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Negative feedback control

Watch this to explore negative feedback controls for Higher Biology (…put another log on the fire)

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Do you remember January 2021? One of the coldest winter spells on record for central Scotland. The country froze – snow and ice everywhere. It was brutal.

Well, my heating packed in and for four days we had no heat in our radiators. Not good at all. I think the cats kept me alive.

The problem was a broken thermostat – the thermometer in the heating system failed to measure the temperature and adjust the heating accordingly. It just wasn’t detecting the temperature in the house, so the boiler wasn’t being activated and we were rather cold as a result.

Well, our metabolic systems in our body are controlled a bit like this and many are constantly increased, or decreased, to keep our internal environment working at its optimum level. This is done using a process called negative feedback control.

So let’s raise the temperature and find out more. This is Thinkfour.

Negative feedback control is the way our bodies first detect any changes in our metabolic rates. This is done in areas called receptors. The receptor then determines if the change is an increase, or a decrease, and sends out a message, either a chemical message such as a hormone, or a nerve impulse.

These messages go to specific areas of the body, called effectors, which bring about changes that will either raise, or lower, the rate of the reactions. If the receptor has detected an increase in the system, the effector then lowers it back to normal, or raises it if it has fallen. This keeps our body functioning at its optimum level and gives us a stable internal environment.

So, for any negative feedback system, remember to look for the receptor, the message and the effectors that then change it.

There are many systems under negative feedback control, such as water balance, which is monitored by our hypothalamus. The hormone ADH is sent to our kidneys, which are the effectors that get rid of the appropriate amount of water to keep it balanced in our blood. Controlling glucose levels in the blood involves two hormones, insulin and glucagon, which keep our blood sugar constant via the liver. The system we need to know in more detail is the control of our body temperature, by a process called thermoregulation. It involves our hypothalamus in the brain, the receptor, which determines the temperature of the blood as it flows through it. A nerve impulse, the message, is then sent to the skin, which is the effector. The skin then carries out various responses to either help raise or lower our body temperature back to its normal level at 37°C.

First, let’s assume you have done a lot of exercise and your blood temperature has gone up. The first thing the skin does is to vasodilate the blood vessels that lead to the skin surface, they get wider. More blood flows close to the surface and more heat is lost by radiation, lowering the blood temperature.

That is why we go red when we do strenuous exercise. We also sweat more. The heat radiating out of our body evaporates the water in our sweat, which also helps to cool the skin surface. These two effects combine to lower the blood temperature. The opposite happens if we get cold, but there are a few added extras.

If we get too cold, our skin blood vessels vasoconstrict, they get narrower, so that less blood flows close to the surface and less heat radiates out. This is why our skin often looks pale when we are cold. Sweating is also reduced to an absolute minimum, so it seems that we stop sweating. We also get goosebumps.

This is when the muscles connected to each of our hairs contract and the hairs stand up, trapping a layer of warm air close to the skin surface and giving us lots of little bumps on the skin. Our skeletal muscles also rapidly contract and relax, we shiver. This heats the blood as it flows through the muscles.

These effects work if we get slightly hot or cold, but outside either extreme, the systems can’t work, so we either get heat exhaustion or hypothermia. So we stay warm, or we cool down and life just ticks along nicely.

Negative feedback works just like a central heating system, keeping our body at a constant temperature, adjusting if it gets too hot, or too cold.

Maintaining a constant internal environment so that we can survive and function normally whether we’re in a hot, humid country, or here in Glasgow on a cold, frosty January morning.

Talking of that, the nights are fair drawing in… I’ll need to turn my thermostat up when I get home later. Keep warm!

This was Thinkfour. Thanks for watching.

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