BIOL 122 - Chapter 18: Blood Outflow and The Endocrine System (Part 3 - Hormone Interactions)

Chapter 18: Blood Outflow and The Endocrine System (Part 3 - Hormone Interactions)

Mechanisms of Hormone Action

  • The response of a hormone may include several effects:

    • Synthesis of New Molecules: Hormones can promote the production of new molecules in target cells.

    • Changing Permeability of the Cell Membrane: Hormones may alter how easy it is for substances to enter or exit the cell.

    • Stimulating Transport of a Substance: This refers to the movement of substances into or out of the cell, controlled by hormones.

    • Altering the Rate of Metabolic Actions: Hormones can increase or decrease the metabolic processes within the cell.

    • Causing Contraction of Smooth or Cardiac Muscle: Certain hormones can trigger muscle contractions, which are crucial for various physiological functions.

  • It's important to note that the responses to the same hormone may differ depending on two key factors:

    • The Hormone Itself: Different hormones may activate different pathways and responses.

    • The Target Cell: The specific type of cell responding to the hormone can influence the outcome.

Factors That Influence Hormone Responsiveness

  • The responsiveness of a target cell to a hormone depends on:

    • Hormone Concentration: The amount of hormone available in the bloodstream.

    • Number of Hormone Receptors: The presence and abundance of receptors on the target cell, which can affect the cell's sensitivity to the hormone.

    • Influence of Other Hormones: The presence of other hormones can either enhance or inhibit a particular hormonal response.

Hormone Interactions

  • There are different types of hormonal interactions that can influence biological effects:

    • Permissive Effect:

    • Definition: The action of one hormone on a target cell requires a simultaneous or recent exposure to a second hormone. This shows that one hormone can enhance the effect of another.

    • Synergistic Effect:

    • Definition: The combined effect of two hormones acting together is greater than the effect of either hormone acting alone. This indicates a cooperative relationship between hormones.

    • Antagonistic Effect:

    • Definition: This occurs when one hormone opposes the action of another hormone. For example, one hormone may counteract the effects of another, regulating homeostasis.

Control of Hormone Secretion

  • Hormone secretion is regulated by various mechanisms:

    • Signals from the Nervous System: The nervous system can trigger hormone release based on immediate physiological needs.

    • Chemical Changes in the Blood: Alterations in blood chemistry (e.g., ion concentrations) can stimulate hormone secretion.

    • Other Hormones: Hormones can regulate the secretion of other hormones, creating a complex interplay between different endocrine signals.

Feedback Mechanisms in Hormone Regulation

Negative Feedback System

  • A negative feedback loop is a self-regulating mechanism that reverses a particular stimulus to maintain homeostasis:

    1. High Level of Ca2+ in Blood: Stimulates thyroid gland parafollicular cells to release more calcitonin (CT).

    2. Calcitonin (CT): Inhibits osteoclast activity, resulting in decreased blood Ca2+ levels.

    3. Low Level of Ca2+ in Blood: Stimulates parathyroid gland chief cells to release more parathyroid hormone (PTH).

    4. Parathyroid Hormone (PTH): Promotes release of Ca2+ from bone extracellular matrix into blood and decreases the loss of Ca2+ in urine, which increases blood Ca2+ levels.

    5. Calcitriol: Stimulates increased absorption of Ca2+ from foods, further increasing blood Ca2+ levels.

    6. Hormone output effectively reverses the particular stimulus affecting homeostasis.

Positive Feedback System

  • In a positive feedback loop, hormone output reinforces and encourages the initial stimulus:

    • Example: Oxytocin, a hormone that stimulates contractions of the uterus, also promotes further release of oxytocin during labor. As uterine contractions increase, more oxytocin is released, which amplifies the contractions.

Examples of Negative Feedback

  • Which of the following is an example of a negative feedback system?

    • Scenario 1: During labor, uterine contractions stimulate the release of oxytocin, which strengthens the contractions of smooth muscle in the uterine wall. (This is an example of positive feedback, not negative.)

    • Scenario 2: A decrease in blood pressure and heart function during severe blood loss. (This may not directly demonstrate a feedback loop, but is indicative of imbalance in homeostasis.)

    • Scenario 3: Formation of a blood clot (a scab) after a person has received a cut to the arm. (This is also an example of positive feedback during the clotting process.)

    • Scenario 4: Scratching at an itchy skin rash, which makes the rash spread and itch even more. (This exemplifies a negative consequence rather than a feedback mechanism.)

    • Conclusion: None of these scenarios are clear examples of a negative feedback system, as they instead showcase positive feedback mechanisms or pathological responses.