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

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

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

     

 

 

Effect-of-osmosis-on-animal-cells

 

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

 

 

The Role of ADH

 

  • 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

     

 

 

 

ADH-and-osmoregulation-1-1ADH-and-osmoregulation-2-1

 

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

 

 

Urine

 

  • 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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