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science biology - B4

  • Created by: Ella Mian
  • Created on: 13-05-11 11:55

Bilology B4 - Homeostasis

Our bodies need to control things like body temperature and water level to keep them from changing. The process of keeping things the same is called homeostasis.

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Biology B4 - Homeostasis

Keeping things the same


Homeostasis is how the body keeps conditions inside it the same. Scientists describe it as the maintenance of a constant internal environment.

Two examples of things that the body keeps the same are:

  • body temperature at 37°C
  • the amount of water inside our body

Keeping these two the same is not always easy when the outside environment is changing constantly. But it is important so that all our cells function properly.Strenuous exercise, or living in a hot or cold environment, affect our body temperature and water balance.

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How we keep things the same

  • First, we need receptors to detect when things such as temperature change.

  • Then we need a processing centre to receive this information and coordinate our response.

  • Finally, we need effectors to produce a response that ensures our body temperature stays at 37°C.

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Biology B4 - Homeostasis

It is easier to understand how this works by using a model. Think of an incubator in a premature baby unit:

The incubator needs sensors to monitor the temperature. It also requires a computer or processing centre to monitor and process the data from the sensors and switch the heater on or off. When the incubator is too cold, the heater switches on. When it is too hot, the heater switches off. In this way, it maintains an almost constant temperature within the incubator.

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Biology B4 - Homeostasis

Negative feedback - Higher

Negative feedback ensures that, in any control system, changes are reversed and returned back to the set level.

For example, negative feedback keeps our body temperature at a constant 37°C. If we get too hot, blood vessels in our skin vasodilate (become larger) and we lose heat and cool down. If we get too cold blood vessels in our skin vasoconstrict (become smaller), we lose less heat and our body warms up. Negative feedback makes sure this happens.

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The other factors also controlled in the body by negative feedback are:

  • blood oxygen levels
  • salt levels
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the importance of homeostasis

Cells depend on the body environment to live and function. Homeostasis keeps the body environment under control and keeps the conditions right for cells to live and function. Without the right body conditions, certain processes (eg osmosis) and proteins (eg enzymes) will not function properly.

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Why is homeostasis important for cells?

Living cells depend on the movement of chemicals around the body. Chemicals such as oxygen, carbon dioxide and dissolved food need to be transported into and out of cells. This is done by the processes of diffusion and osmosis, and these processes depend on the body's water and salt balance, which are maintained by homeostasis.

Cells depend on enzymes to speed up the many chemical reactions that keep the cell alive and make it do its job. These enzymes work best at particular temperatures, and so again homeostasis is vital to cells as it maintains a constant body temperature. 

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Biology B4 - Homeostasis


Particles in liquids and gases move about randomly in all directions.

In an area of high concentration, particles will escape from the concentrated area to places where there are fewer or no particles. Very few particles leave an area of low concentration to go to an area where the concentration is higher.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. This is described as moving down a concentration gradient.

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Biology B4 - Homeostasis

Examples of diffusion

Homeostasis maintains the correct body conditions in order for diffusion to take place.

Remember: Particles continue to move from a high to a low concentration until all the particles are evenly and randomly distributed.

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Diffusion in the lungs

In the lungs, the blood will continue to take in oxygen from the alveolar air spaces, provided there is more oxygen in the air spaces than in the blood. The oxygen diffuses across the alveolar walls into the blood. The circulation takes the oxygen-rich blood away and replaces it with blood that is low in oxygen.

In the exam, you may be asked to give two examples of diffusion in organisms. Make sure you know an example from plants as well as from animals.

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Biology B4 - Homeostasis


Osmosis is simply a special type of diffusion. It occurs when water molecules pass through a partially permeable membrane. Some membranes in plant and animal cells allow certain particles to pass through them but not others. They are partially permeable membranes.

During osmosis, more water molecules pass from the pure water into the dilute solution than pass back the other way. This is because there is a higher concentration of water molecules in the pure water than in the solution. This results in more water molecules diffusing across the concentration gradient from the water to the solution. Eventually, the level on the more concentrated side of the membrane will rise, while that on the less concentrated side falls.

Osmosis is the overall movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane. This is still like diffusion, as the water is moving from a higher concentration of water to a lower concentration of water. When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero, and the system is in equilibrium.

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Biology B4 - Homeostasis

Higher only

If red blood cells are placed in pure water, water enters them by osmosis and the red blood cells swell up and burst.

If cells are placed in a concentrated solution, water leaves them by osmosis and they are unable to function.

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Biology B4 - Homeostasis

Enzymes are proteins that speed up chemical reactions in our cells.Enzymes work best at their optimum temperature. This is why homeostasis is important - to keep our body temperature at a constant 37°C. As the temperature increases, so does the rate of chemical reaction. This is because heat energy causes more collisions, with more energy, between the enzyme molecules and other molecules. However, if the temperature gets too high, the enzyme is denatured and stops working.Graph showing the effect of temperature on enzyme reactions (

Graph showing the effect of temperature on enzyme reactions

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Biology B4 - Homeostasis

One enzyme - one job

Enzymes are specific. Only molecules with the correct shape can fit into the enzyme. Just like only one key can open a lock, only one type of enzyme can speed up a specific reaction. This is called the lock and key model.

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Biology B4 - Homeostasis

Enzymes - Higher

The important part of an enzyme is called the active site. This is where specific molecules bind to the enzyme and the reaction occurs.

Anything that changes the shape of the active site stops the enzyme from working. This is similar to a key that opens a door lock. It does not matter what a key handle looks like, but if you change the shape of the ‘teeth’ the key no longer works.

