B4- Homeostasis

Changing to Stay the Same

Keeping conditions inside your body the same is called homeostasis.
Your body works hard to-
1. Keep a constant body temperature.
2. Keep correct levels of water and salt.
3. Control amounts of nutrients (eg glucose).
4. Take in enough O2 for respiration.
5. Get rid of toxic waste (CO2 and urea).
All control systems have a receptor (detects the stimuli) , processing centre (receives the information) and an effector (produces an automatic response).
An incubator has a temperature centre (the receptor), a thermostat with a switch (processing centre) and a heater (effector).
Some of your temperature control is automatic (sweating) but some you control (putting a coat on).

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Negative feedback- any change in a system that results in an action that reverses the change (for example, if the temperature in a fridge goes up, it is cooled down).
Antagonistic effectors are effectors with 2 opposite effects. For example, when you drive through a 30mph zone, you don't want to go fast or slow. The brake and the accelerator are antagonistic effectors.

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Enzymes are catalysts that speed up chemical reactions in living organisms. They are proteins- an enzyme's shape (determined by amino acid chains) is important.
Some enzymes break apart molecules and some join them together.
Molecules must fit exactly into the active site- key in a lock, the lock-and-key model.
At body temperature, chemical reactions would be too slow to keep you alive. The temperature must be raised- molecules have more energy, move faster, collide more often and react more easily.
High temperatures change the shape of an enzyme- the enzyme becomes denatured, and can never change back.
The temperature at which an enzyme works best is the optimum temperature- we rely on enzymes to works at our body temperature.

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If your environment is hotter than you, it transfers heat to you. You also gain heat through respiration.
For your temperature to remain constant , heat gained= heat lost.
Your extremities are colder than your core, they have a bigger surface area compared to their size.
When you are cold, circulation to extremities is redirected to the core- warm
There are sensitive receptors in the brain that detect blood temperature.
The Hypothalamus is the processing centre for temperature, the cerebral hemispheres make conscious decisions to warm up or cool down.
Shivering is an automatic response- when you shiver, muscle cells contract and respire, releasing energy.
When you are too hot, nerve impulses cause the sweat glands to secrete sweat, which cool you down when it evaporates from your skin.
To cool down, your blood vessels vasodilate- they allow more blood to flow into capillaries on the skin, so more energy is transferred.
Vasoconstriction is opposite, less blood reaches the capillaries, less energy lost.

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Molecules in gases and liquids move randomly- they spread out.
More molecules move away from where they are concentrated. The molecules diffuse from areas of high concentration to low concentration.
Diffusion is a passive process- it doesn't require energy.
Partially permeable membranes are cell membranes that let some molecules through, but not others.
The overall flow of water from a dilute to a more concentrated solution across a partially permeable membrane is osmosis.
Active transport is a way of moving molecules that cannot move through diffusion. Cells use energy from respiration to transport molecules across the membrane.
Land animals lose water to the environment and the water must be replaced. If not, the body fluids become too concentrated and the cells stop.

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Keeping a steady water level is done by balancing water inputs and outputs.
Kidneys control the water balance by changing the amount of urine you make.
Kidneys deal with water homeostasis and excretion- getting rid of toxic waste products from chemical reactions in cells.
These 2 jobs are linked as you use water to flush out waste eg urea.
Liver cells make urea when they break down unusable amino acids. It diffuses into the blood and is carried around the body.
Urea is filtered out as it is poisonous in large concentrations.
Kidneys sieve small molecules out of blood (water, sugar, urea and salt ions), keeping in large molecules (proteins) and blood cells.
Kidneys reabsorb useful chemicals into the blood- glucose for respiration, as much salt and water as needed.
The rest of the filtered chemicals go to bladder to make up urine.

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Salt concentration of blood determine show much water your kidneys reabsorb and how much is excreted in urine.
Salt concentration of blood can become high because of excess sweating, not drinking enough water or eating salty food.
Caffeine= lots of dilute urine. Alcohol has even greater effect.
Ecstasy= little urine-may make person drink too much water.
Receptors in hypothalamus detect salt concentration in blood changes.
If salt concentration is too high, the hormone ADH is released from the pituitary gland. If salt concentration is low, no ADH is released.
ADH travels in the blood to kidney tubules (the effectors). ADH affects the amount of water reabsorbed. More ADH= more water.

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Heat stroke happens when your body cannot lose heat fast enough. You may have become dehydrated and produce too little sweat.
Fever, prolonged exercise, over-exposure to Sun and Ecstasy lead to heat stroke.
Core body temp. over 42c affect the hypothalamus, your temp. control system fails.
Rapid cooling is essential- sponging, fan, ice.
Hypothermia occurs when core body temp. falls below 35c- body heat cannot be replaced as it is lost.
Babies have large surface area compared to their volume, lose heat quickly.
Core must be heated but not skin and limbs; this increases blood flow to them, causing further heat loss.

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