Homeostatic Loops

Hi. It's Mr. Andersen and in this podcast I'm going to show you how humans use feedback loops to maintain homeostasis. And so in a house we use a thermostat. We basically set it at 70 degrees fahrenheit. If the temperature goes below that your heater kicks on. If it goes above that your heater kicks off. And so this is a homeostatic loop. Basically all homeostatic loops are going to have a receptor. In this case it would be the thermostat. And then they're going to have effectors. The effectors are going to be the parts that are effected by changes in the stimuli. So let's go back to the thermostat. Let's say the temperature goes above 70 degrees. Then it's going to turn off. And let's say it goes below that, then it's going to turn it on. And so that's going to raise the temperature. And so basically every homeostatic loop is going to have this figure eight shape. And we're going to have the receptors in the middle and the effectors on either side. And so when you're learning biology there are actually four homeostatic mechanisms that are super important. The first one is how we maintain temperature, stable internal temperature. How we thermoregulate. And so in humans the standard is 37 degrees celsius. The thermostat for us is the hypothalamus. And so it sits right there above the roof of your mouth. It's in the lower portion of the brain. And so it's going to be measuring the temperature of your blood. And so if the temperature goes above that, and so if it's going to go higher than that, then we're going to send a message to the capillaries and to the sweat glands. And so what are we going to do? Well if it's the capillaries what we're going to do is those are going to dilate. In other words they're going to get larger. So we can carry more blood near the surface of our skin to lose more heat. What are the sweat glands going to do? They're just simply going to sweat. How does the message get from the hypothalamus out here to the capillaries and sweat glands? That's going to be nerves. Nerves that are sending that message out to the capillaries and to the sweat glands. Basically they lower the temperature until it hits that set point. But let's say it goes too low. Then what's going to happen? If it goes lower then 37 degrees, then we are going to trigger muscles. And those are going to shiver. We're going to trigger the capillaries and those are going to constrict. Or they're going to vasoconstrict. Basically holding heat closer to our body. And then if we're talking about the skin we might get some goosebumps. And so what's the purpose of that? It's basically our skin kind of pulling itself in tight. And so all of these things are going to work together to raise the body temperature until it hits 37 degrees. And so here's that characteristic again. Figure 8 shape. Okay. Let's go to the next one then. Next one is blood glucose. Blood glucose is super important because glucose remember is the fuel that fuels living cells. And so blood glucose should stay at about 90 milligrams per 100 milliliters. What's going to be sensing that is going to be the pancreas. And there are some important cells on the surface of the pancreas called the islets of langerhans. And basically what happens is if the insulin or excuse me, if the blood glucose goes higher than this. So if it goes up then we're going to secrete insulin. And that will actually come from some cells called the beta cells. So the beta cells are going to be on the surface of the pancreas. And they're going to secrete insulin. What that's going to do to the body cells is the body cells are going to take in glucose. How does that work? Well to be more specific, it basically the insulin sets up a signal transduction pathway that opens up a protein called glut. It's G L U T. It just stands for glucose transport. And that's going to allow the glucose to go into the cell. What's going to happen at the liver? Well at the liver the glucose isn't going to go necessarily into the cell and be used for fuel. It's actually going to be converted in the liver to glycogen. So we're going to take that glucose which is a monosaccharide. And we're going to string it together to make a polysaccharide which is glycogen so we can use it later. So what are these two things going to do? Together they're going to lower the blood glucose. But let's say it goes too low. What's going to happen? Well if it goes too low then we're going to secrete another hormone called glucagon. That's going to be secreted by the alpha cells in the pancreas. And that's essentially going to the liver. The liver now is going to take glycogen. And that glycogen that we stored earlier and it's going to convert that into glucose. Now we have more glucose in the blood and that's going to raise that blood glucose level until we hit a point. And so it's going to keep going and going and going like this. Alright. Let's go to the third one. Third one is going to be blood calcium. Blood calcium is super important because we use calcium in the firing of our nerves. Especially at the synapses. Or even muscles use calcium as well. So we have to have the perfect amount. 10 milligrams per 100 milliliters. So basically in this case it's not the pancreas but it's going to be the thyroid. And the thyroid sits right here. It's actually going to have two glands within it. There's going to be the thyroid which looks almost like a butterfly. And then we're going to have the parathyroid. Parathyroid are going to be these little glands within the thyroid. And so these are the two hormones that we're using this time. Let's say our blood calcium level goes too high. So if it goes higher than this, then we're going to secrete a chemical called calcitonin. Calcitonin is going to be secreted by the thyroid. So the thyroid is going to give off calcitonin. It's a part of the endocrine system. That hormone is going to diffuse. And it's going to effect the kidney. It's basically going to tell the kidney that we have too much blood calcium. And so you can just, you can secrete that. You can let it go. It's going to end up taking off in our urine. Or it's going to effect the bones. And it's going to deposit that calcium. So calcium CA2+ is going to get deposited in the bones. What are those two things going to do? They're going to lower the blood calcium level until it goes below this point. Let's say it goes too low. What's going to happen next? Well that parathyroid gland is going to secrete a hormone called PTH. So that's going to come from the parathyroid. Parathyroid hormone is what it's called. Which is easy to remember because it comes from the parathyroid. What it's going to do is going to effect a number of different parts of our body. If we go to the intestine. The intestine remember, part of our digestive system. Basically the message to the intestine is we need more calcium. And so it's going to pull more calcium out of our food. What's it going to do to the kidney? It's going to hold on to that calcium. It's not going to excrete that. And then in the bones it's going to release that calcium. So those calcium ions are going to be released from the bone right into the blood supply. All three of these things are going to increase the blood calcium levels. And so here's that feedback loop. We're almost there. Let's go to number four. What's the fourth one? That's osmolarity. Osmolarity is going to be the concentration of the blood. Remember if it's hypertonic or hypotonic we're going to have serious problems when it comes to cells. And so it should stay around 300 milliosmoles per liter. Let's say that it goes up. So if it goes higher than that, this is something that students are sometimes confused with. If it's going higher than that, let's say it's 500 or 600, this is a human that is now dehydrated. They have way more of the solute for a unit of solution. And so we're in trouble here. And so who's sensing that? Well it's going to be the hypothalamus is going to sense it. But it's going to send a message to the pituitary. Pituitary gland then is going to release a chemical called ADH. ADH stands for antidiuretic hormone. So it's a hormone that's going to stop our body from releasing water. That's going to effect the kidney. The nephron of the kidney. It's mostly going to effect the collecting tubule of the nephron. But what's the message to the kidney? The message to the kidney is that we need more water. So it's going to hold on to water and we're not going to lose as much of that water in our urine. And so if your urine looks really really concentrated or really really yellow in color, that's basically an increase in ADH coming from the pituitary. It's telling your body hold on to that. So it's eventually going to drop off. We also can have trouble if our osmolarity goes too low. In other words it's too dilute. Well now we're going to not send a new hormone. We're just going to send less of the antidiuretic hormone form the pituitary. We're going to lower the amount of antidiuretic hormone. What does that do to the kidney? It actually allows us to lose more of that water. And what's that going to do? It's going to increase the osmolarity. So we have the fourth type of a homeostatic loop. And all of these things are helping our body maintain that internal stable environment. And I hope that all is helpful.