0610 · 14.4Homeostasis

Homeostasis and negative feedback

0:00Stand in the snow, and your body temperature is about 37 degrees Celsius. Stand in a desert at midday, and your body temperature is still about 37 degrees. Eat a big bag of sweets, or skip lunch, and the amount of sugar in your blood soon settles back to almost exactly where it was. Outside, conditions change all the time. Inside, your body keeps things remarkably steady. How? By the end of this video you will be able to describe homeostasis, explain how negative feedback keeps conditions close to a set point, describe how insulin, glucagon and the liver control blood glucose, outline how Type 1 diabetes is treated, identify the parts of the skin, and explain how mammals keep a constant body temperature.

0:42The syllabus describes homeostasis as the maintenance of a constant internal environment. The internal environment means the conditions inside the body that surround your cells: things like the temperature, the concentration of glucose in the blood, and the amount of water. Why does it need to stay constant? Because your cells depend on it. The chemical reactions in your cells are controlled by enzymes, and enzymes work best at around body temperature.

1:07Too hot, and they are denatured. Too cold, and the reactions slow down. Your cells need a steady supply of glucose for respiration. And if the water or glucose concentration changes too much, water moves into or out of cells by osmosis, and they can be damaged. So keeping the inside constant keeps the cells working. For the extended syllabus, you need to explain how homeostasis works, using the idea of negative feedback with reference to a set point.

1:33So let's start with the set point. The set point is the ideal value that the body is trying to keep something at. For body temperature, the set point is about 37 degrees Celsius. The actual value is never perfectly still. It drifts a little above and a little below. What matters is that it is always pulled back towards the set point. Negative feedback is the way it is pulled back.

1:56When a condition moves away from the set point, receptors detect the change. Then the body makes a response that reverses the change, bringing the condition back towards the set point. And once it is back, the response is switched off. It is called negative feedback because the response always goes in the opposite direction to the change. If something goes up, the response brings it down. If something goes down, the response brings it up.

2:19A good everyday example is a heating system with a thermostat. You set the thermostat to 20 degrees. That is the set point. If the room cools below 20 degrees, the thermostat detects it and switches the heater on. The room warms up. Once it reaches 20 degrees, the thermostat switches the heater off. If the room then gets too warm, the heater stays off and the room cools. The temperature keeps swinging a little above and below 20, but it never moves far away.

2:45Your body does the same thing with its internal environment.

Blood glucose control and diabetes

2:48Let's see negative feedback at work in the control of blood glucose. Glucose is the sugar your cells use in respiration. Its concentration in the blood needs to stay close to its set point. After you eat a meal containing carbohydrate, glucose is absorbed from your gut into the blood, and the blood glucose concentration rises. And when you exercise, or go a long time without food, your cells use glucose up, so the blood glucose concentration falls.

3:14Both of these have to be corrected. The control involves three things: the pancreas, the liver, and two hormones, insulin and glucagon. The pancreas monitors the blood glucose concentration. It is also the gland that makes both hormones. And the liver is the target organ, which stores glucose or releases it. First, when blood glucose is too high. The pancreas detects the rise, and it secretes insulin into the blood.

3:40Insulin travels to the liver. Insulin causes the liver to take in glucose from the blood and convert it into glycogen. Glycogen is a large, insoluble carbohydrate that the liver can store. Insulin also makes muscle cells and other body cells take up more glucose from the blood. So the blood glucose concentration decreases, back towards the set point. That is the core point you need to remember: insulin decreases blood glucose concentration.

4:04Now, when blood glucose is too low. The pancreas detects the fall, and it secretes glucagon into the blood. Glucagon travels to the liver. Glucagon causes the liver to break down its stored glycogen into glucose, and to release that glucose into the blood. So the blood glucose concentration increases, back towards the set point. Notice the negative feedback. A rise in glucose causes a response that lowers it. A fall in glucose causes a response that raises it.

4:32And once the concentration is back at the set point, the pancreas stops secreting extra hormone. Insulin and glucagon work as a pair, pulling in opposite directions. The words glucose, glycogen and glucagon look very alike, so be precise. Glucose is the sugar in the blood. Glycogen is the store of glucose in the liver. And glucagon is the hormone that tells the liver to break glycogen down. What happens when this system breaks down?

4:59In Type 1 diabetes, the cells in the pancreas that make insulin are destroyed by the body's own immune system. So the pancreas cannot make enough insulin. After a meal, the liver is never told to store the extra glucose, and the blood glucose concentration becomes much too high. Glucose even appears in the urine. Without treatment, a person with Type 1 diabetes becomes very ill. The syllabus asks you to outline the treatment of Type 1 diabetes.

5:24There are three main parts. First, insulin. People with Type 1 diabetes inject insulin, or use a small pump, to replace the insulin that their pancreas cannot make. Second, monitoring. They test their blood glucose concentration regularly, often by pricking a finger or using a small sensor, so they know how much insulin they need. And third, diet and exercise. They manage how much carbohydrate they eat, and when, and they take account of exercise, because exercise uses up glucose.

5:52The aim is to keep the blood glucose concentration as close to normal as possible.

Skin structures and temperature detection

5:57Now let's turn to the second big example: body temperature. The skin plays the main part, so first you need to identify its structures in diagrams. There are seven you need to know. Hairs grow out of the skin. Each hair has a tiny muscle attached to its base called a hair erector muscle, which can pull the hair upright. Sweat glands are coiled tubes deep in the skin, which make sweat and release it on to the surface through a pore.

6:22Receptors detect stimuli such as temperature, and they are connected to sensory neurones, which carry impulses to the brain. Blood vessels run through the skin, including arterioles, which are small arteries, and capillaries near the surface. And under the skin is a layer of fatty tissue. The syllabus asks you to describe temperature control in terms of insulation, sweating, shivering and the role of the brain. Let's start with the brain, because it controls everything else.

