Homeostasis and Feedback Loops Study Guide

Fundamentals of Homeostasis

  • Homeostasis ($hoh-mee-oh-STAY-sis$) is defined as the existence and maintenance of a relatively constant environment within the body.
  • The body is continuously exposed to new conditions as it undergoes everyday processes; changes in external environmental conditions can lead to changes in internal body conditions.
  • Internal conditions are referred to as variables because their values are not constant. To maintain homeostasis, the body actively regulates responses to changes in these variables.
  • Examples of variables include:
    • Body temperature.
    • Volume of body fluids.
    • Chemical content of body fluids.
    • pH of body fluids.
  • For cells to function normally, variables must be maintained within a narrow range called the normal range.
  • Homeostatic mechanisms maintain body conditions near an ideal normal value or set point.
  • Physical conditions do not stay perfectly stable at the set point; instead, they increase and decrease slightly around it. For instance, the average body temperature is 98.6 ∘F98.6\,^{\circ}F, but it typically fluctuates minimally, often staying within 1 ∘F1\,^{\circ}F above or below normal.
  • In a typical daily cycle (as shown in Figure 1.4), temperature might vary between roughly 98.4 ∘F98.4\,^{\circ}F and 98.8 ∘F98.8\,^{\circ}F depending on the time of day.
  • Homeostasis is maintained as long as body conditions remain within their normal range.

Systems and Biological Feedback Loops

  • The body’s network of organ systems, including the digestive, respiratory, cardiovascular, and urinary systems, works in coordination to keep the internal environment constant.
  • These systems ensure that cells receive sufficient oxygen and nutrients and that waste products do not reach toxic levels.
  • Disease states can disrupt these processes; modern medicine aims to understand these disturbances to reestablish a normal range of values. Failure to do so can result in death.
  • Homeostasis is regulated through feedback loops, which allow a process to be regulated by its outcome.
  • Feedback loops consist of three primary components:
    1. Receptor: Monitors the value of a variable by detecting stimuli.
    2. Control Center: Often a part of the brain (e.g., the hypothalamus), it determines the set point for the variable and receives input from the receptor.
    3. Effector: Stimulated by the control center, it generates the response that adjusts the value of the changed variable.
  • A stimulus is defined as the changed variable that initiates a homeostatic mechanism.

Negative-Feedback Mechanisms

  • Negative-feedback mechanisms are the most common way the body maintains homeostasis.
  • In this context, "negative" means "to decrease." A negative-feedback mechanism occurs when any deviation from the set point is made smaller or resisted.
  • The response by the effector stops once the variable returns to its set point.
  • It is a common misconception that negative feedback only refers to the decrease of a parameter; in reality, both the increase and decrease of parameters (like blood glucose) are regulated by negative feedback to bring them back to the set point.
  • Example: Maintenance of Body Temperature:
    • Normal body temperature is critical for allowing molecules and enzymes to maintain their shape for optimal function.
    • Extreme heat can change the shape of body molecules (similar to how egg whites turn from transparent fluid to white solid solids when cooked), preventing normal function.
    • Process involved in cooling the body:
      1. Receptors in the skin (thermoreceptors) detect an increase in temperature.
      2. The Hypothalamus (control center) receives this information and compares it against the set point.
      3. The control center stimulates effectors, such as sweat glands and blood vessels.
      4. Sweat glands produce sweat and blood vessels in the skin dilate to increase blood flow to the surface.
      5. The body cools down.
      6. Once the temperature returns to the set point, the control center stops stimulating the effectors (sweating stops).

Positive-Feedback Mechanisms

  • Positive-feedback mechanisms occur when a response to the original stimulus results in the deviation from the set point becoming even greater.
  • In this context, "positive" means "to increase."
  • While most positive-feedback mechanisms are part of a normal physiological process, they can sometimes be detrimental.
  • Example: Blood Clotting:
    • During blood loss, a chemical called thrombin is produced.
    • Thrombin stimulates the production of even more thrombin to form a clot.
    • The process is self-limiting because the chemicals needed for clot formation eventually deplete in the area of blood loss.
  • Example: Childbirth:
    1. The large fetus stretches the uterus near the end of pregnancy.
    2. Stretching stimulates sensory neurons to signal the control center (posterior pituitary gland).
    3. The control center releases Oxytocin, which causes uterine smooth muscles to contract.
    4. Contractions push the baby against the opening, causing more stretch.
    5. Increased stretch signals the control center to further increase contractions.
    6. The cycle ends only when the baby is delivered and the stretch stimulus is removed.
  • Example: Detrimental Positive Feedback (Heart Failure):
    • Extreme blood loss causes blood pressure to drop significantly.
    • The heart receives inadequate blood delivery for its own muscle function.
    • The heart pumps less blood, which further decreases blood pressure.
    • This lead to even less blood reaching the heart muscle, causing a self-propagating cycle of decreasing blood pressure that can end in death.

Flexibility of Homeostatic Set Points

  • The maintenance of a normal range does not mean variables are fixed at the same value at all times. Deviations can be beneficial based on activity.
  • Example: Blood Pressure During Exercise:
    • During exercise, muscle tissue requires more oxygen.
    • The normal homeostatic range for blood pressure increases (Figure 1.8).
    • This is not a nonhomeostatic condition but a resetting of the normal range to a higher and broader level to meet increased metabolic demand.
    • Once exercise stops, the set point and range return to resting conditions.

Case Study 1.1: Orthostatic Hypotension

  • Patient Profile: Molly, a 75-year-old widow.
  • Symptoms: Fever and chills for 2 days; felt dizzy, fainted, and fell upon rising to go to the bathroom.
  • Diagnosis: Orthostatic hypotension.
  • Definition: Orthostasis means "to stand"; hypotension means "low blood pressure." It is a significant drop in blood pressure when moving from a lying position to standing.
  • Mechanism: Gravity causes blood to pool in veins below the heart, resulting in less blood returning to the heart and lower cardiac output.
  • Contributing Factors in the Elderly: Age-related decreases in neural and cardiovascular responses; dehydration from fever-induced sweating and decreased fluid intake, which lowers blood volume.

Questions & Discussion

  • What is the normal response to a decrease in blood pressure on standing?
    • Normally, receptors detect the drop, and the control center increases heart rate and constricts blood vessels to return blood pressure to the normal range.
  • What happened to Molly's heart rate just before she fainted? Why did Molly faint?
    • Her heart rate likely increased as a negative-feedback response to low blood pressure. She fainted because this compensation failed due to dehydration and age, resulting in insufficient blood flow (oxygen) to the brain.
  • How did Molly's fainting and falling help establish homeostasis?
    • Fainting caused her to fall to the floor, placing her body in a horizontal position. This neutralized the effect of gravity on blood pooling, allowing blood to return to the heart more easily and restoring adequate blood pressure to the brain.
  • What effect would swimming in cool water have on body temperature regulation?
    • Cool water would stimulate self-regulatory mechanisms to conserve and generate heat, such as constricting skin blood vessels and initiating shivering. If negative feedback failed to return the temperature to normal, the body could suffer from hypothermia.
  • Ashley’s respiratory rate increases rapidly while running an 800-meter race. Does this represent negative or positive feedback?
    • This is negative feedback. The increased respiratory rate is a response to the stimulus of increased carbon dioxide levels and decreased oxygen levels in the blood. The response (increased breathing) works to bring these levels back toward the homeostatic set point required for the higher metabolic activity level.