work together
• Example: the skin
Organ Systems
• Organ systems are two or more individual
organs that work together, performing a
common function
• Human organ systems are represented in
Table 31-1, p. 645