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### Course: Biology archive>Unit 12

Lesson 4: Feedback

# Homeostasis

Learn how organisms maintain homeostasis, or a stable internal environment.

## Key points

• Homeostasis is the tendency to resist change in order to maintain a stable, relatively constant internal environment.
• Homeostasis typically involves negative feedback loops that counteract changes of various properties from their target values, known as set points.
• In contrast to negative feedback loops, positive feedback loops amplify their initiating stimuli, in other words, they move the system away from its starting state.

## Introduction

What's the temperature in the room where you're sitting right now? My guess would be that it's not exactly $98.6{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{F}$/ $37.0{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{C}$. Yet, your body temperature is usually very close to this value. In fact, if your core body temperature doesn't stay within relatively narrow limits—from about $95{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{F}$/ $35{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{C}$ to $107{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{F}$/ $41.7{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{C}$—the results can be dangerous or even deadly.${}^{1}$
The tendency to maintain a stable, relatively constant internal environment is called homeostasis. The body maintains homeostasis for many factors in addition to temperature. For instance, the concentration of various ions in your blood must be kept steady, along with pH and the concentration of glucose. If these values get too high or low, you can end up getting very sick.
Homeostasis is maintained at many levels, not just the level of the whole body as it is for temperature. For instance, the stomach maintains a pH that's different from that of surrounding organs, and each individual cell maintains ion concentrations different from those of the surrounding fluid. Maintaining homeostasis at each level is key to maintaining the body's overall function.
So, how is homeostasis maintained? Let's answer this question by looking at some examples.

## Maintaining homeostasis

Biological systems like those of your body are constantly being pushed away from their balance points. For instance, when you exercise, your muscles increase heat production, nudging your body temperature upward. Similarly, when you drink a glass of fruit juice, your blood glucose goes up. Homeostasis depends on the ability of your body to detect and oppose these changes.
Maintenance of homeostasis usually involves negative feedback loops. These loops act to oppose the stimulus, or cue, that triggers them. For example, if your body temperature is too high, a negative feedback loop will act to bring it back down towards the set point, or target value, of $98.6{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{F}$/ $37.0{\phantom{\rule{0.167em}{0ex}}}^{\circ }\text{C}$.
How does this work? First, high temperature will be detected by sensors—primarily nerve cells with endings in your skin and brain—and relayed to a temperature-regulatory control center in your brain. The control center will process the information and activate effectors—such as the sweat glands—whose job is to oppose the stimulus by bringing body temperature down.
Of course, body temperature doesn't just swing above its target value—it can also drop below this value. In general, homeostatic circuits usually involve at least two negative feedback loops:
• One is activated when a parameter—like body temperature—is above the set point and is designed to bring it back down.
• One is activated when the parameter is below the set point and is designed to bring it back up.
To make this idea more concrete, let's take a closer look at the opposing feedback loops that control body temperature.

## Homeostatic responses in temperature regulation

If you get either too hot or too cold, sensors in the periphery and the brain tell the temperature regulation center of your brain—in a region called the hypothalamus—that your temperature has strayed from its set point.
For instance, if you’ve been exercising hard, your body temperature can rise above its set point, and you’ll need to activate mechanisms that cool you down. Blood flow to your skin increases to speed up heat loss into your surroundings, and you might also start sweating so the evaporation of sweat from your skin can help you cool off. Heavy breathing can also increase heat loss.
On the other hand, if you’re sitting in a cold room and aren’t dressed warmly, the temperature center in the brain will need to trigger responses that help warm you up. The blood flow to your skin decreases, and you might start shivering so that your muscles generate more heat. You may also get goose bumps—so that the hair on your body stands on end and traps a layer of air near your skin—and increase the release of hormones that act to increase heat production.
Notably, the set point is not always rigidly fixed and may be a moving target. For instance, body temperature varies over a 24-hour period, from highest in the late afternoon to lowest in the early morning.${}^{2}$ Fever also involves a temporary increase in the temperature set point so that heat-generating responses are activated at temperatures higher than the normal set point.${}^{3}$

## Disruptions to feedback disrupt homeostasis.

Homeostasis depends on negative feedback loops. So, anything that interferes with the feedback mechanisms can—and usually will!—disrupt homeostasis. In the case of the human body, this may lead to disease.
Diabetes, for example, is a disease caused by a broken feedback loop involving the hormone insulin. The broken feedback loop makes it difficult or impossible for the body to bring high blood sugar down to a healthy level.
To appreciate how diabetes occurs, let's take a quick look at the basics of blood sugar regulation. In a healthy person, blood sugar levels are controlled by two hormones: insulin and glucagon.
Insulin decreases the concentration of glucose in the blood. After you eat a meal, your blood glucose levels rise, triggering the secretion of insulin from β cells in the pancreas. Insulin acts as a signal that triggers cells of the body, such as fat and muscle cells, to take up glucose for use as fuel. Insulin also causes glucose to be converted into glycogen—a storage molecule—in the liver. Both processes pull sugar out of the blood, bringing blood sugar levels down, reducing insulin secretion, and returning the whole system to homeostasis.
Glucagon does the opposite: it increases the concentration of glucose in the blood. If you haven’t eaten for a while, your blood glucose levels fall, triggering the release of glucagon from another group of pancreatic cells, the α cells. Glucagon acts on the liver, causing glycogen to be broken down into glucose and released into the bloodstream, causing blood sugar levels to go back up. This reduces glucagon secretion and brings the system back to homeostasis.
Diabetes happens when a person's pancreas can't make enough insulin, or when cells in the body stop responding to insulin, or both. Under these conditions, body cells don't take up glucose readily, so blood sugar levels remain high for a long period of time after a meal. This is for two reasons:
• Muscle and fat cells don't get enough glucose, or fuel. This can make people feel tired and even cause muscle and fat tissues to waste away.
• High blood sugar causes symptoms like increased urination, thirst, and even dehydration. Over time, it can lead to more serious complications.${}^{4,5}$

