Homeostasis

• Homeostasis


  The tendency to resist change in order to maintain a stable, relatively constant internal environment
• Homeostasis


  • Typically involves negative feedback loops that counteract changes of various properties from their target values, known as set points
  • In contrast to negative feedback loops, positive feedback loops amplify their initiating stimuli, in other words, they move the system away from its starting state
• The tendency to maintain a stable, relatively constant internal environment is called homeostasis
• The body maintains homeostasis for many factors in addition to temperature, such as the concentration of various ions in the blood, pH, and the concentration of glucose
• Homeostasis is maintained at many levels, not just the level of the whole body
• Maintaining homeostasis

  1. Biological systems are constantly being pushed away from their balance points

  2. Homeostasis depends on the ability of the body to detect and oppose these changes

  3. Maintenance of homeostasis usually involves negative feedback loops that act to oppose the stimulus that triggers them
• Negative feedback loop


  A loop that acts to oppose the stimulus, or cue, that triggers it
• Negative feedback loop for body temperature regulation

  • Sensors (nerve cells) detect high temperature and relay it to the temperature-regulatory control center in the brain
  • The control center activates effectors (sweat glands) to bring body temperature down

  
• Homeostatic circuits usually involve at least two negative feedback loops: one activated when a parameter is above the set point to bring it back down, and one activated when the parameter is below the set point to bring it back up
• Homeostatic responses in temperature regulation

  1. When body temperature is too high, blood flow to skin increases, sweat glands secrete fluid, and heavy breathing increases heat loss

  2. When body temperature is too low, blood vessels constrict, sweat glands don't produce sweat, and shivering generates heat to warm the body

  
• The set point for body temperature is not always rigidly fixed and may be a moving target, such as varying over a 24-hour period or increasing during fever
• Disruptions to feedback mechanisms can disrupt homeostasis and lead to disease, as seen in diabetes where a broken feedback loop makes it difficult to regulate blood sugar levels

  
• Insulin


  A hormone that decreases the concentration of glucose in the blood by stimulating cells to take up glucose and store it as glycogen

  
• Glucagon


  A hormone that increases the concentration of glucose in the blood by stimulating the liver to break down glycogen and release glucose

  
• Diabetes happens when the pancreas can't make enough insulin or cells stop responding to insulin, leading to high blood sugar levels

  
• Positive feedback loops


  • Unlike negative feedback loops, positive feedback loops amplify the starting signal and are usually found in processes that need to be pushed to completion, not when the status quo needs to be maintained
  • An example is the positive feedback loop that drives childbirth, where the baby's head pressing on the cervix triggers the release of oxytocin, which increases contractions and further stimulates oxytocin release

  
• Endotherms


  • Maintain body temperature through internal metabolic activity
  • Mammals and birds

  
• Ectotherms


  • Rely on external factors, such as basking in the sunlight or retreating to the shade, in maintaining ideal body temperature
  • Amphibians, reptiles, and fish
• All animals' internal temperatures fall within a few degrees of one another
• Surface-to-volume ratio

  Influences the maintenance of body temperature
• Larger body volume


  More heat is produced by metabolic activity
• Smaller animals


  More surface area relative to the overall body volume
• Animals in cold climates


  • Compact, stocky bodies and short legs, tails, and ears
• Animals in warm climates

  • Slim bodies and long appendages
• Ectotherms are more dependent on behavioral devices for temperature regulation and do not share the endotherms' ability to use internal mechanisms

  
• Behavioral strategies for regulating temperature

  1. Move to a different environment

  2. Adjust body position

  3. Huddle together

  4. Change weight, color, and composition of fur/clothing

  
• Endothermic responses to heat and cold

  1. Shivering

  2. Blood vessel constriction

  3. Increased thyroid hormone release

  4. Increased metabolic activity in brown fat cells
• Core temperature


  The temperature maintained for vital organs within the head and torso

  
• Disturbances in the body's ability to maintain the average core temperature point can result in hot flashes experienced by nearly 80 percent of women in months or years surrounding menopause
• Spinal cord


  Does not respond to heat or cold until an animal's core temperature is as much as two to three degrees away from the set point
• Hypothalamus


  Initiates compensation whenever core temperature deviates as little as 0.01 degrees from the ideal set point
• Solutes


  Molecules dissolved in a fluid
• Electrolytes


  Solutes that break into ions and dissolve, such as sodium, calcium, potassium, chloride, magnesium, and bicarbonate
• Intracellular fluid


  About two-thirds of the body's water found within cells
• Extracellular fluid


  The remaining one-third of the body's water, including the blood supply and the interstitial fluid surrounding the body's cells
• Extracellular fluid has higher concentrations of sodium and chloride, and intracellular fluid has higher potassium concentrations
• Isotonic


  Two solutions with equal concentrations of solutes
• Nephrons

  The complex system in the kidneys that filters blood and removes impurities and excess water and sodium
• Water loss from the body

  1. Urination

  2. Breathing

  3. Perspiration

  4. Evaporation through the skin

  5. Defecation
• Osmotic thirst


  Thirst that occurs in response to cellular dehydration and drops in the intracellular fluid volume