6:46The brain acts as the control centre. A part of the brain called the hypothalamus contains receptors that monitor the temperature of the blood flowing through it. The brain also receives impulses from temperature receptors in the skin, along sensory neurones. It compares the temperature with the set point of 37 degrees. If the body is too hot or too cold, the brain sends impulses along motor neurones to the effectors, the sweat glands, the muscles and the blood vessels, to bring the temperature back.

7:14That is negative feedback again.

Sweating, shivering and insulation

7:16When the body is too hot, one response is sweating. The sweat glands produce more sweat, which comes out on to the surface of the skin. The water in the sweat evaporates. Evaporation needs heat energy, and it takes that energy from the skin. So as the sweat evaporates, it cools the skin, and heat is lost from the body. The key word is evaporation. Sweat that just drips off without evaporating does not cool you much.

7:41When the body is too cold, one response is shivering. Shivering is rapid, automatic contraction and relaxation of skeletal muscles. Contracting muscles respire faster, and respiration releases heat energy. So shivering warms the body up. At the same time, sweating is reduced, so less heat is lost by evaporation. Next, insulation. An insulator is a material that reduces the loss of heat. The layer of fatty tissue under the skin is a good insulator, and it reduces heat loss from the body all the time.

8:12Hair is the other form of insulation. When the body is cold, the hair erector muscles contract, and they pull the hairs upright. The upright hairs trap a layer of air next to the skin. Air is a poor conductor of heat, so this layer of trapped air reduces heat loss. When the body is hot, the hair erector muscles relax, the hairs lie flat, and less air is trapped.

8:33In furry mammals, this makes a big difference. In humans, we have so little body hair that the main effect you notice is goosebumps.

Vasodilation and exam answers

8:41And the last response, for the extended syllabus, involves the blood vessels. The syllabus asks you to describe vasodilation and vasoconstriction of the arterioles supplying the skin surface capillaries. Remember, an arteriole is a small artery, and here the arterioles carry blood to the capillaries near the surface of the skin. When the body is too hot, the arterioles supplying the skin surface capillaries widen. This is called vasodilation. Vaso means vessel, and dilation means widening.

9:11The muscles in the walls of the arterioles relax, so the arterioles get wider, and much more blood flows through the capillaries near the surface of the skin. Heat is lost from this blood to the surroundings by radiation. That is why your skin looks red and flushed when you are hot. When the body is too cold, the arterioles supplying the skin surface capillaries narrow. This is called vasoconstriction.

9:32The muscles in the walls of the arterioles contract, so the arterioles get narrower, and much less blood flows through the capillaries near the surface. The warm blood is kept deeper in the body, so less heat is lost by radiation. That is why your skin can look pale when you are cold. Be careful here, because this is one of the most common mistakes in the whole topic. It is the arterioles that widen and narrow, not the capillaries.

9:55And the blood vessels do not move up or down in the skin. They stay exactly where they are. What changes is how much blood flows through the surface capillaries. Let's see how this can be tested. A typical question says: a person runs a race on a hot day, and their body temperature starts to rise. Describe how the body brings their temperature back to normal. Build the answer around negative feedback.

10:17First, detection: receptors in the brain detect that the blood temperature is above the set point, and receptors in the skin detect the rise too. Then the responses. The sweat glands produce more sweat, and the evaporation of the sweat removes heat from the skin. The arterioles supplying the skin surface capillaries dilate, so more blood flows near the surface and more heat is lost by radiation. The hair erector muscles relax, so the hairs lie flat.

10:42And then the result: the temperature falls back to the set point, and the responses switch off. Another question might give a graph of blood glucose after a meal and ask you to explain the shape. The glucose rises as carbohydrate is absorbed. The pancreas detects the rise and secretes insulin. Insulin causes the liver to convert glucose into glycogen, and the cells to take up glucose. So the concentration falls back to the set point.

Common mistakes and recap

11:08Now some common mistakes. First, saying the capillaries dilate, or move closer to the surface. It is the arterioles that dilate or constrict, and no vessel moves. Second, saying that insulin converts glucose into glycogen. Insulin is the message. The liver does the converting. Say that insulin causes the liver to convert glucose into glycogen. Third, mixing up glucagon and glycogen. Glucagon is the hormone. Glycogen is the store. Fourth, saying that sweat cools you just by being on the skin.

11:41The cooling comes from evaporation, which takes heat energy from the skin. And fifth, forgetting to mention the set point and negative feedback in a homeostasis question. The examiner wants to know that the change is detected, that the response reverses it, and that it returns the value to the set point. Let's recap. Homeostasis is the maintenance of a constant internal environment. It works by negative feedback: when a condition moves away from its set point, the change is detected and a response reverses it, bringing it back to the set point.

12:14When blood glucose is too high, the pancreas secretes insulin, which decreases the blood glucose concentration, by causing the liver to convert glucose into glycogen. When blood glucose is too low, the pancreas secretes glucagon, which causes the liver to break down glycogen into glucose. Type 1 diabetes is treated with insulin injections, regular blood glucose monitoring, and managing diet and exercise. The skin contains hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue.

12:45The brain monitors body temperature and controls the responses. When we are too hot, we sweat, and the evaporation cools us, and the arterioles supplying the skin surface capillaries dilate, so more heat is lost by radiation. When we are too cold, we shiver, the hairs are raised to trap air, the fatty tissue insulates us, and the arterioles constrict, so less heat is lost. So whether you are standing in the snow, or in the desert, or eating a whole bag of sweets, your body is constantly measuring, correcting and bringing things back to where they should be.