B4 - Homeostasis

B4 - Homeostasis revision notes

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Homeostasis - Keeping a constant internal environment such as temperature and water levels.

Receptors - Detects stimuli.
Processing centres - Recieves information and coordinates responses.
Effectors - To produce responses.

Negative feedback - Any change in a control system leads to an action that reverses a change. Negative feedback between the effector and the receptor makes this happen.

Antagonistically - Some effectors have opposite effects to eachother, they work antagonistically.

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Enzymes - Speed up cheimical reactions in cells. They are protein molecules. Every type of enzyme has a different shape and speeds up a particular reaction. Molecules that take part in reaction must fit into enzyme's Active site. This is called the lock and key model.

Below 37oC - Few collisions between enzyme molecules and reacting molecules. Reactions are too slow.

At 37oC -
Collisions happen more often and collisions have more energy, so reaction is faster.

Above 37oC - Enzymes stop working. They Denature.

When enzymes denature, the shape of the active site changes. Molecules no longer fit into active site, therefore reaction cannot happen. Changes in pH cause the same problem.

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Keeping body temperature constant

Respiration - Releases energy from food for your body to use. You can also gain energy from your environment, if it is hotter than you.

The body's core is hotter than it's extremities. At the core, energy is transferred to the blood. At the extremities, blood transfers energy to tissues.

Temperature receptors in the skin detect the temperature outside of the body.

Temperature receptors in the brain detect the blood's temperature. (in the hypothalamus)

A processing centre in the brain (in the hypothalamus) receives information from receptors and triggers responses in effectors.

Effectors, for example sweat glands and muscles.

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Keeping body temperature constant

At high temperatures, the body cools down by;

Making sweat - When sweat evaporates, energy is transferred from skin to sweat
Vasodilation - Blood vessels that supply the skin's capillaries get wider. More blood flows through and the body loses more energy.

At low temperatues, the body warms up by;
Shivering - Caused by muscles contracting fast. Respiration in muscle cells increase. Respiration releases some energyas heat to nearby tissue.
Vasoconstriction - Blood vessels get narrower. Less blood flows through, and body loses less energy.

Vasodilation and Vasoconstriction are examples of effectors that work antagonistically.

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Keeping body temperature constant

Heat stroke

When core body temperature rises above 42oC.
Hot,dry skin because sweating stops. Fast pulse rate because of dehydration and stress. Dizziness and confusion because nerve cells in the brain are damaged.
Treat with sponge with water, put near a fan, ice in groin and under arms.
When it is hot you sweat more which causes dehydration. when dehydrated you sweat less, so body temperature rises out of control.


When core body temperature falls below 35oC.
Shivering, confusion, slurred speech, loss of co-ordination. Coma below 30oC. Death below 28oC. Insulate patient, warm gently with towels, give warm drinks. Body loses energy faster than it gains energy.

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Getting substances into and out of cells

Diffusion - When liquid or gas molecules move from an area of high concentration to an area of low concentration. It is a Passive process - it requires no energy.

Cell membranes are partially permeable - Water molecules can get through their small holes.

Osmosis - More water molecules diffuse through partially permeable membranes from a dilute solution to a more concentrated solution.

Sometimes a cell needs to take in molecules such as glucose that have a higher concentration inside the cell than outside. The cell uses Active transport. Respiration provides energy for active transport.

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Controlling water levels

Cells only work properly if the concentration of their contents are correct. If too much water moves into an animal cell, it may rupture. If too much water moves out of an animal cell, the solutions in the cell become too concentrated and the cell cannot work properly.

Kidneys control water levels. They also get rid of waste products such as excretion.


Filter out small molecules from the blood (Water, Urea, Glucose and Salt). Blood cells and protein molecules stay in the blood.
They send all the urea, some water and salt, to the bladder. This is Urine and is stored in the bladder and later excreted.
They reabsorb molecules that the body needs - including all the glucose and some water and salt - back into the blood.

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Controlling water levels

Hot days - sweat more, kidneys produce less urine. Also make less urine after exercise, if not drunk much water or eaten salty food.

Receptors in the hypothalamus detect changes in salt concentration in blood plasma. If salt concentration is too high, to processing centre makes the pituitary gland release ADH into the blood stream. The ADH travels to the kidneys. The more ADH that arrives, the more water the kidneys reabsorb.So the more concentrated the urine.

Drinking alcohol leads to big volumes of dilute urine, so you may get dehydrated - this is because alcohol stops the pituitary gland releasing ADH.

Taking ecstasy leads to small volumes of concentrated urine. This is because Ecstasy makes the pituitary gland release more AHD.

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