Edexcel IGCSE Biology 复习笔记 2.9.5 Regulating Water Content in the Blood

• Osmoregulation is the process of maintaining water and salt concentrations (osmotic balance) across membranes within the body
• It is an example of homeostasis in the human body

The importance of osmoregulation

• The cytoplasm of all cells is largely composed of water, as is the blood plasma
• Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis
• If body cells lose or gain too much water by osmosis they do not function efficiently:
  • Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting)
  • Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death

   

The effect of water concentration on body cells due to osmosis

Water content of the body

• There are two sources of water in the body:
  • Water produced as a result of aerobic respiration
  • Water in the diet

   

• Water is lost from the body in the following ways:
  • Via the lungs during exhalation (breathing out)
  • Lost from the skin as sweat (along side mineral ions and urea)

   

• Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys

• Water reabsorption occurs along the nephron tubules in the kidneys
• The control of water reabsorption by the tubules is an example of negative feedback
• If the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed
  • This is controlled by the hormone ADH

   

• The pituitary gland in the brain constantly releases a hormone called ADH
• How much ADH is released depends on how much water the kidneys need to reabsorb from the filtrate
• ADH affects the permeability of the tubules to water
• If the water content of the blood is too high:
  • The pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water)
  • As a result, the kidneys produce a large volume of dilute urine

   

• If the water content of the blood is too low:
  • The pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water)
  • As a result, the kidneys produce a small volume of concentrated urine

   

The kidney controls water levels using the hormone ADH. It is an example of a negative feedback mechanism.

• Urine produced by the kidneys contains a mixture of
  • Urea
  • Excess mineral ions
  • Excess water

   

• The colour and quantity of urine produced in the body can change quickly
  • Large quantities of urine are usually pale yellow in colour because it contains a lot of water and so the urea is less concentrated
  • Small quantities of urine are usually darker yellow / orange in colour because it contains little water and so the urea is more concentrated

   

• There are various reasons why the concentration of urine will change, including:
  • Water intake - the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced
  • Temperature - the higher the temperature the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
  • Exercise - the greater the level of exercise, the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced

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