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B2.4 How do our bodies keep a healthy water balance?

B2.4 How do our bodies keep a healthy water balance?

HOMEOSTATIS is the maintenance of a constant internal environment – It is achieved by balancing bodily inputs and outputs, using the NERVOUS SYSTEM and HORMONES to control the process. Examples if things that the body keeps the same are:

  • Body temperature at 37˚C
  • The amount of water inside our body

Automatic control systems throughout the body maintain a range of factors at steady levels and that this is required for cells to function properly.

For homeostasis to work, these control systems need to have:

  • RECEPTORS to detect changes in the environment
  • PROCESSING CENTRES to receive information and coordinate responses automatically
  • EFFECTORS to produce the response

NEGATIVE FEEDBACK ensures that, in any control system, changes are reversed and returned back to the set level. For EXAMPLE when receptors detect that the temperature in the body has increased above a certain level, the processing centre (brain) sends signals to the effectors (in this case sweat glands) to produce sweat to cool down the body. This process is where the steady state of the body is adjusted to reverse the change.

For the cells of our body to work properly, it is important that their water contained is maintained at the correct level. This means our body must maintain a balance between the water we take in and water we lose – Water is INPUT (gained) from drinks, foods and respiration and it is OUTPUT (lost) through sweating, breathing, faeces and the excretion of urine.

The KIDNEYS play a vital role in balancing levels of water, waste and other chemicals in the blood.

The brain monitors water content constantly and causes the kidney to adjust the concentration and volume of urine produced-

  • When the water level of our BLOOD PLASMA is LOW, more water is reabsorbed back into the blood and the urine becomes more CONCENTRATED
  • When the water level of our BLOOD PLASMA is HIGH, less water is reabsorbed back into the blood and the urine becomes more DILUTE

The amount of water that needs to be reabsorbed depends on a number of factors:

  • External temperature
    • High → Concentrated urine
    • Low → Dilute urine
  • Level of exercise
    • High → Concentrated urine
    • Low → Dilute urine
  • Fluid intake
    • High → Dilute urine
    • Low → Concentrated urine
  • Salt intake
    • High → Dilute urine
    • Low → Concentrated urine

The concentration of urine is controlled by a hormone called ADH, which is released into the bloodstream by the PITUITARY GLAND.

When the level of water in the blood is too low, ADH is released and this causes concentrated urine to be produced. This is because the hormone causes the kidney to make MORE PERMEABLE, allowing water to be reabsorbed.

When the level of water in the blood is too high, ADH is NOT released. The kidney becomes LESS PERMEABLE and this causes dilute, watery urine to be produced.

Effects of Alcohol on water balance:

  • Alcohol causes the kidneys to produce a greater volume of more dilute urine. This can lead to DEHYDRATION
  • This is because alcohol suppresses (restrains) ADH production

 

Effect of Ecstasy on water balance:

  • Ecstasy causes the kidneys to produce smaller volume of less dilute urine. This can result in the body having TOO MUCH water
  • Ecstasy increases ADH production which means the kidneys reabsorb water