Science B4

Revision for B4 


Homeostasis - What is Homeostasis?

Homeostasis is where our body controls things in our body such as; body temperature and the amount of water in our body. Homeostasis is the process of keeping things the same. 

Homeostasis is how the body keeps conditions inside it the same. It is described as the maintenance of a constant temperature. 
Keeping things the same like body temperature and amount of water inside our body isn't always easy due to the environment outside as that is changing constantly. But it is important that all our cells function. Strenuous exercise or living in a hot or cold environment affects our body temperature and water balance. 

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Homeostasis - How we keep things the same?

To keep things the same in our body we need a variety of things.

  • Firstly we need receptors to detect when things such as temperature changes. This is usually done in the brain or skin.
  • Then we need a processing centre to recieve this information and coordinate our response. This is usually done in the hypothalamus.
  • Finally we need effectors to produce a response that ensures our body temperature stays at 37oC. This is done in the blood capilarries or skin. 

It is easier to understand how this works by using a model. Think of an incubator in a premature 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, and 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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Homeostasis - Negative Feedback

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

For example: negative feedback keeps our body temperature at a constant 37degrees. If we get too hot our blood vessels in our skin vasodilate (become larger) and we lose heat and cool down. If we get too cold our blood vessels in our skin vasoconstrict (become smaller). We lose less heat and warm up. Negative feedback makes this happen.

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

  • blood oxygen levels
  • salt levels 
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Homeostasis - 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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Homeostasis - Why is Homeostasis important for cel

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 processed of diffusion and osmosis, and these processes depend on the body's water and salt level, 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 a certain temperature, and so again homeostasis is vital to cells as it maintains a constant body temperature.

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Homeostasis - Diffusion

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

In all 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 with low concentration. This is described as moving down a concentration gradient.

Homeostasis maintains the correct body condition in order for diffusion to take place.
Remember: Particles continue to move from a high to low concentration until all the particles are evenly and randomly distributed. 

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Homeostasis - Diffusion in the lungs

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

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Homeostasis - Osmosis

Osmosis is simply a special type of diffusion. It occurs when water molecules pass through a partially permeable membrane.Some membranes in plants 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 pure water into dilute solution then pass back the other way. This is because there is higher concentration of water molecules in the pure water than in the solution. This results in more water molecules diffusing across the 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 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 0, and the system is in equilibrium. If red blood cells enter the pure water, water enters them by osmosis and the 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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Homeostasis - Enzymes

Enzymes are protein that speed up chemical reaction in our cells.

Enzymes work best at their optimum temperature. This is why homeostasis is important to keep our body at a constant temperature of 37degrees. As the temperature increases so does the rate of chemical reaction. This is because the heat energy cause 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. 

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Homeostasis - One Enzyme One Job

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

The important part of an enzyme is called the active site. This is where the specific molecules bind to the enzyme and the reaction takes place.
Anything that changes the shape of the active site stops the enzyme from working. This is similar to a key the 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 the pH. This is why enzymes 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 degrees will cause a permanent change to the shape of the active site. This is why enzymes stop working when they are heated. We say that they have become denatured. 

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Homeostasis - Active Transport

Active transport is the process by which dissolved molecules move across a cell membrane from a lower to 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.
In humans, active transport takes place during the digestion of food in the small intestine. Carbohydrates are broken down into simple sugars such as glucose. The glucose is absorbed by active transport into the villi, to be passed into the bloodstream and taken around the body.  

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Homeostasis - Maintaining Body Temperature

The hypothalamus is in the processing centre in the brain that controls body temperature. It does this by triggering changes to the effectors such as; sweat glands and muscle controlling body hair.

Heat stroke can come when the body gets too hot; and hypothermia becomes when the body gets too cold.  

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Homeostasis - Temperature Control

The temperature control is the process of keeping the body at a constant temperature of 37 degrees.

Our body can only stay at a constant temperature if the heat we generate is equal to the heat in which we lose.
Although our core temperature must be 37 degrees our fingers and toes can be colder. This is because the energy is transferred from the blood as it travels to the fingers and toes.

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Homeostasis - How our body maintains a constant te

Temperature receptors in the skin detect the changes in the external temperature. They pass this information onto 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 the body remains at a constant temperature of 37 degrees.
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 the heat loss from the body's surface.

  • Hairs on the skin trap more warmth when they are stood up, and less if they lay down. Tiny muscles in the skin quickly pull the hairs upright to reduce heat loss, or lay them down to increase heat loss.
  • If the body is too hot, the glands in the skin secrete sweat onto the surface to increase the heat loss by evaporation. This cools down the body. Sweat secretion slows when the body temperature returns to normal.
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Homeostasis - Heat Stroke

Heat stroke is caused by an uncontrolled increase in body temperature.

Causes - 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 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.

Symptoms - the symptoms of heat stroke are; increase body temperature, hot/dry skin, rapid heartbeat, increased or decreased  blood pressure, headache, confusion, unconsciousness.

Treatment -  move person to a shady/cool place, cool the person by covering them with a damp sheets or spraying them with water, cool the person with a fan, seek medical help.

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Homeostasis - Hypothermia

Hypothermia occurs when the core temperature falls below 35 degrees. It happens when the body is losing heat faster than it can make it. 

Causes - 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.

Symptoms - The symptoms of hypothermia are; violent shivering which stops when hypothermia becomes more severe, confusion, difficult in movement, memory loss, slurred speech, tiredness, slow shallow breathing, weak pulse.

Treatment - Move the person somewhere warm, change them out of wet clothing, wrap them in warm clothing, give them warm drinks not alcohol, give them warm food which is rich in carbohydrates.

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Homeostasis - Blood and Temperature Control

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 the 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 or warm blood near the surface of the skin, and reduces heat loss.
This is why some people's skin looks paler when the feel too cold. 

Signal 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 col. They respond by shivering. The rapid contraction of muscles during shivering results in heating being produced during respiration. This heat then warms up surrounding tissues.

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Homeostasis - 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 we 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 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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Homeostasis - What the Kidneys do?

Blood is brought to the kidneys to be filtered, then returned, to be circulated around the body. As the blood passes through the kidneys, all the small molecules are filtered out of the blood.

This includes molecules of; salt, water, glucose, urea (a waste product from the breakdown of proteins)

The kidneys reabsorb all of the glucose and as much water and salt the body needs, putting them back into the blood. This leaves some of the 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 body's water balance. They also control:

  • The level of salts in the blood
  • The excretion of urea and other metabolic waste
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Homeostasis - How is the Water Balance Maintained?

The kidneys control 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 the urine is more dilute.

The level of water in our blood plasma can be very 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 so our blood plasma becomes more concentrated.
Fluid Intake - the more we drink the more dilute the blood plasma gets. 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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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 more dilute urine. This can result in the body having too much water.

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Homeostasis - How the Kidneys are Controlled?

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 blood stream, which travels in the blood to the kidneys. 

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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Homeostasis - When there is too little water

  • Hypothalamus detects too little water in blood
  • Pituitary gland releases ADH 
  • Kidneys maintain blood water level
  • So less water is lost in urine (urine is more concentrated)
  • Blood water levels return to normal
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Homeostasis - When there is too much water

  • Hypothalamus detects more water in the body
  • Pituitary gland release less ADH
  • Kidneys reduce blood water level
  • So more water reaches bladder (urine is more dilute)
  • Blood water level returns to normal
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Homeostasis - How Alcohol and Ecstasy affects 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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