YC

Feedback Control of Circulation

Feedback Control of Circulation

  • Instructor: Dr. Peter McFawn
  • Course: PHYL1001: Physiology How the Human Body Works 2023
  • Room: G.01 Anatomy Building
  • Contact: Ph: 6488 3341 | Email: peter.mcfawn@uwa.edu.au

Lecture Outline

  • Control of Blood Vessels
  • Control of the Heart
  • Inotropy
  • Baroreceptor Reflex
  • Other Short Term Control Systems
  • Long Term Regulation of Blood Pressure

Aims I

  • Local Metabolites, Autonomic Nerves, Adrenaline, Vasopressin: Understand their actions on blood vessels.
  • Inotropy/ Inotropic: Definition and significance.
  • Cardiac Function Curve: Draw and explain effect of inotropic agents.
  • Autonomic Stimulation of Heart: Describe its actions.
  • Baroreceptor and Baroreceptor Reflex: Definition and description.

Aims II

  • Baroreceptor Reflex Function: At rest, during standing, and in haemorrhage.
  • Central Ischemic Response: Definition and implications.
  • Vasopressin Role: In blood pressure control.
  • Long Term Blood Pressure Regulation: Relationship with ECF volume.

Control of Systemic Arterial Pressure

  • Increased muscle work causes:
  • Reduction in local nutrients.
  • Increase in local waste products.
  • Local metabolite changes lead to:
  • Dilation of arteriolar smooth muscle.
  • Increased local blood flow, known as active hyperaemia.
  • Blood flow redistributes cardiac output effectively to working tissues.

Control of Blood Vessels

  • Regulators: Local metabolites, mediators, hormones, autonomic nerves influence blood vessel tone, tissue perfusion, and total peripheral resistance.
  • Higher O2, lower O2 effects.
  • Adrenaline: Effects on blood vessels.

Sympathetic Control of Blood Vessels

  • Increased sympathetic tone leads to:
  • Constriction of vascular smooth muscle.
  • Increased peripheral resistance and venous return via veno-constriction.
  • Noradrenaline released from sympathetic fibers affects a-receptors for muscle contraction.

Parasympathetic Control of Blood Vessels

  • Limited effect on most blood vessels.
  • Vasodilator nerves minimally affect overall resistance.

Who Wins?

  • During exercise:
  • Sympathetic activation and adrenaline cause vasoconstriction of arterioles.
  • Local metabolites cause vasodilation exclusively in active tissues.
  • Results in increased cardiac output to active tissues, while reducing blood flow to non-active areas.

Cardiac Function Curve

  • Frank-Starling Law: Increased cardiac output boosts activity levels.
  • Increasing venous return enhances stroke volume, critical for maintaining arterial pressure.

Action of Sympathetic Stimulation

  • Enhances cardiac contractility.
  • At rest, heart exhibits sympathetic tone with minimal parasympathetic impact.

Intrinsic vs Extrinsic Regulation

  • Venous return influences intrinsic cardiac contraction.
  • Autonomic nerves and hormones provide extrinsic regulation affecting contractility.
  • Inotropic Agents: Sympathetic stimulation and adrenaline.

Neuronal Control of the Heart

  • Sympathetic nerves target the entire heart, increasing heart rate and contractility.
  • Parasympathetic nerves mainly affect SA/AV nodes and the atria, slow heart rate with less inotropic effect.

Blood Pressure Control

  • Chronotropy (Heart Rate) affects Cardiac Output.
  • Inotropy (Stroke Volume) affects Cardiac Output.
  • Vaso-tone and Veno-tone influence total peripheral resistance and arterial blood pressure.

The Baroreceptor Reflex

  • Location: Baroreceptors in carotid and aortic arteries detect pressure changes.
  • Sensitivity: Carotid baroreceptors are more sensitive than aortic ones.

Blood Volume and Haemorrhage

  • Decreased blood volume leads to decreased blood pressure.
  • Baroreceptors respond with increased sympathetic activity:
  • Elevated heart rate and force.
  • Improvement in blood pressure and vasotone.

Baroreceptor Reflex Mechanism

  • Reduced arterial pressure leads to decreased baroreceptor activity.
  • Induces a series of responses to restore heart rate, stroke volume, and overall blood pressure.

Blood Pooling

  • Results in venous return and pressure drops:
  • Increase in capillary blood pressure leading to potential swelling.

Standing and Baroreceptor Stimulation

  • Blood volume shifts a portion from the heart/brain towards the feet, reducing cardiac output/blood pressure.
  • Baroreceptor Reflex Activation: Increases heart rate, contractility, resistance, and venous return to recover blood pressure.

Antidiuretic Hormone: Vasopressin

  • Role: Regulates blood volume and pressure through absorption and vasoconstriction.
  • Impact on ECF volume, osmolarity, and blood pressure.

Central Ischemic Response

  • Triggered by reduced brain blood supply.
  • Occurs at blood pressures below 60 mmHg, capable of raising blood pressure significantly while limiting blood flow to specific tissues.

Long Term Blood Pressure Control

  • Blood volume directly influences venous return, stroke volume, and cardiac output.
  • Maintaining blood volume is crucial for blood pressure stability, influenced by fluid intake and renal function.

Summary

  • Local Blood Flow Control: Maintained by arterial pressure and local metabolites.
  • Regulation Mechanisms:
  • Vasoconstriction and resistance changes.
  • Veno-constriction enhances stroke volume.
  • Inotropic influences on stroke volume.
  • Chronotropic changes affect cardiac output.
  • Baroreceptor Functionality: Key in various physiological changes (standing, exercise, blood loss).
  • Acknowledgment of broader mechanisms such as vasopressin and central ischemic response for blood pressure management.