2.5: Homeostasis and Feedback

Steady State

This device looks simple, but it controls a complex system that keeps a room at a steady temperature. The device allows the occupant to set the thermostat to the desired temperature (the set point), and the heating and cooling systems maintain that programmed temperature. A thermostat is a commonly cited model of how living systems, including the human body, maintain a steady state called homeostasis.

Figure \(\PageIndex{1}\): Thermostat (CC0).

What is Homeostasis?

Homeostasis is the maintenance of a system, such as the human body, in a relatively stable state. In the body, cells, tissues, organs, and organ systems maintain many variables within narrow ranges compatible with life. Keeping a stable internal environment requires continuous monitoring and constant adjustments to maintain balance.

Setpoint and Normal Range

For any given variable, such as body temperature or blood glucose level, there is a particular setpoint that is the physiological optimum value.

For example, the setpoint for human body temperature is about 37 ºC (98.6 ºF). As the body works to maintain homeostasis of temperature or other internal variables, 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 normal range. For example, the normal range for body temperature in adults is about 36.5 to 37.5 ºC (97.7 to 99.5 ºF).

Maintaining Homeostasis

Homeostasis is normally maintained in the human body by an extremely complex balancing act. Even when the variable remains within its normal range, maintaining homeostasis requires at least four interacting components: a stimulus, a sensor, a control center, and an effector.

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

The left side of Figure \(\PageIndex{2}\) is a general model illustrating how the four components interact to maintain homeostasis.

1. The stimulus activates the sensor.
2. The sensor then activates the control system that regulates the effector.

From the diagrams, you can see that maintaining homeostasis involves feedback: data that feeds back to control a response. Feedback may be negative or positive. All the feedback mechanisms that maintain homeostasis use negative feedback. Biological examples of positive feedback are much less common.

The right side of Figure \(\PageIndex{2}\) illustrates body temperature: high body temperature may stimulate the brain's temperature regulatory center to activate the sweat glands, lowering body temperature. When body temperature reaches the normal range, it acts as negative feedback to stop the process.

Figure \(\PageIndex{2}\): Maintaining homeostasis through feedback requires a stimulus, sensor, control center, and effector (OpenStax CC-BY 4.0).

Negative Feedback

In a negative feedback loop, feedback reduces an excessive response and keeps a variable within the normal range. Examples of processes controlled by negative feedback include body temperature regulation and blood glucose control.

Negative Feedback Example: Body Temperature

Body temperature regulation involves negative feedback, whether it lowers the temperature or raises it (Figure \(\PageIndex{3}\)).

Cooling Down

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

• Blood vessels in the skin dilate (vasodilation) to allow more blood from the warm body core to flow toward the skin's surface, so heat can be radiated into the environment.
• As blood flow to the skin increases, sweat glands are activated, leading to increased sweating (diaphoresis). When sweat evaporates from the skin surface into the surrounding air, it carries heat with it.
• Breathing becomes deeper, and the person may breathe through the mouth rather than the nose. This increases heat loss from the lungs.

Figure \(\PageIndex{3}\): The hypothalamus plays a major role in temperature regulation (CC0).

Heating Up

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

• Blood vessels in the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body. This reduces heat loss from the surface.
• As the temperature falls, random signals are sent to skeletal muscles, triggering contractions. This causes shivering, which generates a small amount of heat.
• The thyroid gland may be stimulated by the brain (via the pituitary gland) to secrete more thyroid hormones. This hormone increases cellular metabolic activity and heat production throughout the body.
• The adrenal glands may also be stimulated to secrete the hormone adrenaline. This hormone causes the breakdown of glycogen (the carbohydrate used for energy storage in animals) to glucose, which can be used as an energy source. This catabolic chemical process is exothermic, meaning it produces heat.

Negative Feedback Example: Blood Glucose

In the control of blood glucose levels, certain endocrine cells in the pancreas, called alpha and beta cells, detect blood glucose levels. Then they respond appropriately to keep blood glucose within the normal range.

