7.8 Homeostasis and Feedback – Human Biology (2024)

Created by CK-12 Foundation/Adapted by Christine Miller

This device (Figure 7.8.1) looks simple, but it controls a complex system that keeps a home at a steady temperature — it’s a thermostat. The device shows the current temperature in the room, and also allows the occupant to set the thermostat to the desired temperature. A thermostat is a commonly cited model of how living systems — including the human body— maintain a steady state called homeostasis.

is the condition in which a system (such as the human body) is maintained in a more or less steady state. It is the job of, , , and throughout the body to maintain many different variables within narrow ranges compatible with life. Keeping a stable internal environment requires continually monitoring the internal environment and constantly making adjustments to keep things in balance.

Set Point and Normal Range

For any given variable, such as bodytemperatureorbloodglucose level, there is a particularthat is the physiological optimum value.The set point forhuman bodytemperature, for example, is about 37 degrees C (98.6 degrees F). As the body works to maintain for temperature or any other internal variable, the value typically fluctuates around the set point. Such fluctuations are normal, as long as they do not become too extreme. The spread of values within which such fluctuations are considered insignificant is called the. In the case of body temperature, for example, the normal range for an adult is about 36.5 to 37.5 degrees C (97.7 to 99.5 degrees F).

A good analogy for set point, normal range, and maintenance of homeostasis is driving. When you are driving a vehicle on the road, you are supposed to drive in the centre of your lane — this is analogous to the . Sometimes, you are not driving in the exact centre of the lane, but you are still within your lines, so you are in the equivalent of the . However, if you were to get too close to the centre line or the shoulder of the road, you would take action to correct your position. You’d move left if you were too close to the shoulder, or right if too close to the centre line — which is analogous to our next concept, to maintain .

Maintaining Homeostasis

is normally maintained in the human body by an extremely complex balancing act. Regardless of the variable being kept within its normal range, maintaining homeostasis requires at least four interacting components: stimulus, sensor, control centre, and effector.

1. Theis provided by the variable being regulated. Generally, the stimulus indicates that the value of the variable has moved away from the set point or has left the normal range.
2. The monitors the values of the variable and sends data on it to the control centre.
3. The matches the data with normal values. If the value is not at the set point or is outside the normal range, the control centre sends a signal to the effector.
4. The is an organ, gland, muscle, or other structure that acts on the signal from the control centre to move the variable back toward the set point.

Each of these components is illustrated in Figure 7.8.2. The diagram on the left is a general model showing how the components interact to maintain homeostasis. The diagram on the right shows the example of body temperature. From the diagrams, you can see that maintaining homeostasis involves feedback, which is data that feeds back to control a response. Feedback may be negative (as in the example below) or positive. All the feedback mechanisms that maintain homeostasis use . Biological examples of positive feedback are much less common.

Negative Feedback

In a, feedback serves to reduce an excessive response and keep a variable within the .Twoprocesses controlled by negative feedbackarebody temperature regulation and control ofbloodglucose.

Body Temperature

Body temperature regulation involves , whether it lowers the temperature or raises it, as shown in Figure 7.8.3 and explained in the text that follows.

Cooling Down

The human body’s temperature regulatory centre is the in the brain. When the hypothalamus receives data from sensors in the skin and brain that body temperature is higher than the , it sets into motion the following responses:

• Blood vesselsin the skin dilate (vasodilation) to allow morebloodfrom the warm body core to flow close to the surface of the body, soheat can be radiated intothe environment.
• As blood flow to the skin increases, sweat glands in the skin are activated to increase their output of sweat (diaphoresis). When the sweat evaporates from the skin surface into the surrounding air, it takesheatwith it.
• Breathingbecomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increasesheatloss from the lungs.

Heating Up

When the brain’s temperature regulatory centre receives data that body temperature is lower than the set point, it sets into motion the following responses:

• Blood vesselsin the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body, which reduces heat loss from the surface.
• As temperature falls lower, random signals toskeletal musclesare triggered, causing them to contract. This causes shivering, which generates a small amount of heat.
• Themay be stimulated by the brain (via the pituitary gland) to secrete more thyroidhormone. This hormone increases metabolic activity and heat production incellsthroughout the body.
• Themay also be stimulated to secrete thehormone . This hormone causes the breakdown of (thecarbohydrateused forenergystorage in animals) to , which can be used as an energy source. This catabolic chemical process is , or heat producing.

Blood Glucose

In controllingthe blood glucose level, certain endocrinecellsin thepancreas(called alpha and beta cells) detect the level of glucose in the blood. They then respond appropriately to keep the level of blood glucose within the normal range.

• If the blood glucose level rises above the normal range, pancreatic beta cells release thehormoneinsulin into the bloodstream. Insulin signals cells to take up the excess glucose from the blood until the level of blood glucose decreases to the normal range.
• If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormoneglucagoninto the bloodstream. Glucagon signals cells to break down stored glycogen to glucose and release the glucose into the blood until the level of blood glucose increases to the normal range.

Homeostasis and Negative/Positive Feedback, Amoeba Sisters, 2017.

In a, feedback serves to intensify a response until an end point is reached. Examples of processes controlled by positive feedback in the human body include blood clotting and childbirth.

Blood Clotting

When a wound causes bleeding, the body responds with a positive feedback loop to clot the blood and stop blood loss. Substances released by the injured blood vessel wall begin the process of blood clotting. Platelets in the blood start to cling to the injured site and release chemicals that attract additional platelets. As the platelets continue to amass, more of the chemicals are released and more platelets are attracted to the site of the clot. The positive feedback accelerates the process of clotting until the clot is large enough to stop the bleeding.

