Homeostasis (pg. 49 & 50)

• Homeostasis is the maintenance of constant internal environment.
• Water content - you need to keep a balance between the water you gain and the water you lose.
• Body temperature - you need to get rid of excess body heat when you're hot, but retain heat when the environment is cold.
• Water is taken into the body as food and drink and is lost in the following ways:

  1. through the skin as sweat.
  2. via the lungs in breath.
  3. via the kidneys as urine.
• On a hot day, or when you're exercising, you sweat a lot. You will produce less urine, but this will be more concentrated. You will also lose more water through your breath when you're exercising because you're breathing faster.
• On a cold day, or when you're not exercising, you don't sweat much. You'll produce more urine, which will be pale (since the waste carried in the urine is more diluted).
• The enzymes in the human body work best at around 37ºC.
• A part of the brain is sensitive to the blood temperature in the brain, and it receives messages from temperature receptors in the skin that provide information about skin temperature. Based on the signals from these receptors, your CNS can activate the necessary effectors to make sure your body temperature is adequate.
• When you're too hot:

  1. Lots of sweat is produced. When it evaporates it transfers energy from your skin to the environment, cooling you down.
  2. Blood vessels close to the surface of the skin widen: vasodilation. It allows more blood to flow near the surface, so it can transfer more energy into the surroundings.
  3. Hairs lie flat.
• When you're too cold:

  1. Very little sweat is produced.
  2. Blood vessels near the surface of the skin constrict: vasconstriction. This means less blood flows near the surface, so less energy is transferred to the surroundings.
  3. You shiver, which increases your rate of respiration, which transfers more energy to warm the body. Exercise does the same.
  4. Hairs stand on end to trap an insulating layer of air which helps keep you warm.
• Organisms with bigger surface area to volume ratios can gain or lose heat faster because there is more area for the heat to transfer across.
• Small organisms lose body heat more easily in hot climates and reduces the chance of them overheating. It also means that they're very vulnerable in cold environments.
• Animals living in cold conditions have a compact (rounded) shape to keep their surface area to a minimum, reducing heat loss.