0:00 In this video we're going to talk about homeostasis, this is an overview. 0:13 Although the environment around an organism changes, the organism maintains 0:17 relatively 0:17 stable internal conditions. 0:20 This ability to maintain internal stability is called homeostasis. 0:25 For example, if a person stands in a cold, wintery night or hot sub-Saharan 0:31 Africa, the 0:32 person is able to maintain a normal internal body temperature between 36 to 37 0:39 degrees 0:39 or 97 to 99 degrees Fahrenheit through homeostasis. 0:45 The main mechanism to maintain a homeostatic environment is through negative 0:49 feedback. 0:50 Negative feedback is where the body senses change and activates mechanisms that 0:55 negate 0:55 or reverses it. 1:00 The term homeostasis was coined by an American physiologist Walter Cannon to 1:04 explain this 1:05 tendency to maintain internal stability of the body. 1:08 However, the internal stability that is maintained is not absolute. 1:12 Rather, the internal state is maintained between a limited range and it is 1:17 useful to use the 1:18 term dynamic equilibrium. 1:21 And a good example of this is the internal body temperature, which is usually 1:25 maintained 1:25 between 36 degrees and 37 degrees, for example. 1:30 Again, the main mechanism to maintain a homeostatic environment is through 1:36 negative feedback. 1:37 So let's look at how the body maintains normal body temperatures using negative 1:43 feedback. 1:44 The stimulus is, for example, an increase in body temperature because perhaps 1:51 the person 1:52 is exercising or it's super hot outside. 1:56 This increase in body temperature will be detected by the brain, which is the 2:01 control 2:01 center and will activate mechanisms to lose heat to keep your body cool, and 2:08 this is done 2:08 via the hypothalamus. 2:13 The hypothalamus will send signals out that will cause vasodilation, which is 2:19 widening 2:19 of the vessels. 2:21 When blood vessels of the skin dilate, warm blood flows closer to the body 2:27 surface and 2:27 loses heat to the surrounding air. 2:30 If this is not enough to return your temperature to normal, sweating occurs 2:35 through activation 2:36 of sweat glands and sweating is the evaporation of water from the skin and has 2:41 a powerful cooling 2:42 effect. 2:46 Now you can imagine when it is hot outside you are sweating and appear flushed 2:52 from the 2:52 dilation of the superficial blood vessels in the skin. 2:56 All these mechanisms are causing the body to reduce the body's internal 3:02 temperature. 3:02 When the body's internal temperature returns to normal, the brain's hypothal 3:08 amus, heat 3:09 loss center, shuts off, and this is done through negative feedback. 3:14 The negative feedback essentially stops the hypothalamus's heat loss center 3:20 because the 3:20 body does not need to lose any more heat. 3:25 Conversely, if the stimulus causes a decrease in body temperature, such as the 3:30 cold weather 3:31 outside, let us just say your body temperature drops much below 36 degrees 3:39 Celsius. 3:40 The brain, which is the control center, activates heat conserving mechanisms. 3:48 The first to be activated is vasoconstriction, a narrowing of the blood vessel 3:54 in the skin, 3:55 which serves to retain warm blood deeper in your body and minimize heat loss 4:01 from the 4:02 skin. 4:06 If this is not enough, the brain activates shivering muscle contractions, trem 4:11 ors that 4:11 will generate heat. 4:15 These mechanisms increase body temperature and when the body temperature is 4:19 back to normal, 4:20 there will be a negative feedback to the brain telling it to shut off the heat- 4:25 promoting 4:26 centers. 4:28 Body temperature will increase and the hypothalamus heat-promoting center will 4:35 shut off. 4:36 That was an example of how negative feedback works to maintain an internal 4:41 stability and 4:42 we used internal body temperatures. 4:47 While negative feedback is the main driver of homeostasis, there is also 4:50 something called 4:51 positive feedback, which is a self-amplifying cycle in which a physiological 4:56 change leads 4:57 to an even greater change in the same direction. 5:02 In a way, positive feedback tries to maintain homeostasis, but often with a 5:07 price. 5:08 Let's take a look at a positive feedback example, which is a woman 5:13 breastfeeding lactation. 5:15 A baby's suckling on the mother's nipple will activate mechanoreceptors in the 5:19 nipple. 5:20 The receptors will send signals to the brain via neurons, which are the brain 5:25 cells, telling 5:25 the brain's pituitary gland to release two important hormones, prolactin and 5:33 oxytocin. 5:35 Prolactin is a hormone which stimulates milk production in the breast tissue. 5:39 The oxytocin is a hormone which stimulates muscle contraction, smooth muscle 5:45 contraction 5:45 of the breast, allowing the milk produced to be ejected out of the nipple. 5:51 The milk ejected is taken by the baby and the whole process continues. 5:56 The baby's suckling will activate mechanoreceptors which will stimulate the 6:00 brain. 6:01 This is a positive feedback loop because nothing is being suppressed here. 6:06 It is amplifying a response. 6:08 And as you can see, positive feedback is amplifying a response in the same 6:13 direction, whereas negative 6:14 feedback negates the response to maintain internal stability. 6:20 Positive feedback can be dangerous because of this self-amplifying capability, 6:25 which 6:25 can quickly change the internal state of the body to something far from its 6:29 homeostatic 6:30 set point. 6:32 So I hope you enjoyed this video on homeostasis. 6:34 We looked at the two main mechanisms. 6:37 So I hope you enjoyed this video on homeostasis. 6:40 The main mechanism of homeostasis is through negative feedback, but also there 6:44 's something 6:44 called positive feedback which is important to know. 6:47 Thank you for watching.
