Lecture Notes on Homeostasis Loop for Regulating Mean Arterial Pressure (MAP)

Overview

• Controlled Variable: Mean Arterial Pressure (MAP)
• Scenario: Large drop in MAP due to blood loss (e.g., injury)
• Emergency Situation: Blood pressure dropping, requires immediate regulation

Sensors

• Baroreceptors:
  • Located in the kidney, aortic arch, and carotid artery
  • Monitor blood pressure and detect changes
  • Ensure adequate blood delivery to heart and brain

Integrating Center

• Central Nervous System (CNS):
  • Integrates information from baroreceptors
  • Key areas:
    • HT/PPG (Hypothalamic-Pituitary Axis)
    • Cardiovascular Control Center (CVCC) in the medulla oblongata (brainstem)

Effector Tissues

• Vascular Circular Smooth Muscle of blood vessels
• SA Node of the Heart:
  • Autorhythmic cells
  • Myocardial pumping cells
• Kidney

Efferent Output and Effector Action

• HT/PPG (Hypothalamic-Pituitary Axis):

  • Low MAP or high plasma osmolarity triggers ADH (Vasopressin) secretion
  • ADH Effects:
    • Vasoconstriction: Increases peripheral vasoconstriction (squeeze blood into heart-brain loop)
    • Kidney: Increases water conservation

• Cardiovascular Control Center (CVCC):

  • Increased sympathetic output
  • Sympathetic Effects:
    • NE from sympathetic neurons and EPI from adrenal medulla
    • Adrenergic Receptors:
      • Blood vessels: Vasoconstriction (alpha adrenergic receptors)
      • SA node of the heart: Increased heart rate
      • Pumping cells: Increased stroke volume
    • Sympathetic influence on kidney: Reduces blood flow (promotes water conservation)

Kidneys as Baroreceptors

• Low Renal Blood Pressure:
  • Increases secretion of renin
  • Renin converts angiotensinogen to ANG I
  • ANG I converted to ANG II by ACE
  • ANG II Effects:
    • Vasoconstriction: Increased peripheral vasoconstriction
    • Hypothalamus: Increases ADH release and thirst
    • Adrenal Cortex: Increases aldosterone synthesis and release

Baroreceptor Reflex

• Immediate Response:
  • Peripheral vasoconstriction and increased heart rate and stroke volume
• Slower Response:
  • Kidney's action (water conservation)

Outcome

• Goal: Bring MAP back up to a safer level
• Consideration: If significant fluid loss, may not return to normal MAP but will improve from emergency levels

Study Tip

• Practice: Use notes template to reproduce homeostasis loop without referencing notes
• Challenge: Create a loop for increased MAP

Preparation

• Understand the mechanisms and steps involved in regulating MAP during both increases and decreases in blood pressure as a preparation for quizzes.