Childbirth

Figure 7.8.6 shows the positive feedback loop that controls childbirth. The process normally begins when the head of the infant pushes against the cervix. This stimulates nerve impulses, which travel from the cervix to the hypothalamus in the brain. In response, the hypothalamus sends the hormone to thepituitary gland,which secretes it into the bloodstream so it can be carried to the uterus. Oxytocin stimulates uterine contractions, which push the baby harder against the cervix. In response, the cervix starts to dilate in preparation for the passage of the baby. This cycle of positive feedback continues, with increasing levels of oxytocin, stronger uterine contractions, and wider dilation of the cervix until the baby is pushed through the birth canal and out of the body. At that point, the cervix is no longer stimulated to sendnerve impulsesto the brain, and the entire process stops.

Normal childbirth is driven by a positive feedback loop. Positive feedback causes an increasing deviation from the normal state to a fixed end point, rather than a return to a normal set point as in homeostasis.

Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do,may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease — or even death.is an example of a disease caused by homeostatic imbalance. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention can help restore homeostasis and possibly prevent permanent damage to the organism.

Normal aging may bring about a reduction in theefficiencyof the body’s control systems, which makes the body more susceptible to disease.Older people, for example, may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop serious heat-induced illnesses, such as heat stroke.

is diagnosed in people who have abnormally high levels of blood glucose after fasting for at least 12 hours. A fasting level of blood glucose below 100 is normal. A level between 100 and 125 places you in the pre-diabetes category, and a level higher than 125 results in a diagnosis of diabetes.

Of the two types of diabetes, is the most common, accounting for about 90 per cent of all cases of diabetes in the United States. Type 2 diabetes typically starts after the age of 40. However, because of the dramatic increase in recent decades in obesity in younger people, the age at which type 2 diabetes is diagnosed has fallen. Even children are now being diagnosed with type 2 diabetes. Today, about 3 million Canadians (8.1% of total population) are living with diabetes.

You may at some point have your blood glucose level tested during a routine medical exam. If your blood glucose level indicates that you have diabetes, it may come as a shock to you because you may not have any symptoms of the disease. You are not alone, because as many as one in four diabetics do not know they have the disease. Once the diagnosis of diabetes sinks in, you may be devastated by the news. Diabetes can lead to heart attacks, strokes, blindness, kidney failure, nerve damage, and loss of toes or feet. The risk of death in adults with diabetes is 50 per cent greater than it is in adults without diabetes, and diabetes is the seventh leading cause of death of adults. In addition, controlling diabetes usually requires frequent blood glucose testing, watching what and when you eat, and taking medications or even insulin injections. All of this may seem overwhelming.

The good news is that changing your lifestyle may stop the progression of type 2 diabetes or even reverse it. By adopting healthier habits, you may be able to keep your blood glucose level within the normal range without medications or insulin. Here’s how:

• Loseweight. Anyweightloss is beneficial. Losing as little assevenper cent of yourweightmay be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.
• Exerciseregularly.You should try toexercisefor at least 30 minutes, five days a week. This will not only lower your blood sugar and help your insulin work better, but it will also lower yourblood pressureand improve yourhearthealth. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.
• Adopt a healthy diet. Decrease your consumption of refined carbohydrates, such as sweets and sugary drinks. Increase your intake of fibre-rich foods, such as fruits, vegetables, and whole grains. About one-quarter of each meal should consist of high-protein foods, such as fish, chicken, dairy products, legumes, or nuts.
• Control stress.Stress can increase your blood glucose and also raise yourblood pressureand risk ofheartdisease. When you feel stressed out, dobreathingexercises or take a brisk walk or jog.Try to replace stressful thoughts with more calming ones.
• Establish a support system.Enlist the help and support of loved ones, as well as medical professionals, such as a nutritionist and diabetes educator. Having a support system will help ensure that you are on the path to wellness, and that you can stick to your plan.

Attributions

Figure 7.8.1

Nest_Thermostat by Amanitamano on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.

Figure 7.8.2

Negative_Feedback_Loopsby OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.

Figure 7.8.3

Body Temperature Homeostasisby OpenStax College, Biology is used under a CC BY 4.0 license.

Figure 7.8.4

Homeostasis_of_blood_sugar by Christinelmiller on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.

Figure 7.8.5

Positive_Feedback_Diagram_Blood_Clotting by Elliottuttle on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 7.8.6

Pregnancy-Positive_Feedback by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.

References

Amoeba Sisters. (2017, September 7). Homeostasis and negative/positive feedback. YouTube. https://www.youtube.com/watch?v=Iz0Q9nTZCw4&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure1.10Negative feedback loop [digital image/ diagram]. In Anatomy and Physiology (Section 1.5). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-5-homeostasis

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 1.11Positive feedback loopnormal childbirth is driven by a positive feedback loop [digital image/ diagram]. In Anatomy and Physiology (Section 1.5). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-5-homeostasis

Cognito. (2018, December 18). GCSE Biology – Homeostasis #38. YouTube. https://www.youtube.com/watch?v=XMsJ-3qRVJM&feature=youtu.be

Mayo Clinic Staff. (n.d.). Type 2 diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193

OpenStax CNX. (2016, March 23). Figure 4 The body is able to regulate temperature in response to signals from the nervous system [digital image]. In OpenStax, Biology (Section 33.3). https://cnx.org/contents/GFy_h8cu@10.8:BP24ZReh@7/Homeostasis

Whats Up Dude. (2017, September 20). Homeostasis – What is homeostasis – What is set point for homeostasis – Homeostasis in the human body. YouTube. https://www.youtube.com/watch?v=LSgEJSlk6W4&feature=youtu.be