In this lesson, you will learn the structures that allow for homeostasis and the two ways in which homeostasis regulates itself. Specifically, this lesson will cover:
Recall that homeostasis is a state of internal equilibrium or balance maintained by living things through their unique physiology. In this lesson, you will learn about the general anatomical structures and physiological processes that make this maintenance possible.
Maintaining homeostasis requires that the body constantly monitors internal conditions, processes all changes to determine if a response is necessary, and creates an appropriate response when needed. This system is commonly called the homeostatic reflex arc. It contains three components, two signal pathways, and is associated with two changes in the internal environment. Each of these functions in a set order of events.
Generic Homeostatic Reflex Arc - Indicating stimulus, sensor, afferent pathway, control center, efferent pathway, effector, and effect.
step by step
Using the image above, follow the set order of events (steps) below in this homeostatic reflex arc.
A change occurs in an external or internal physiological condition such as body temperature, blood pressure, nutrient levels, and more. This is called a stimulus.
A specialized cell (or group of cells) called a sensor monitors physiological conditions and detects the stimulus.
Once the stimulus is detected, the sensor sends a signal along the afferent (incoming) pathway towards the control center.
The control center receives and processes the sensory signal to determine if a response to the stimulus is needed, and if so, what response is appropriate.
If a response is needed, the control center sends a signal along the efferent (outgoing) pathway towards the effector.
The effector is a specialized cell that receives the response signal and executes the response.
The response that an effector carries out, called an effect, makes a change in the physiological condition
Each physiological condition has a specific value that it remains stable around. This value is called the set point. The set point for human body temperature is 37°C (98.6°F). Each condition can fluctuate slightly above or slightly below its set point without activating a homeostatic reflex arc, within what is called its normal range. If the condition fluctuates beyond the normal range, a homeostatic reflex arc is activated to work on returning the condition value back to the set point. Once a homeostatic reflex arc is activated, it will only turn off when the condition has returned to the normal range or the body dies.
terms to know
Stimulus
Any change in an external or internal physiological condition.
Sensor
The component of a homeostatic reflex arc that monitors internal physiological conditions.
Afferent Pathway
The pathway that incoming sensory signals travel along from the sensor to the control center.
Control Center
A component of a homeostatic reflex arc that processes sensory signals and, when appropriate, generates response signals.
Efferent Pathway
The pathway that outgoing response signals travel along from the control center to the effector.
Effector
A specialized cell that receives the response signal and executes the response.
Effect
A change in an internal condition caused by a homeostatic reflex arc.
Set Point
A value that an internal physiological condition fluctuates around.
Normal Range
The values around a set point that an internal physiological condition can be without a homeostatic response.
2. Feedback
The effect created by a homeostatic reflex arc creates feedback that will influence the condition value and the homeostatic reflex arc itself. The feedback can either oppose the original stimulus or enhance it. Both methods work to return the condition value to its set point.
2a. Negative Feedback Loop
A homeostatic reflex arc where the effect opposes the original stimulus (a counterbalance) is called a negative feedback loop. This is the predominant way the body maintains homeostasis.
EXAMPLE
Body temperature, blood glucose, pupil size, and many other conditions are regulated by negative feedback. In this example, you will use body temperature as the condition of focus.
When the internal body temperature rises above its set point (37°C, 98.6°F), a stimulus of increased temperature is created. A sensor detects this change in temperature and sends a signal along the afferent pathway to a control center. If the temperature is still within the normal range, no response is created. However, if the temperature increases above the normal range, the control center will send at least one signal along an efferent pathway to an effector. One possible effector may be sweat glands which will execute the effect of sweating which opposes the original stimulus by working to lower the body temperature. This, therefore, is homeostatic regulation via negative feedback. You can see this negative feedback loop in the image below.
In a negative feedback loop, a stimulus—a deviation from a set point—is resisted through a physiological process that returns the body to homeostasis. (a) A negative feedback system has five basic parts. (b) Body temperature is regulated by negative feedback.
2b. Positive Feedback Loop
A homeostatic reflex arc where the effect enhances the original stimulus is called a positive feedback loop. This type of feedback is rare and is only seen in two individual processes in the human body—blood clotting and female labor.
EXAMPLE
In this example, you will use labor and childbirth as the condition of focus.
Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Furthermore, the events of childbirth, once begun, must progress rapidly to a conclusion or the life of the person giving birth and the baby are at risk. The extreme muscular work of labor and delivery is the result of a positive feedback loop (see illustration below).
Positive Feedback Loop - Normal childbirth is driven by a positive feedback loop. A positive feedback loop results in a change in the body’s status, rather than a return to homeostasis.
The first contractions of labor (the stimulus) begin to push the baby. The lower end of the uterus contains stretch-sensitive cells (the sensors) that monitor the degree of stretching. These cells send messages to the brain (the control center), which in turn causes a gland at the base of the brain (the effector) to release a hormone into the bloodstream which causes stronger contractions (the effect). Increased pushing of the baby down the birth canal causes even greater stretching of the uterus. The cycle of stretching, hormone release, and increasingly more forceful contractions is the positive feedback loop and stops only when the baby is born. At this point, the stretching halts, stopping the release of hormones.
EXAMPLE
A second example of positive feedback centers on reversing extreme damage to the body.
Following a penetrating wound, the most immediate threat is excessive blood loss and organ failure or death. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area, saving the life of the person in which it occurs.
make the connection
If you're taking the Anatomy & Physiology I Lab course simultaneously with this lecture, it's a good time to try the Lab: Homeostatic Control: How does the human body keep itself in balance? in Unit 2 of the Lab course. Review the lab-to-lecture crosswalk if you need more information. Good luck!
terms to know
Negative Feedback Loop
A homeostatic reflex arc in which the effect opposes (counterbalances) the original stimulus. This is the predominant way the body maintains homeostasis.
Positive Feedback Loop
A homeostatic reflex arc in which the effect enhances the original stimulus. This type of feedback is rare and is only seen in two individual processes in the human body - blood clotting and female labor.
summary
In this lesson, you recalled that homeostasis is a state of internal equilibrium or balance maintained by living things through their unique physiology. You learned about how the homeostatic reflex arc makes this balance possible by constantly monitoring, processing, and responding to changes as needed. You then followed an example of the basic components and step-by-step process of a homeostatic reflex arc. You also learned that an effect created by a homeostatic reflex arc creates one of two types of feedback (negative or positive feedback loops) and you had a chance to identify both feedback loops with some examples.
The pathway that incoming sensory signals travel along from the sensor to the control center.
Control Center
A component of a homeostatic reflex arc that processes sensory signals and when appropriate generates response signals.
Effect
A change in an internal condition caused by a homeostatic reflex arc.
Effector
A specialized cell that receives the response signal and executes the response.
Efferent Pathway
The pathway that outgoing response signals travel along from the control center to the effector.
Negative Feedback Loop
A homeostatic reflex arc in which the effect opposes (counterbalances) the original stimulus. This is the predominant way the body maintains homeostasis.
Normal Range
The values around a set point that an internal physiological condition can be without a homeostatic response.
Positive Feedback Loop
A homeostatic reflex arc in which the effect enhances the original stimulus. This type of feedback is rare and is only seen in two individual processes in the human body—blood clotting and female labor.
Sensor
The component of a homeostatic reflex arc that monitors internal physiological conditions.
Set Point
A value that an internal physiological condition fluctuates around.
Stimulus
Any change in an external or internal physiological condition.
