1. Section 3: Feedback Mechanisms
Preview
Bellringer
Key Ideas
Mechanisms of Control
Controlling Breathing Rate
Controlling Blood-Glucose Level
Summary

2. Bellringer
Write a short explanation of how you think a thermostat on a furnace helps to keep the room temperature at a comfortable level.
Answer the following question:
How does the human body act like a thermostat?

3. Key Ideas How do feedback mechanisms help maintain homeostasis?
How does a feedback mechanism control a person’s breathing rate?
How does a feedback mechanism maintain the concentration of glucose in the bloodstream?

4. Mechanisms of Control
A body’s internal state varies around a certain average value
Homeostasis maintains a dynamic equilibrium in the body.

5. Mechanisms of Control, continued
Your body has a variety of feedback mechanisms that detect deviations in the body’s internal environment and make corrections.
A feedback mechanism is a mechanism in which the last step in a series of events controls the first step.
A feedback mechanism has three parts.

6. Mechanisms of Control, continued
First, a receptor, such as a nerve, detects change in the internal or external environment.
Second, a control center, such as the brain, selects a response to the information from the receptor.
Third, an effector, such as a muscle or gland, carries out the response. This effect is detected by the receptor, which starts the process again.

7. Mechanisms of Control, continued
Homeostasis is maintained mostly through negative feedback.
In negative feedback, the final step in a series of events inhibits the first step in the series.
In positive feedback, a change in the body causes even more change in the same direction.

8. Visual Concept: Comparing Positive and Negative Feedback
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9. Mechanisms of Control, continued
Positive feedback occurs in blood clotting.
One clotting factor activates another in a cascade that leads quickly to the formation of a clot.
Another example of positive feedback occurs in females, when rising levels of estrogen cause another hormone to be secreted until an egg is released from an ovary.

10. Mechanisms of Control, continued
Antagonistic hormones work in opposing pairs to control levels of important substances.
The release of each hormone of the pair is regulated by negative feedback.
A pair of antagonistic hormones helps to keep blood sugar from getting too high or too low in the body.

11. Controlling Breathing Rate
The concentration of oxygen (O2) and carbon dioxide (CO2), and the pH of the internal environment are components of homeostasis.
Your brain monitors the blood’s pH, which is affected by the concentration of CO2 in the blood.
When the brainstem senses a change in the blood’s pH, it adjusts the body’s breathing rate.

12. Controlling Breathing Rate, continued
Blood pH can fall when you exercise.
During exercise, your muscle cells need more energy.
Mitochondria in muscle cells perform cellular respiration at a faster rate to provide that energy.
This causes CO2 and water to be released into the bloodstream.

13. Controlling Breathing Rate, continued
In the bloodstream, an enzyme catalyzes the reaction of carbon dioxide and water to form carbonic acid, H2CO3.
The carbonic acid breaks up to form a bicarbonate ion, HCO3-, and a hydrogen ion, H+.
The higher the concentration of the hydrogen ion in blood, the more acidic blood becomes.
Acidic blood has a low pH.

14. Controlling Breathing Rate, continued
Cells on the surface of the brainstem are sensitive to the acidity of the fluid that bathes the brain.
This fluid is strongly affected by the blood’s acidity.
When blood acidity is high, nerve centers in the brainstem signal muscles that control breathing to contract at a faster rate.
This makes the breathing rate increase.

15. Controlling Breathing Rate, continued
A faster breathing rate removes carbon dioxide from the body more quickly.
This lowers the blood’s acidity, or raises the blood’s pH.
When the blood’s pH returns to normal, the brainstem will signal the muscles that control breathing to slow down.

16. Visual Concept: Blood and the Transport of Carbon Dioxide
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17. Controlling Breathing Rate, continued
As you sleep, blood pH can rise.
When your body isn’t moving, cellular respiration slows in your cells.
Nerve cells in your brainstem signal muscles to slow your breathing rate, so less CO2 is released.
This returns the blood’s pH to normal.

18. Controlling Breathing Rate, continued
The amount of oxygen in the blood does not affect breathing rate, unless oxygen levels are dangerously low.
Receptors in the arteries that connect the heart to the lungs are sensitive to blood oxygen concentrations.
If these receptors sense a sharp drop in oxygen, they send an emergency signal to the brainstem to speed breathing rate.

19. Controlling Breathing Rate, continued
Without enough oxygen, brain cells begin to die.
Because even small decreases in oxygen levels can kill brain cells, the body relies on carbon dioxide levels to control the breathing rate.

20. Controlling Blood-Glucose Level
Your body’s cells need glucose to perform cellular respiration.
When you digest a large meal, a large amount of glucose is introduced into your body in a short period of time.
Two hormones, insulin and glucagon, control the level of glucose in the bloodstream.

21. Controlling Blood-Glucose Level, continued
Insulin and glucagon are released by the pancreas.
These hormones act antagonistically to regulate blood- glucose level.
Insulin lowers blood glucose levels by communicating with the liver to convert glucose into glycogen that can be stored for future use.

22. Controlling Blood-Glucose Level, continued
When glucose levels fall below the normal range, glucagon stimulates the liver to break down glycogen.
This adds more glucose to the bloodstream.
This feedback system regulates blood-glucose levels, so they do not stay too high after a meal and do not drop too low between meals.

23. Maintaining a Normal Blood-Glucose Level

24. Summary
To maintain homeostasis, your body has a diversity of feedback mechanisms that detect deviations in the body’s internal environment and make corrective actions.
When the brainstem senses a change in the blood’s pH, it adjusts the breathing rate.
Two hormones that are released by the pancreas, insulin and glucagon, control the level of glucose in a person’s bloodstream.