Biology Unit 2

B4- Biology Unit 2: - Homeostasis

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  • Created by: Ella
  • Created on: 31-03-11 16:32


Homeostasis = maintenance of a constant internal environment.

This is achieved by:

  • Balancing Bodily inputs and outputs
  • Removing waste products

Your body has automatic control systems which maintain steady levels of:-

  • temperature
  • water (hydration)       --> These factors enable you to function properly.
                                               Failure of homeostasis results in death :(

Homeostasis can be affected by exercise and temperature.

Homeostasis can also be affected by blood, oxygen levels and salt levels.

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Factors Affecting Homeostasis

  • Strenuous exercise causes your body temperature to increase and water is therefore lost through sweat.
  • Your body needs to get rid of excess heat and replace lost water so that your system will continue to work correctly.

In HOT climates:- Body temperature rises --> water level drops.

In COLD climates:- Body loses heat --> hypothermia.


  • Acuba Divers: equipment used to make sure oxygen is kept at correct level. Wetsuits maintain body temperature.
  • Mountain Climbers: breathing apparatus- blood oxygen kept at correct level.
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Controlling Conditions

Artificial homeostasis- needed when the bodies control systems don't work correctly.

Incubators- help premature babies to survive by controlling temperature and oxygen levels .

Artificial systems and body systems both have...... :D

  • receptors - sensors to detect stimuli
  • processing centres- to recieve information and coordinate responses.
  • effectors- which automatically produce the response.

Negative feedback- maintenance of a steady state by reversing the change in conditions.

Antagonistic effectors- act as opposites to one another.

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Water Balance


The kidneys maintain our body's water balance by controlling the water concentration of blood plasma. The kidneys also control salt levels and the excretion of urea. Water that is not put back into the blood is excreted in our urine.

Water Balance:-

Our bodies take in water from food and drinks. We even get some water when we respire by burning glucose to release energy.

We lose water in sweat, faeces, urine and when we breathe out (on a cold day you can see this water as it condenses into vapour).

For the cells of our body to work properly, it is important that their water content is maintained at the correct level. This means our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys.

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The Kidneys

Humans have two kidneys. They are bean-shaped organs, approximately 11.5cm long, that are situated in the abdominal cavity, just below the ribcage, one on either side of the spine.

Location of kidneys in abdominal cavity (

(Location of kidneys in abdominal cavity) --> Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body.

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What do Kidneys do?

What do kidneys do?

As the blood passes through the kidneys, all the small molecules are filtered out of the blood.

This includes molecules of:

  • water
  • salt
  • glucose
  • urea (a waste product from the breakdown of proteins)

The kidneys then reabsorb all of the glucose and as much water and salt as the body needs, putting them back into the blood. This leaves some water and salt, and all of the urea, which is now called urine. The urine passes from the kidneys to the bladder, where it is stored prior to being excreted from the body.

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Kidneys Cont. & Maintaining Water Balance

The kidneys do more than just control the body’s water balance. They also control:

  • The level of salts in the blood.

  • The excretion of urea and other metabolic waste.

How is water balance maintained? :O

The kidneys maintain our water balance by producing urine of different concentrations.

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 our urine is more dilute.

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Blood Plasma & Drugs

The level of water in the blood plasma can vary depending on:

  • External temperature - when it is hot, we sweat more and lose water, thereby making the blood plasma more concentrated.

  • Amount of exercise - if we exercise, we get hot and increase our sweating, so we lose more water and the blood plasma becomes more concentrated.

  • Fluid intake - the more we drink, the more we dilute the blood plasma. The kidneys respond by producing more dilute urine to get rid of the excess water.

  • Salt intake - salt makes the plasma more concentrated. This makes us thirsty, and we drink more water until the excess salt has been excreted by the kidneys.

Drugs that affect water balance:-


Alcohol causes the kidneys to produce a greater volume of more dilute urine. This can lead to dehydration.


Ecstasy causes the kidneys to produce a smaller volume of less dilute urine. This can result in the body having too much water.

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