Chapter 9: Internal Regulations

• Homeostasis


  Refers to temperature regulation and other biological processes that keep body variables within a fixed range
• Homeostatic processes in animals

  • Trigger physiological and behavioral activities that keep certain variables within a set range
  • The range is so narrow that we refer to it as a set point, a single value that the body works to maintain
• Negative feedback


  Processes that reduce discrepancies from the set point
• The concept of homeostasis is not fully satisfactory, because the body does not maintain complete constancy
• Changes in the body

  • Body temperature is about half a Celsius degree higher in mid-afternoon than in the middle of the night
  • Most animals maintain a nearly constant body weight from day to day, but add body fat in fall and decrease it in spring
• Allostasis


  The adaptive way in which the body anticipates needs depending on the situation, avoiding errors rather than just correcting them
• Basal metabolism


  The energy used to maintain a constant body temperature while at rest
• Ectothermic animals

  • Amphibians, reptiles, and most fish depend on external sources for body heat instead of generating it themselves
  • Their body temperature is nearly the same as the temperature of their environment
  • They lack physiological mechanisms of temperature regulation such as shivering and sweating, but they can regulate their body temperature behaviorally
• Endothermic animals


  • Mammals and birds generate enough body heat to remain significantly above the temperature of the environment
  • They use physiological mechanisms to keep their core temperature nearly constant
  • Endothermy is costly, especially for small animals
• Cooling the body when air is warmer than body temperature

  1. Evaporation

  2. Licking themselves and panting

  3. If air is humid, moisture does not evaporate

  4. Sweating without drinking leads to dehydration
• Increasing body heat in a cold environment

  1. Shivering

  2. Decreased blood flow to the skin

  3. Fluffing out fur to increase insulation
• Behavioral mechanisms are better than physiological mechanisms for regulating temperature
• Ectothermic animals are vulnerable to freezing if their body temperature drops below the freezing point of water
• Mechanisms used by ectothermic animals to survive winters

  • Burrowing or finding sheltered locations
  • Fish and insects stocking their blood with glycerol and other antifreeze chemicals
  • Wood frogs withdrawing most fluid from their organs and blood vessels and storing it in extracellular spaces, having chemicals that cause ice crystals to form gradually, and having extraordinary blood-clotting capacity
• Advantages of constant high body temperature in endothermic animals

  • Enables rapid movement without excessive fatigue even in a cold environment
  • Warmer muscles allow faster running with less fatigue
• Maintaining a higher body temperature requires more fuel and energy, and beyond about 41°C (105°F), proteins begin to break their bonds and lose their useful properties
• Our body temperature of 37°C is a trade-off between the advantages of high temperature for rapid movement and the disadvantages of high temperature for protein stability and energy expenditure
• Preoptic area/anterior hypothalamus (POA/AH)


  The brain area that controls the physiological mechanisms of temperature regulation such as shivering, sweating, changes in heart rate and metabolism, and changes in blood flow to the skin
• How the POA/AH regulates temperature

  1. Receives input from temperature receptors in the skin, organs, and hypothalamus

  2. Reacts most vigorously if the skin and hypothalamus are both hot or both cold

  3. Receives input from the immune system, causing shivering, increased metabolism, and other processes that produce a fever
• After damage to the POA/AH, mammals can still regulate body temperature, but less efficiently
• Fever


  An increased set point for body temperature, directed by the hypothalamus, not something an infection does to the body
• Certain types of bacteria grow less vigorously at high temperatures than at normal mammalian body temperatures, and the immune system works more vigorously at an increased temperature
• A fever above about 39°C (103°F) in humans does more harm than good, and a fever above 41°C (109°F) is life-threatening
• Thirst


  The mechanism of water regulation in different species
• Water regulation strategies in different species

  • Beavers and other animals that live in rivers or lakes drink plenty of water, eat moist foods, and excrete dilute urine
  • Most gerbils and other desert animals go through life without drinking at all, gaining water from their food and having adaptations to avoid losing water
• Temperature of 37C


  A compromise between competing considerations
• Preoptic area and anterior hypothalamus (POA/AH)


  • Critical for temperature control
  • Cells there monitor their own temperature and that of the skin and organs
  • When they receive input indicating an infection, they initiate responses that produce a fever
• All animals rely partly on behavioral mechanisms for temperature regulation
• A moderate fever helps an animal combat an infection
• Thirst


  Mechanism of water regulation
• Strategies for maintaining water

  • Beavers and other animals that live in rivers or lakes drink plenty of water, eat moist foods, and excrete dilute urine
  • Most gerbils and other desert animals go through life without drinking at all, gain water from their food, have adaptations to avoid losing water, unable to sweat, avoid the heat of the day by burrowing under the ground, highly convoluted nasal passages minimize water loss when they exhale
• Humans vary their strategy depending on circumstances
• Conserving water

  1. Excreting more concentrated urine

  2. Decreasing sweat
• Vasopressin


  Hormone released by the pituitary gland that raises blood pressure by constricting blood vessels<|>Also known as antidiuretic hormone (ADH) because it enables the kidneys to reabsorb water from urine and make the urine more concentrated
• Humans drink more than they need and excrete the excess
• Osmotic thirst


  Result of eating salty foods<|>Caused by higher concentration of solutes outside cells than inside, drawing water from cells into extracellular fluid
• Osmotic pressure


  Tendency of water to flow across a semipermeable membrane from the area of low solute concentration to the area of higher concentration
• Osmotic thirst

  • Certain neurons detect their own loss of water and trigger osmotic thirst
  • Kidneys excrete more concentrated urine to rid the body of excess sodium and maintain water
  • Brain has receptors around the third ventricle that detect osmotic pressure and sodium content of blood
  • OVLT receives input from receptors in digestive tract to anticipate osmotic need
• Subfornical organ

  • Has one population of neurons that increases thirst and another that suppresses it
  • Axons combine with input from OVLT, stomach, and elsewhere to provide input to hypothalamus
• Hypovolemic thirst


  Thirst caused by losing fluid by bleeding or sweating<|>Triggered by the hormone angiotensin II, which increases when blood pressure falls