WEBVTT 00:00:08.410 --> 00:00:12.750 Soon after a meal, your digestive system breaks down the food you have eaten into a simple 00:00:12.750 --> 00:00:17.439 sugar called glucose. Glucose is absorbed from the gut into the bloodstream, causing 00:00:17.439 --> 00:00:22.689 your blood sugar level to increase. In healthy individuals, feedback mechanisms in the body 00:00:22.689 --> 00:00:27.990 bring blood sugar levels back to normal. In some people, this process breaks down, resulting 00:00:27.990 --> 00:00:29.840 in diabetes. 00:00:29.840 --> 00:00:33.860 In this video, we'll take a closer look at feedback loops, how they tie into the body's 00:00:33.860 --> 00:00:39.390 mechanism of internal regulation, and what happens when these mechanisms fail. 00:00:39.390 --> 00:00:44.399 This video is part of the Information Flow series. A system is shaped and changed by 00:00:44.399 --> 00:00:48.579 the nature and flow of information into, within, and out of the system. 00:00:48.579 --> 00:00:54.930 Hi, my name is Leah Okumura and I am a Technical Instructor in the Biology Department at MIT. 00:00:54.930 --> 00:00:58.370 Before watching this video, you should be familiar with the concept that your body is 00:00:58.370 --> 00:01:03.399 a tightly regulated environment. After watching this video, you will be able identify the 00:01:03.399 --> 00:01:08.020 general components of a feedback loop, examples of negative and positive feedback loops in 00:01:08.020 --> 00:01:15.020 the body, and describe how feedback loops are vital to healthy function and survival. 00:01:17.160 --> 00:01:21.440 Our bodies rely heavily on feedback loops to control and regulate important biochemical 00:01:21.440 --> 00:01:23.160 and physiological functions. 00:01:23.160 --> 00:01:28.500 There are two types of feedback loops in biology: negative feedback loops, and positive feedback 00:01:28.500 --> 00:01:34.110 loops. These act to either reduce or amplify the changes that occur in a given system. 00:01:34.110 --> 00:01:39.110 Let's see how feedback loops can be applied to thermoregulation. Imagine walking into 00:01:39.110 --> 00:01:43.800 a room that has an air conditioner on at full blast. After a few minutes, you find yourself 00:01:43.800 --> 00:01:49.420 shivering from the cold. Does the shivering serve a purpose? Yes! The rapid muscle contractions 00:01:49.420 --> 00:01:56.420 from shivering generate heat within the body and warm you back up. Now imagine taking a 00:01:56.670 --> 00:02:01.720 walk outside on a very hot and sunny day. After a few minutes under the hot sun, you 00:02:01.720 --> 00:02:07.479 are perspiring profusely. What purpose does sweating serve? As sweat evaporates from your 00:02:07.479 --> 00:02:12.590 skin, it helps to cool you down. 00:02:12.590 --> 00:02:15.970 Thermoregulation serves to control our body's temperature much like a thermostat regulating 00:02:15.970 --> 00:02:20.900 the temperature of a room. From this diagram, you can see how thermoregulation follows a 00:02:20.900 --> 00:02:25.710 simple loop. Because the response in this case is always to reverse a given change in 00:02:25.710 --> 00:02:30.840 body temperature, we call this a negative feedback loop. Positive feedback loops are 00:02:30.840 --> 00:02:35.360 just the opposite. When a change occurs in a system, a positive feedback loop acts to 00:02:35.360 --> 00:02:40.760 increase or exacerbate it. Labor preceding childbirth is a classic example of a positive 00:02:40.760 --> 00:02:45.190 feedback loop. Contractions of the uterus during childbirth stimulate the release of 00:02:45.190 --> 00:02:50.970 a hormone, oxytocin, which in turn induces more uterine contractions. The self-amplifying 00:02:50.970 --> 00:02:55.760 nature of the positive feedback loop is repeated over and over with increasing intensity until 00:02:55.760 --> 00:03:02.180 the baby is born. After birth, contractions stop and the feedback loop ceases. Here are 00:03:02.180 --> 00:03:07.370 a few well-known physiological processes and parameters that involve feedback loops. Pause 00:03:07.370 --> 00:03:12.010 the video here and determine which ones involve negative feedback and which ones involve positive 00:03:12.010 --> 00:03:18.180 feedback. 