Homeostasis 2022 Sum HP

  1. Homeostasis is the maintenance of a stable internal environment (temperature, pH, blood levels of nutrients and ions, and many other physiological variables).

  2. Homeostasis takes the majority of the body’s energy and design (~80%).

Maintaining homeostasis is vital because:

  1. Proper internal environmental conditions are required to allow enzymes to function.

  2. Life requires particular chemical reactions to happen at particular times.

  3. Living things cause the right reactions to happen at the right times by producing enzymes to catalyze the desired reactions.

  4. Each enzyme works as a catalyst because of its specific 3-dimensional shape.

  5. Enzymes are proteins (long chains of amino acids). This is analogous to saying “My bicycle is carbon fiber”:  Protein is the kind of material, the enzyme is what is built from that material.  Just as you can build nearly endless varieties of words with 26 letters, your cells can build many thousands of different proteins using the 20 common amino acids. Some of these proteins are enzymes, each of which catalyzes a different reaction

  6. When you change a protein’s environment, you change its shape. When you change a protein’s shape, you change its function. This is a recurring theme in biology.

  7. Therefore: When the internal environment of a cell changes, its enzymes change shape and no longer work properly. Dysfunction and death follow.

  8. Homeostasis is the maintenance of those cellular conditions that encourage proper enzyme shape and therefore proper action.

Homeostatic and allostatic variables

It is impossible, in this changeable world, to hold every internal variable constant.

  1. A homeostatic variable in a physiological mechanism is the variable that the system is designed to maintain near its desired level (setpoint).

  2. Typically, the homeostatic variable is the one most critical to maintain for life and health.

  3. Homeostatic variables may be recognized by their tendency to remain relatively stable, despite challenges that would tend to cause them to change.

  4. An allostatic variable in a physiological system is a variable that is intentionally adjusted in order to maintain a more constant level of the homeostatic variable.

  5. The allostatic variables can change without causing harm; and are changed on purpose to hold a homeostatic variable more constant.

  6. Allostatic variables may be recognized by the fact that they change readily when needs or conditions change. Allostatic variables change to keep the homeostatic variable of the system stable.

  7. For example, when your body starts to chill, you may constrict blood vessels in the skin in order to reduce heat loss.

  8. In this system, the level of vasoconstriction is allostatic, because it is being changed on purpose to help maintain body temperature. The levels of signals such as hormones that cause vasoconstriction are allostatic too, for the same reason.

  9. In this system, the body temperature is the homeostatic variable; it’s the variable you are working to keep close to its setpoint.

  10. These definitions only apply to homeostatic systems. If there is no variable being maintained, the variables are neither homeostatic nor allostatic.

Mechanisms for maintaining homeostasis

  1. Negative feedback mechanisms are frequently used to maintain homeostasis.  Negative feedback systems follow this form:

Stimulus

Reduces                        Causes

Response

  1. A system is negative feedback if the stimulus causes the response, and the response tends to reduce or eliminate the stimulus. The response doesn’t necessarily have to be successful at getting rid of the stimulus, since other factors may interfere; but it does have to be designed to cause a reduction of the stimulus.

  2. In many systems designed to maintain homeostasis, the stimulus will be a difference between the actual value of a homeostatic variable (such as body temperature) and its setpoint.

  3. For example, in the temperature control system, the setpoint is at 37  °C. If the body were currently at 36 °C, the fact that you were 1 °C too cool would be a stimulus that triggers homeostatic responses to warm you.

  4. The response is some action that is performed by a muscle, gland, or organ (called the effector). Note the response is what the effector does, not the eventual outcome:  A muscle contracting is the response, the leg kicking is the eventual outcome.

  5. Most (but not all) negative feedback systems also have the following characteristics: These characteristics don’t define the system as being negative feedback (the response reducing the stimulus is the defining character), but they are often true.

  6. They promote stability by maintaining conditions.

  7. They are self initiating. The fact that the variable is not at the setpoint serves as the stimulus and initiates the response.

  8. They are self limiting. Once the variable is back at the setpoint, the stimulus is gone so the response ends.

  9. Many body systems are controlled by dual negative feedback responses.

  10. One feedback loop is activated if the variable rises too high. A separate loop is activated if the variable sinks too low.

  11. An example of dual negative feedback controls: Your hypothalamus determines the setpoint for your body temperature.

  12. The stimulus of you being too warm causes the response of sweating, which reduces the stimulus.

  13. The stimulus of you being too cool causes responses such as shivering, which reduce the stimulus.

  14. The diagram below shows another example of how dual negative feedback controls can work; in this case detailing what happens when the variable of basal metabolic rate leaves its setpoint. Image thanks to OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

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    Shows a control system where the hypothalamus ultimately directs the release of thyroid hormones, whose main action is to raise metabolic rate. When metabolic rate is too low, the system activates more vigorously to release more thyroid hormone. When metabolic rate is high, the activity of the system decreases and reduces thyroid hormone output.

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  17. Positive feedback mechanisms also occur in the body, but are much less commonly used to maintain homeostasis. A system  shows positive feedback if the stimulus causes the response, and the response enhances the stimulus.

Stimulus

Enhances                        Causes

Response

  1. Positive feedback systems usually (but not always) also have these characteristics:

  2. They tend to drive a variable away from a setpoint.

  3. They are not self-limiting. They have to be interrupted by something outside the system to stop them.

  4. They are good for amplification, to produce a strong, quick response.

  5. Example of a ==positive feedback mechanism==: During the process of childbirth, the hormone oxytocin is released.  This acts as a stimulus to cause the response of uterine contractions. Uterine contractions result in the production of a chemical signal that encourages more oxytocin production.

  6. ‘==No feedback’== mechanisms occur when the stimulus causes a response, but the response does nothing to impact the stimulus.

Stimulus Does not Causes Affect

An example of a no feedback response: High levels of testosterone during late adolescence in males act as a stimulus to cause the response of shutting down growth hormone production; but the stopping of growth hormone has no impact on further testosterone release.

Key point:  Physiology is all about Mechanisms, HOW things are accomplished. Therefore, concentrate on the direct mechanism of the system rather than the ultimate goal.

Example: Blood clotting is a positive feedback mechanism because the stimuli of clotting factor activations cause a response which ultimately activates more of the clotting factors; although one starts out bleeding and the clot reduces the bleeding.

Really Key Point:  Needs are not mechanisms.

  1. Saying something ‘needs to happen’ does not explain why it happens. Needs do not explain why things happen.

  2. Mechanisms explain why things happen. What is a mechanism? A step-by-step process that details how something gets done. If a mechanism for a reaction is in place, the reaction will occur (regardless of if it is ‘needed’); if there is no mechanism, the reaction cannot occur regardless of one’s ‘need’.

  3. The only real function of ‘needs’ in physiology is to provide selective pressures that have driven evolution to produce mechanisms.

Key points about homeostasis:

  1. Maintenance of a stable internal environment (homeostasis) is critical to life and health and so is the goal of many physiological systems.
  2. Homeostatic variables are the variables whose stability is most important to life and health. The values of homeostatic variables don’t change much in the face of challenges that would tend to push them away from their setpoints.
  3. Allostatic variables are variables that are intentionally changed in order to keep the homeostatic variable of the system more stable.
  4. Negative feedback responses are a major kind of mechanism for maintaining homeostasis. In a negative feedback system, the stimulus causes a response which reduces the stimulus.
  5. Positive feedback responses are when the stimulus causes a response which enhances the stimulus.
  6. Some cause and effect mechanisms have no feedback at all because the response does not impact the stimulus that caused it.
  7. In physiology, mechanisms for how things happen are key, not why they need to happen.