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

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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

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