TR

Homeostasis

Introduction

  • Size and shape affect the way an animal interacts with its environment
  • Rate of exchange is proportional to a cell’s surface area while amount of exchange material is proportional to a cell’s volume
  • More complex organisms have highly folded internal surfaces for exchanging materials
  • In vertebrates, the space between cells is filled with interstitial fluid, which allows for the movement of material into and out of cells

Organization of Animal Bodies

  • All animal cells share similarities in the ways in which they
    • Exchange materials with their surroundings
    • Obtain energy from organic nutrients
    • Synthesize complex molecules
    • Reproduce themselves
    • Detect and respond to signals in their immediate environment

Tissues are classified into four main categories: epithelial, connective, muscle, and nervous

Epithelial Tissue

  • Epithelial tissue: covers the outside of the body and lines the organs and cavities within the body
    • It contains cells that are closely joined
  • The shape of epithelial cells may be
    • cuboidal (like dice)
    • columnar (like bricks on end)
    • squamous (like floor tiles)
  • The arrangement of epithelial cells may be
    • simple (single cell layer)
    • stratified (multiple tiers of cells)
    • pseudostratified (a single layer of cells of varying length)

Connective Tissue

  • Connect, anchor, and support
  • Includes blood, adipose, bone, cartilage, loose and dense connective tissue
  • Form extracellular matrix around cells
    • provides scaffold for attachment
    • Protects and cushions
    • mechanical strength
    • Transmit information

Nervous Tissue

  • Nervous tissue: initiate and conduct electrical signals from one part of the animal’s body to another
  • Neuron: single nerve cell
  • Electrical signals produced in a nerve cell may stimulate or inhibit other cells to
    • Initiate new action potentials in other neurons
    • Stimulate muscle to contract
    • Stimulate glands to release chemicals

Vertebrates

  • In vertebrates, the fibers and foundation combine to form six major types of connective tissue:
    • Loose connective tissue: binds epithelia to underlying tissues and holds organs in place
    • Cartilage: a strong and flexible support material
    • Fibrous connective tissue: is found in tendons and ligaments
    • Tendons: attach muscles to bones
    • Ligaments: connect bones at joints
    • Adipose tissue: stores fat for insulation and fuel
    • Blood: composed of blood cells and cell fragments in blood plasma
    • Bone: mineralized and forms the skeleton

Coordination and Control

  • Control and coordination within a body depend on the endocrine system and the nervous system
    • Endocrine system: transmits chemical signals called hormones to receptive cells throughout the body via blood
    • A hormone may affect one or more regions throughout the body
    • Hormones are relatively slow acting, but can have long-lasting effects
    • Nervous system: transmits information between specific locations
    • The information conveyed depends on a signal’s pathway, not the type of signal
    • Nerve signal transmission is very fast
    • Nerve impulses can be received by neurons, muscle cells, endocrine cells, and exocrine cells

Feedback Control

  • Feedback control: maintains the internal environment in many animals
  • Animals manage their internal environment by regulating or conforming to the external environment
    • Regulator: uses internal control mechanisms to moderate internal change in the face of external, environmental fluctuation
    • Conformer: allows its internal condition to vary with certain external changes
    • Animals may regulate some environmental variables while conforming to others

Homeostatic Control Systems

  • Set point: normal value for controlled variable

  • Sensor: monitors particular variable

  • Integrator: compares signals from the sensor to set point

  • Effector: compensates for deviations between actual value and set point

  • Example: body temperature in mammals

  • Organisms use homeostasis to maintain a 

    “steady state” or internal balance regardless 

    of external environment

  • Acclimatization: homeostasis can adjust to changes in external environment

  • Thermoregulation: adaptation to the thermal environment

  • Osmoregulation: adaptation to the osmotic environment

  • Excretion: strategies for the elimination of waste products of protein catabolism

Thermoregulation

  • Homeostatic processes for thermoregulation involve form, function, and behavior
  • Thermoregulation: the process by which animals maintain an internal temperature within a tolerable range
    • Endothermic animals: generate heat by metabolism; birds and mammals are endotherms
    • Ectothermic animals: gain heat from external sources ectotherms include most invertebrates, fishes, amphibians, and nonavian reptiles
    • In general, ectotherms tolerate greater variation in internal temperature, while endotherms are active at a greater range of external temperatures
    • Endothermy is more energetically expensive than ectothermy
  • Heat regulation in mammals often involves the integumentary system: skin, hair, and nails 
  • Five adaptations help animals thermoregulate:
    • insulation = skin, fur, feathers, blubber
    • circulatory adaptations= vasodilation and vasoconstriction
    • cooling by evaporative heat loss
    • behavioral responses
    • adjusting metabolic heat production

Behavioral Responses

  • In winter, many animals bask in the sun or on warm rocks.
  • In summer, many animals burrow or move to damp areas.
  • Some animals migrate to more suitable climates.

Physiological Thermostats and Fever

  • Thermoregulation is controlled by a region of the brain called the hypothalamus
  • The hypothalamus triggers heat loss or heat generating mechanisms
  • Fever is the result of a change to the set point for a biological thermostat

Negative Feedback

  • Negative feedback: the variable being regulated brings about responses that move the variable in the opposite direction
  • ex: Decrease in body temperature leads to responses that increase body temperature
  • May occur at cellular or molecular level
  • Also prevents homeostatic responses from overcompensating

Positive Feedback

  • Positive feedback: reinforces the direction of change
  • Far less common
  • Accelerates a process
    • Explosive system
  • ex: Birth in mammals

Feedforward Regulation

  • Feedforward regulation: animal’s body begins preparing for a change in some variable before it occurs
  • Anticipatory
  • Speeds up homeostatic responses and minimizes deviations from the set point
  • Many result from or are modified by learning