B4: Homeostasis Maintenance of a constant environment within a body

Keeping our cells happy Cells work best at certain temperatures and with the right amount of hydration (water present) The levels of glucose, salts, urea, oxygen and carbon dioxide must also be controlled The control of these factors is automatic and occurs via feedback systems.

Where we are and what we do affect our levels… Which internal factors are being stressed in each picture?

Feedback loops The effectors adjust the quantity if the processing centre notices a difference between the sensor reading and the set-point and uses effectors to modify the values Processing centre receptor stimuli

Negative feedback loops High “readings” bring about a reduction and low levels result in a rise Draw a similar flow diagram to show the chain of events if the car is too slow

Antagonistic effectors The use of two effectors which have opposite effects makes the control of systems much more sensitive (a push-pull approach!). E.g. blood sugar levels are controlled by two hormones –glucagon (raises sugar levels) –insulin (lowers sugar levels) which we met in C3 (Food Matters) Blood sugar is too low Blood sugar is Too high Glucagon increases Insulin decreases Glucagon decreases Insulin increases Blood sugar levels return to the set point

Another example: Body temperature… temperature is too low temperature is too high shivering increases sweating decreases shivering decreases sweating increases temperature returns to the set point

Some machines use feedback loops too! If the incubator gets too hot measured by the thermostat (sensor) then the heater automatically turns off. If too cold the heater turns on The temperature stays almost constant Draw a decision chart for the incubator

Temperature control The core temperature must remain constant (36.8 o C in humans) but the extremities (e.g. limbs) can withstand more variation Credit: TONY MCCONNELL/SCIENCE PHOTOLIBRARY Where is the most heat (i) lost and (ii) retained in the diagram? Use your answers to decide which would be the (iii) warmest and (iv) coolest parts of the body. What differences in the diagram would you see if the person had a fever?

How is the body temperature maintained? To maintain the body’s temperature… (a) capillaries of the skin can be opened to lose heat and closed to retain heat (b) shivering due to increased respiration warms up the surrounding tissues (c ) sweating causes cooling as the water evaporates Set point 37 o C Set point 37 o C Blood too hot Blood too cool Shivering and vasoconstriction of capillaries in the skin sweating and vasodilation of capillaries in the skin The hypothalamus of the brain has temperature receptors which detect the blood temperature and automatically trigger the effectors. Which are the effectors in the diagram?

Can’t take the heat? If we get too hot the feedback loop does not work as well since we dehydrate and can’t produce as much sweat. The body will need immediate first aid to lower its’ temperature or else death is possible. Heat stroke is an uncontrolled increase in body temperature First aid

Chill out! Cold, wet and windy weather can all lower the body temperature of poorly prepared travellers. Hypothermia (where heat loss is faster than heat production) occurs if the core temp drops below 35 o C Frostbite damage to hands

Hypothermia first aid Keep dry and sheltered Insulate Warm with drinks and food unless unconscious Monitor pulse and breathing CPR if necessary What might happen if a hot water bottle was applied to their limbs?

Why do we have an optimum temperature? Most chemical reactions in our body rely on enzymes (biological catalysts made of proteins) They need a reasonably high temperature to allow enough collisions with other molecules (substrates) but not too high in case they denature (no longer work)

Enzymes The substrate fit perfectly into the active site of the enzyme like a key into a lock! The shape of the enzyme is due to folding and weak bonding between chains. Too much heating OR a change in pH breaks these weak bonds so the active site changes shape (denatured!) Active site

On a cellular level The levels of substrates and other chemicals must be maintained in cells using one of the transport methods below… Diffusion e.g. O 2, CO 2, dissolved food Osmosis e.g. water Active transport (requires the use of an energy rich molecule) e.g. transport of glucose

Diffusion

Osmosis Semi-permeable membrane (allows the water through but not the large solute!) Low solute concentration High solute concentration Only the water moves!

Uses of water Can you name 3 ways water is gained for our body? Losses include… sweating, breathing, excretion of urine, faeces

Controlling our hydration Our kidneys help to regulate our levels of… –urea ( a waste substance from protein breakdown) –Water –Salts –Sugars by stopping their loss in urine

kidneys Kidneys produce urine ( a mixture of water, urea and occasionally salts) Bladder stores urine (until a convenient moment!) Small molecules filtered from the blood and the wanted ones are reabsorbed

Bladder control… it’s all in the mind! The pituitary gland uses a hormone called ADH (anti diuretic hormone) to control hydration levels via the bloodstream. Uses a negative feedback loop where the blood plasma concentration determines the release of the hormone. Hormone makes the kidney more permeable to water allowing it to reabsorb more. Pituitary gland

Urine: the colour of samples indicate the level of hydration High plasma conc ADH Released by pituitar y More water reabsorbe d low plasma conc ADH not release d less water Reabsorbe d by kidneys Dilute urine Concentrated urine Which factors may affect your urine colour?

Factors affecting hydration levels: External temperature Level of exercise Water intake Salt intake Drug taking… –Alcohol suppresses ADH so more dilute urine is formed dehydrating the body How might hangovers be explained? –Ecstasy causes too much ADH to be formed so too much water is absorbed… this can cause the brain to to swell, water intoxication and death as with the case of Leah Betts How might the conditions in the night club have made the effect of ecstasy worse for Leah?