00:03:18.180 --> 00:03:22.350 Negative feedback loops minimize deviations within a system, keeping its parameters close 00:03:22.350 --> 00:03:27.790 to a desired set-point. The control of blood glucose, blood pressure, and breathing rates 00:03:27.790 --> 00:03:31.650 rely heavily on negative feedback loops. 00:03:31.650 --> 00:03:35.450 Positive feedback loops, on the other hand, tend to destabilize a system by amplifying 00:03:35.450 --> 00:03:41.600 a stimulus towards an extreme. This is important in irreversible processes such as action potential 00:03:41.600 --> 00:03:45.560 generation, blood clotting, lactation, and ovulation. 00:03:45.560 --> 00:03:52.560 There are many other examples of negative and positive feedback loops in the human body. 00:03:57.030 --> 00:04:01.540 Now let's take a closer look at specific components involved in a feedback loop. 00:04:01.540 --> 00:04:06.310 The receptor is the component that detects and measures changes in a given parameter. 00:04:06.310 --> 00:04:12.910 The receptor then relays this information to a control center. The control center compares 00:04:12.910 --> 00:04:18.259 the measured parameter to a desired set point. Based on the extent of deviation, the control 00:04:18.259 --> 00:04:24.710 center decides on the appropriate response and sends signals to an effector. Effectors 00:04:24.710 --> 00:04:30.120 can be muscles, organs, or any other component that receives signals from the control center. 00:04:30.120 --> 00:04:36.990 In response to these signals, the effectors can either enhance or reduce the deviation. 00:04:36.990 --> 00:04:42.770 If the deviation is enhanced, we have a positive feedback loop. And if it is reduced, we have 00:04:42.770 --> 00:04:49.770 a negative feedback loop. Now let's go back to our thermoregulation example in greater 00:04:53.940 --> 00:05:00.860 detail. Here, the parameter we are controlling is internal body temperature. The receptors 00:05:00.860 --> 00:05:05.970 of temperature in our body are specialized cells called thermoreceptors. Thermoreceptors 00:05:05.970 --> 00:05:10.020 continually monitor our temperature and pass this information to the control center in 00:05:10.020 --> 00:05:15.340 the brain, called the hypothalamus. The hypothalamus compares the measured temperatures to the 00:05:15.340 --> 00:05:22.340 set point of 37 ᵒC. It can distinguish temperature differences as small as a hundredth of a degree! 00:05:22.600 --> 00:05:27.220 The hypothalamus then sends out signals to effectors in the body to initiate corrective 00:05:27.220 --> 00:05:30.880 mechanisms when our temperatures are too high or too low. The effectors react accordingly 00:05:30.880 --> 00:05:34.250 to adjust our body temperature. When we are too hot, signals are sent to activate our 00:05:34.250 --> 00:05:39.740 sweat glands. Blood vessels in the skin are also stimulated to dilate. These changes increase 00:05:39.740 --> 00:05:45.480 heat loss from the skin. When we are too cold, signals are sent to the tissues to increase 00:05:45.480 --> 00:05:51.389 metabolism, and to the muscles to induce shivering. These mechanisms generate heat. Signals are 00:05:51.389 --> 00:05:58.389 also sent to constrict blood vessels and to raise skin hairs to minimize heat loss. 00:06:01.820 --> 00:06:05.520 Both negative and positive feedback loops are equally important for the healthy functioning 00:06:05.520 --> 00:06:10.520 of one's body. Complications can arise from failure of these mechanisms. 00:06:10.520 --> 00:06:14.199 Let's look at blood glucose regulation, and see what happens if the feedback mechanisms 00:06:14.199 --> 00:06:20.340 do not work as they should. Glucose is the primary source of energy for the body's cells. 