Chapter 31

• 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

• Homeostatic mechanisms regulate a variety of conditions in the fluids surrounding cells
– Temperature
– Water and salt levels
– Glucose
– pH
– Oxygen and carbon dioxide

• Animals are grouped according to source of body warmth
– Endotherms
– Ectotherms
• Endotherms generate body heat through metabolic reactions
• Maintain constant body temperature
• Birds, mammals

• Ectotherms derive body heat from environment
• Maintain heat by occupying a constant environment or behaviorally, e.g. basking in sun
• Reptiles, amphibians, fishes, invertebrates

• 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

• 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

• 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 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 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

• 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

Cells  Tissues  Organs  Organ Systems

• 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

• 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

• 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 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

• Attaches to epithelium to form membranes
• Contains protein fibers and syrup-like extracellular fluid
• Surrounds, cushions, and supports most organs
– Example: the dermis

• Contains densely packed collagen fibers that provide strength
• Examples
– Tendons : connect bones to muscles
– Ligaments : connect bones to bones

• 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

• 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 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 are formed from at least two types of tissues that work together
• Example: the skin

• 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