## Positive feedback loops

Homeostatic circuits usually involve negative feedback loops. The hallmark of a negative feedback loop is that it counteracts a change, bringing the value of a parameter—such as temperature or blood sugar—back towards it set point.
Some biological systems, however, use positive feedback loops. Unlike negative feedback loops, positive feedback loops amplify the starting signal. Positive feedback loops are usually found in processes that need to be pushed to completion, not when the status quo needs to be maintained.
A positive feedback loop comes into play during childbirth. In childbirth, the baby's head presses on the cervix—the bottom of the uterus, through which the baby must emerge—and activates neurons to the brain. The neurons send a signal that leads to release of the hormone oxytocin from the pituitary gland.
Oxytocin increases uterine contractions, and thus pressure on the cervix. This causes the release of even more oxytocin and produces even stronger contractions. This positive feedback loop continues until the baby is born.

## Want to join the conversation?

• Can someone explain what is negative feedback? I didn't understand the concept from the article....
• From what I understood, negative feedbacks is your body's response to keep things normal or stable, whereas positive feedbacks exacerbate certain effects on the body by repeating functions deliberately. In essence, negative feedbacks preserve your body's original or 'set' condition and positive feedbacks do the opposite and change you body more by constantly pushing certain types of growth or development in the same direction until something has been accomplished. The example they used was a fetus's head constantly putting more and more pressure on the cervix until birth. Since this is very necessary and important, a positive feedback loops is run: the substance that pushes the fetus' head towards the cervix, oxytocin, is released as a cause of contractions from the uterus, which are themselves a cause of pressure from the fetus' head on the cervix. So the pressure essentially causes contractions in the uterus which stimulate nerve impulses in the brain to release more oxytocin, which further increase the pressure of the fetus' head. Clearly the goal isn't to maintain the fetus' current state but rather push it to the point where it is primed for birth.
• Is the system that regulates pH, homeostasis?
• To be precise, homeostasis is a process/phenomenon not a system.

Homeostasis is actually the process of maintaining a stable internal environment despite changes in the external environment.

There are mechanisms in organisms that regulate pH and this regulation is an example of homeostasis. For example, if you have learned about buffers, then it may help to know that essentially all organisms use buffers (and other mechanisms) to maintain control over the pH — for example the different organelles within eukaryotic cells will consistently have different pHs:
http://book.bionumbers.org/what-is-the-ph-of-a-cell/

Does that help?
• what is pH guys and how does it relate to homeostasis.
• pH is a measure of how acidic or basic a solution is. More specifically, pH=-log[H+], which essentially means that the more positively charged hydrogen ions you have in a volume of solution, the lower the pH is and the more acidic the solution is. You can also check out this video (if you haven’t already seen it): https://www.khanacademy.org/science/high-school-biology/hs-biology-foundations/hs-ph-acids-and-bases/v/introduction-to-ph

pH matters for homeostasis because it’s part of the chemical environment in which biochemical reactions have to take place. For example, enzymes (which help speed up chemical reactions in living things) have ranges of pH where they work best. Because these molecules’ proper functioning is necessary, pH is very important for maintaining homeostasis.

Hope this helps!
• can someone please tell me which organ in the body controls homeostasis?
• Homeostasis is mainly controlled by the organs in the central nervous system and the endocrine system (hormones). Organs in the two systems send commands to other organs in other systems to allow them to carry out certain functions.

Example for the nervous system: You have stepped outside into some snowy weather. It is cold outside, and your body temperature is dropping. The nervous system sends signals to the muscles that tell them to shake. The shaking of the muscles generates heat, keeping you warm.

Example for the endocrine system: Unfortunately, your have not eaten anything for hours. This results in a low blood sugar level. To maintain homeostasis, cells from the pancreas release a hormone known as glucagon, which raises your blood sugar by decreasing the storage of sugar in body cells.

• How can very low temperatures be fatal?
• Low temperatures would mean that the enzymes would be inactive or they may not be able to catalyse as much. This slows down reactions in the body (lowers metabolism), meaning that you may be deprived of essential things such as energy etc, which can eventually lead to complications such as death.
• How does Blood clot relate to Homeostasis?
• Blood clotting is considered part of the Positive Feedback (PF) Loop. This is defined as an effector that will AMPLIFY the effect of the Negative Feedback (NF) Loop. For instance, when there is a hemorrhage (loss of blood), it will cause a sequential activation of clotting factors. Here, a single clotting factor results in the activation of many more clotting factors. This is also known as a PF cascade. This overall process will give the completion of the NF Loop because blood loss was prevented with the clotting factors, resulting in Homeostasis.
• What system controls homeostasis?
• There are many different systems that organisms use to maintain homeostasis.

For example, the processes used for thermoregulation in mammals and birds are very different from the methods used to keep pH relatively constant.