00:06:20.340 --> 00:06:25.430 Thus, in a healthy individual, glucose levels are tightly regulated to maintain a fairly 00:06:25.430 --> 00:06:30.479 constant and optimal supply in the bloodstream. Too little or too much blood glucose could 00:06:30.479 --> 00:06:35.949 be damaging to the body. Circulating levels of glucose are monitored by specialized beta 00:06:35.949 --> 00:06:41.850 cells in islets of Langerhans in the pancreas. In response to high glucose levels, for example 00:06:41.850 --> 00:06:46.430 after a large meal, these same beta cells release a hormone called insulin into the 00:06:46.430 --> 00:06:51.510 bloodstream. Insulin is transported to the rest of the body, whereupon it stimulates 00:06:51.510 --> 00:06:57.690 the cells to take up and adsorb glucose. Insulin also promotes the storage of glucose in muscle 00:06:57.690 --> 00:07:04.050 and liver tissues. These actions act to remove glucose from the blood, thus lowering blood 00:07:04.050 --> 00:07:09.830 glucose levels to the normal set-point. Blood glucose regulation is a classic example of 00:07:09.830 --> 00:07:15.080 negative feedback. Pause the video and identify the receptor, control center, and effectors 00:07:15.080 --> 00:07:22.080 in this particular feedback system. 00:07:24.310 --> 00:07:29.550 The receptors are the beta cells in the pancreas. These same cells also act as the control center 00:07:29.550 --> 00:07:34.880 and send signals to the effectors in the form of insulin. The effectors are the cells of 00:07:34.880 --> 00:07:41.880 the body that increase their uptake and storage of glucose in response to insulin. Now let's 00:07:41.990 --> 00:07:46.360 see how failure of the body to regulate blood glucose levels results in a disease known 00:07:46.360 --> 00:07:52.100 as diabetes. There are two common forms of the disease. 00:07:52.100 --> 00:07:57.490 In Type I diabetes, also known as early-onset diabetes, the insulin-producing beta cells 00:07:57.490 --> 00:08:03.180 of the pancreas are destroyed due to an autoimmune reaction. In this form of diabetes, the body 00:08:03.180 --> 00:08:09.289 is unable to produce insulin. The lack of insulin means that the cells of the body do 00:08:09.289 --> 00:08:14.270 not take up glucose in response to increasing glucose levels. Hence, glucose levels in the 00:08:14.270 --> 00:08:21.270 blood continue to rise. In Type II diabetes, or late-onset diabetes, insulin is still produced 00:08:22.080 --> 00:08:26.930 but the body's cells no longer respond to it. This is often due to years of insulin 00:08:26.930 --> 00:08:33.198 over-production, caused by a high-sugar diet. The inability of cells to use insulin properly 00:08:33.198 --> 00:08:40.120 means that high glucose levels in the blood persist with time. Type I and Type II diabetes 00:08:40.120 --> 00:08:45.019 have very similar symptoms but they arise from different causes. Pause the video and 00:08:45.019 --> 00:08:52.019 identify the parts of the feedback loop that break down in each. 00:08:55.050 --> 00:09:00.649 In Type I diabetes, there is a lack of signaling between the control center and the effectors. 00:09:00.649 --> 00:09:06.970 In Type II diabetes, the effectors fail to respond to the signals from the control center. 00:09:06.970 --> 00:09:11.420 The end result is the same in both forms of the disease - glucose accumulates to toxic 00:09:11.420 --> 00:09:17.129 levels in the blood. If left unchecked, diabetes can lead to kidney failure, blindness, and 00:09:17.129 --> 00:09:22.429 heart disease. Feedback loop breakdown leads to the development of an unstable internal 00:09:22.429 --> 00:09:26.800 environment. This tips the scales towards imbalance, increasing the risk of illness 00:09:26.800 --> 00:09:31.860 and progressive damage to the body. Many other diseases and pathological conditions follow 00:09:31.860 --> 00:09:38.860 a similar progression. 00:09:39.309 --> 00:09:43.660 In this video, you learned about negative and positive feedback loops, their general 00:09:43.660 --> 00:09:49.980 components, and how they regulate the flow of information. We identified feedback loops 00:09:49.980 --> 00:09:56.980 in the body and examined their role in childbirth, thermoregulation, and blood glucose regulation. 00:09:57.100 --> 00:10:02.230 Our body relies on a finely-tuned system of checks and balances to function smoothly. 00:10:02.230 --> 00:10:06.279 Feedback loops, like the ones described in this video, provide this balance. Failure 00:10:06.279 --> 00:10:11.239 of these mechanisms, as in the case of diabetes, can be detrimental. We hope that you will 00:10:11.239 --> 00:10:15.139 apply your knowledge of feedback loops and the consequences of their failure in the study 00:10:15.139 --> 00:10:20.540 of other biological processes and diseases.