The shape of the active site is affected by pH. This is why enzymes will only work at a specific pH, as well as a specific temperature. Change the pH and the enzyme stops working.

Increasing the temperature to 60°C will cause a permanent change to the shape of the active site. This is why enzymes stop working when they are heated. We say they have become denatured.

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Biology B4 - Homeostasis

Active transport - Higher

Active transport is the process by which dissolved molecules move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell.

Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.

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maintaining body temperature

The hypothalamus is the processing centre in the brain that controls body temperature. It does this by triggering changes to effectors, such as sweat glands and muscles controlling body hair. Heat stroke can happen when the body becomes too hot; and hypothermia when the body becomes too cold.

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Biology B4 - Homeostasis

Temperature control

Temperature control is the process of keeping the body at a constant temperature of 37°C.

Our body can only stay at a constant temperature if the heat we generate is balanced and equal to the heat we lose.

Although our core temperature must be 37ºC, our fingers and toes can be colder. This is because energy is transferred from the blood as it travels to our fingers and toes.

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How our body maintains a constant temperature

Temperature receptors in the skin detect changes in the external temperature. They pass this information to the processing centre in the brain, called the hypothalamus.The processing centre also has temperature receptors to detect changes in the temperature of the blood. The processing centre automatically triggers changes to the effectors to ensure our body temperature remains constant, at 37°C. The effectors are sweat glands and muscles. If we are too hot or too cold, the processing centre sends nerve impulses to the skin, which has two ways to either increase or decrease heat loss from the body's surface.

  1. Hairs on the skin trap more warmth if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.

  2. If the body is too hot, glands in the skin secrete sweat onto the surface to increase heat loss by evaporation. This cools the body. Sweat secretion slows when the body temperature returns to normal.

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Biology B4 - Homeostasis

When things go wrong - heat stroke

Heat stroke is an uncontrolled increase in body temperature.


Heat stroke is caused by:

  • high temperatures which cause an increase in sweating. This can lead to dehydration which reduces sweating which then allows the core body temperature to rise.
  • not drinking sufficient water when hot

As the core body temperature rises, the normal mechanisms for controlling body temperature break down. This can lead to a further rise in the core body temperature.

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The symptoms of heat stroke are:

  • increased body temperature
  • hot, dry skin
  • rapid heartbeat
  • increased or decreased blood pressure
  • headache
  • confusion
  • unconsciousness.
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  • Move the person to a cool, shady place.
  • Cool the person by covering them with damp sheets or spraying them with water.
  • Cool the person with a fan.
  • Seek professional medical help.
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Biology B4 - Homeostasis

When things go wrong - hypothermia

Hypothermia occurs when the core body temperature falls below 35°C.

It happens when the body is losing heat faster than it can make it.


Hypothermia is caused by:

  • extreme cold
  • taking sedatives or alcohol when cold
  • certain medical conditions such as heart problems
  • being very young or very old - these people cannot regulate their body temperature very well
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Biology B4 - Homeostasis


The symptoms of hypothermia are:

  • violent shivering, which stops as hypothermia becomes more severe
  • confusion
  • difficulty in moving
  • memory loss
  • tiredness
  • slurred speech
  • slow, shallow breathing
  • weak pulse.
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  • Move the person somewhere warm.
  • Change them out of wet clothing.
  • Wrap them in warm clothing.
  • Give them warm drinks, but not alcohol.
  • Give them food that is rich in carbohydrates.
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Blood and temperature control - Higher

When we are too hot, blood vessels supplying blood to the skin can swell or dilate (vasodilation). This allows more warm blood to flow near the surface of the skin, where the heat can be lost to the air.

This is why some people's skin looks redder when they feel too hot.

When we are too cold the blood vessels supplying warm blood to the skin become narrow or constrict (vasoconstriction). This reduces the flow of warm blood near the surface of the skin, and reduces heat loss.

This is why some people's skin looks paler when they feel too cold.

A very common mistake in exams is to write that the blood vessels move up and down in the skin. The blood vessels do not move during vasodilation and vasoconstriction.

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Biology B4 - Homeostasis

Signals along nerves from the hypothalamus control both vasodilation and vasoconstriction.

Muscles attached to our skeleton can also receive signals from the hypothalamus when we feel too cold. They respond by shivering. The rapid contraction of muscles during shivering results in heat being produced during respiration. This heat then warms up surrounding tissues.

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Biology B4 - Homeostasis

maintaining 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.

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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

Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body.

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

  1. The level of salts in the blood.

  2. The excretion of urea and other metabolic waste.

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Biology B4 - Homeostasis

how is water balance maintained?

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. 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.

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Biology B4 - Homeostasis

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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How the kidneys are controlled - Higher

The concentration of our urine is controlled by a hormone called ADH.

ADH is produced by the pituitary gland that is situated just below the brain. The pituitary gland monitors the concentration of the blood plasma. It releases ADH into the bloodstream, which travels in the blood to the kidneys

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The more concentrated the plasma, the more ADH is released into the blood. When the ADH reaches the kidneys, it causes them to reabsorb more water. This keeps more water in the body and produces more concentrated urine.

When the plasma is more dilute, less ADH is released into the bloodstream. This allows more water to leave the kidneys, producing a more dilute urine.

This method of control is an example of negative feedback.

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Biology B4 - Homeostasis

How alcohol and ecstasy affect ADH

Alcohol suppresses ADH production. This causes the kidneys to produce more dilute urine. It can lead to dehydration.

Ecstasy increases ADH production. This causes the kidneys to reabsorb water. It can result in the body having too much water.

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