[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe what's meant by negative feedback you should then be able to describe the key parts of a negative feedback system and finally you should be able to compare negative feedback with positive feedback in the last video we started looking at homeostasis and we saw that homeostasis is the maintenance of a constant internal environment for example the human body maintains a relatively constant blood pH a relatively constant blood glucose concentration and a relatively constant core body temperature I'm showing you here how the human body temperature can vary over time as you can see body temperature fluctuates around 37° C now a key idea you need to understand is that any parameter will fluctuate slightly around an Optimum level and that's due to small small changes taking place within the body scientists refer to this as a dynamic equilibrium homeostasis operates to minimize these fluctuations and ensure that each parameter remains close to the Optimum to do this homostasis uses a process called negative feedback and any negative feedback system has several key features firstly remember that any parameter will have an Optimum value now any check change in the parameter from the optimum value is referred to as a stimulus so if the parameter rises above the optimum or Falls below the optimum then this is a stimulus the stimulus is detected by a receptor when the receptor detects a stimulus it passes signals to the coordinator now the coordinator can accept information from a range of receptors and the coordinator compares the value from each receptor with the optimum value for that parameter the coordinator then sends instructions to an affector and affectors are often muscles or glands when an affector receives instructions from the coordinator the affector carries out a response for example a muscle May contract or a gland May secrete a substance and the response causes the parameter to change back towards the optimum value these changes in the parameter are then fed back to the receptor so by constantly monitoring changes to the parameter and triggering responses to reverse any changes negative feedback keeps a parameter near the optimum value now a key idea you need to understand is that in any negative feedback system there are separate mechanisms for when the parameter is too high or too low I'm showing you here human body temperature again now we're going to be looking in detail at thermal regulation in a later video so I'm just using this example to illustrate a point imagine that our body temperature rises above the optimum this triggers mechanisms to reduce body temperature for example sweating now imagine that our body temperature starts to fall below the optimum clearly sweating will be stopped however the body also activates different mechanisms which actively increase body temperature back towards the optimum for example we might start shivering so as you can see a rise above the optimum triggers one set of mechanisms and a fall below the optimum triggers a different set of mechanisms having separate mechanisms like this gives homeostasis a high degree of control over the parameter now in this topic we'll be looking at a range of homeostatic systems for example we'll be looking at temperature regulation the regulation of blood glucose concentration and the regulation of blood water potential and all of these involve negative feedback okay now another type of feedback in biology is called positive feedback but you need to understand that positive feedback is not seen in homeostasis in positive feedback if a parameter changes then a response is triggered which increases the change a good example is found in childbirth during childbirth the head of the baby pushes against the cervix of the mother pressure on the cervix triggers the pituitary gland to release the hormone oxytocin oxytocin then triggers contraction of the uterus these contractions cause the baby's head to press harder against the cervix this triggers even more oxytocin to be released and the uterus to contract more strongly and this positive feedback continues until birth takes place now with positive feedback there's usually a clearly defined end point and there are not many examples of positive Fe feedback in biology in contrast negative feedback involves continuous monitoring and adjustment to an ongoing process and there are lots of examples of negative feedback in biology okay so hopefully now you can describe negative feedback and positive feedback [Music]