• If blood glucose levels rise above the normal range, pancreatic beta cells release the hormone insulin into the bloodstream. Insulin signals cells to take up excess glucose from the blood until blood glucose levels return to the normal range.
• If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormone glucagon into the bloodstream. Glucagon signals cells to break down stored glycogen into glucose, which is released into the blood until blood glucose levels return to the normal range.

Feature: Reliable Sources

Teamwork among organ systems allows the human organism to work like a finely tuned machine. Or at least it does until one of the organ systems fails. When that happens, other organ systems interacting in the same overall process will also be affected. This is especially likely if the system affected plays a role in maintaining the process's homeostasis.

An example is Type 1 diabetes. This disorder occurs when the pancreas fails to secrete the hormone insulin. Insulin is normally secreted in response to rising blood glucose levels, and it lowers them back to normal by stimulating cells to take up glucose from the blood.

Learn more about type 1 diabetes. Use several reliable sources to answer the following questions:

1. What causes the endocrine system to fail to produce insulin in type 1 diabetes?
2. Which organ systems are affected by high blood glucose levels if type 1 diabetes is not controlled? What are some of the specific effects?
3. How can blood glucose levels be controlled in patients with type 1 diabetes?

Positive Feedback

In a positive feedback loop, feedback intensifies a response until an endpoint is reached.

Positive Feedback Example: Blood Clotting

When a wound causes bleeding, the body initiates a positive feedback loop to clot the blood and stop bleeding:

• Substances released by the injured blood vessel wall initiate blood clotting.
• Platelets in the blood start to cling to the injured site and release chemicals that attract additional platelets.
• As platelets continue to accumulate, more chemicals are released, and more platelets are attracted to the clot site.
• Positive feedback accelerates clotting until the clot is large enough to stop bleeding.

Positive Feedback Example: Childbirth

The positive feedback loop that controls childbirth is illustrated in (Figure \(\PageIndex{4}\)):

• The process begins when the infant's head pushes against the cervix. This stimulates nerve impulses, which travel from the cervix to the hypothalamus in the brain.
• In response, the hypothalamus signals the pituitary gland to release oxytocin into the bloodstream, which is then 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 send nerve impulses to the brain, and the entire process stops.

Figure \(\PageIndex{4}\): Vaginal childbirth is driven by a positive feedback loop. Positive feedback causes an increasing deviation from the normal state to a fixed endpoint rather than a return to a normal set point (OpenStax CC-BY 4.0).

When Homeostasis Fails

Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, homeostatic imbalance 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. Diabetes is an example of a disease caused by a 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.

Video

How does your body maintain its balance? Review this video on how homeostasis ensures that cells receive an optimal environment for proper functioning.

 

Feature: My Human Body

Diabetes is diagnosed in people who have abnormally high levels of blood glucose after fasting for at least 12 hours. A fasting blood glucose level 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, type 2 diabetes is the most common, accounting for about 90 percent 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 30 million Americans have type 2 diabetes, and another 90 million have pre-diabetes.

You are likely to 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 does 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, and loss of toes or feet. The risk of death in adults with diabetes is 50 percent greater than it is in adults without diabetes, and diabetes is the seventh leading cause of death in adults. In addition, controlling diabetes usually requires frequent blood glucose testing, monitoring what and when you eat, and taking medications, including 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. Here’s how:

• Lose weight. Any weight loss is beneficial. Losing as little as seven percent of your weight may be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.
• Exercise regularly. You should try to exercise for at least 30 minutes, 5 days a week. This will not only lower your blood sugar and help your insulin work better, but also lower your blood pressure and improve your heart health. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.
• Adopt a healthy diet. Reduce your intake of refined carbohydrates, such as sweets and sugary drinks. Increase your intake of fiber-rich foods such as fruits, vegetables, and whole grains. About a 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 your blood pressure and risk of heart disease. When you feel stressed, do some breathing exercises or take a brisk walk or jog. Also, try to replace stressful thoughts with more calming ones.
• Establish a support system. Enlist the help and support of loved ones and medical professionals, such as a nutritionist and a diabetes educator. Having a support system will help ensure you are on the path to wellness and can stick to your plan.