chapter 15

• homeostasis


  maintenance of a stable equilibrium in the conditions inside the body
• receptors and effectors are vital for the body to maintain the body's dynamic equilibrium
• factors in the internal environment that concern homeostasis:

  1. pH of blood
  2. core body temperature
  3. concentrations of urea
  4. sodium ions in the blood
• effectors


  muscles or glands that react to the motor stimulus to bring about a change in response to a stimulus
• feedback systems


  homeostasis depends on sensory receptors detecting small changes, effectors and receptors depend on feedback systems that enable the maintenance of a relatively steady state around a narrow range of conditions
• negative feedback systems


  work to reverse the initial stimulus, important in controlling temperature control and water balance of the body

  
• positive feedback systems


  a change is detected by the receptors and the effectors are stimulated to reinforce that change and increase the response, e.g in the blood clotting cascade and in childbirth
• temperature changes in organisms depend on:

  • exothermic chemical reactions
  • latent heat of evaporation = objects cool down as water evaporates
  • radiation = transmission of EM waves to and from air, water or ground
  • convection = heating and cooling by currents of air or water
  • conduction = heating due to collisions of molecules
• ectotherms


  animals that use their surroundings to warm their bodies so their core temperature is heavily dependent on the environment
• examples of ectotherms


  all invertebrate animals as well as fish, amphibians and reptiles
• ectotherms living in water do not need to thermoregulate as the high heat capacity of water means temperature does not change much, but land living ectotherms struggle more with thermoregulation as temperature of air can vary dramatically throughout the seasons and during 24 hours
• endotherms


  animals that rely on their metabolic processes to warm their bodies and maintain their core temperature
• examples of endotherms


  mammals and birds
• endotherms can survive in a wide range of environments as keeping warm in cold conditions and cooling down in hot conditions are both active processes
• metabolic rate of endotherms is 5 times higher than ectotherms so they need to consume more food to meet their metabolic needs than ectotherms of a similar size
• behavioural responses of ectotherms for thermoregulation

  to warm up they may bask in the sun and orientate their bodies so that maximum surface area is exposed to the Sun, and extend their bodies to increase the surface area, e.g lizards bask for long periods of time to move fast and hunt prey
• ectotherms can also increase body temperature by conduction by pressing their bodies against the warm ground, so they get warmer due to exothermic metabolic reactions, e.g galapagos iguanas contract muscles and vibrate to increase cellular metabolism to raise body temperature
• ectotherms cooling down with behavioural responses


  they shelter from the sun by seeking shade, hiding in the cracks in rocks or hiding in burrows, and they press their bodies against cool, shady stones or move into available water or mud, they orientate their body so that the minimum surface area is exposed to the sun
• ectotherm physiological responses to warming

  dark colours absorb more radiation than light colours, e.g lizards living in colder climates tend to be darker coloured than lizards living in hotter countries so that they get warmer
• ectotherms can also alter their heart rate to increase or decrease the metabolic rate and affect the warming or cooling across the body surfaces
• ectotherms are more valuable to fluctuations in the environment than endotherms, however by using both behavioural and physiological strategies many of them can maintain a stable core temperature, so they will need less food as they can survive in more difficult habitats
• peripheral temperature receptors are in the skin and detect changes in surface temperature
• temperature receptors in the hypothalamus detect the temperature of the blood deep in the body
• temperature of the skin is more likely to be affected by external conditions than the temperature of the hypothalamus
• combination of the peripheral and temperature receptors give the body a greater sensitivity and allow it to respond not only to actual changes in blood temperature but to predict possible problems that could occur from changes in the external environment
• temperature receptors in the hypothalamus act as the thermostat of the body, controlling the responses that maintain the core temperature in a dynamic equilibrium from 1 to 37 degrees
• endotherms use internal exothermic metabolic activities to keep them warm, and energy requiring physiological responses to help them cool down
• endotherms also have passive ways of heating up and cooling down to reduce the energy demands on the body
• like ectotherms, endotherms also bask in the sun and press themselves against hot surfaces to warm up, and go into water and mud and hide in burrows to cool down
• endotherms become dormant through the coldest weathers of the year = hibernation
• some endotherms become dormant through the warmest temperatures of the year to avoid heat stress = aestivation
• even though endotherms have behavioural responses, they mainly rely on physiological adaptations to maintain a stable core temperature regardless of the environmental conditions or the amount of exercise being done
• physiological adaptations of endotherms

  1. peripheral temperature receptors
  2. thermoregulatory centres of the hypothalamus
  3. skin
  4. muscles
• cooling down in endotherms = vasodilation


  arterioles near the surface of the skin dilate when the temperature rises, and the arteriovenous shunt vessels constrict, this forces blood through the capillary networks close to the skin surface so the skin flushes and cools due to radiation, and if skin is pressed against a cool surface then cooling occurs through conduction
• cooling down in endotherms = increased sweating

  as sweat evaporates from the surface of the skin, heat is lost which cools down the blood below the surface, in humans around 1 decimetre cubed of sweat is lost by evaporation in a day, and in hot and hard conditions they can lose up to 12
• in animals that have restricted hair glands due to fur or hair open their mouths and pant when they get hot, losing heat as the water evaporates
• cooling down in endotherms = reducing the insulating effect of hair or feathers


  as temperature rises, erector pili muscles in the skin relax so hair or feathers of the animal lie flat to the skin, which avoids trapping an insulating layer of air, with little effect in humans
• endotherms in hot climates have anatomical adaptations as well as their behavioural and physiological adaptations to minimise the effect of heat such as a large SA:V to maximise cooling
• warming up in endotherms = vasoconstriction


  arterioles near the surface of the skin constrict, arteriovenous shunt vessels dilate so very little blood flows through the capillary networks close to the skin surface, the skin looks pale and very little radiation takes place as blood is kept well below the surface
• warming up in endotherms = decreased sweating


  as core temperature falls, rate of sweat production will stop completely and this reduces cooling by evaporation of water from the skin surface although some evaporation from the lungs still continues