Regulation of Mean Arterial Pressure

Key Concepts

• Mean Arterial Pressure (MAP): Refers to the average pressure in a patient's arteries during one cardiac cycle.
• Systemic Vascular Resistance (SVR): The resistance the heart works against to pump blood; synonymous with afterload.

Effect of Afterload on Stroke Volume

• Afterload/Systemic Vascular Resistance:
  • An increase in afterload (SVR) makes it harder for blood to be ejected, decreasing stroke volume.
  • A decrease in afterload facilitates blood ejection, increasing stroke volume.
• Stroke Volume Impact:
  • Changes in SVR have a greater impact on MAP than on stroke volume.

Mean Arterial Pressure Formula

• SVR and MAP Relationship:
  • Increase in SVR leads to an increase in MAP, despite a potential decrease in stroke volume.
  • Decrease in SVR results in a decrease in MAP.

Regulatory Mechanisms

• Autonomic Nervous System:
  • Parasympathetic System:
    • Decreases heart rate, impacting cardiac output, and thus decreasing MAP.
  • Sympathetic System:
    • Affects heart rate, contractility, and vascular resistance, all contributing to increased MAP.

Sympathetic System Effects

• Heart Rate: Increases due to SA node stimulation.
• Contractility: Increased contractility and stroke volume due to ventricular muscle stimulation.
• Vasoconstriction:
  • Arterioles: Increases SVR and MAP.
  • Veins: Decreases compliance, increasing venous return and MAP.

Monitoring Blood Pressure

• Baroreceptors:
  • Located in the arch of the aorta and carotid bifurcation.
  • Detect changes in arterial wall stretch due to pressure differences.
• Cardiovascular Control Center:
  • Located in the medulla, responds to baroreceptor input.
  • Adjusts sympathetic and parasympathetic output to regulate MAP.

Responses to Blood Pressure Changes

Drop in Blood Pressure

• Sympathetic Response:
  • Increases heart rate via beta-1 receptors.
  • Increases contractility and stroke volume.
  • Causes vasoconstriction in arterioles and veins, increasing SVR and venous return.
• Parasympathetic Response: Reduced activity to increase heart rate.

Increase in Blood Pressure

• Sympathetic Response:
  • Decreases normal beta-1 receptor activity on SA node and ventricles, reducing heart rate and contractility.
  • Causes vasodilation of arterioles and veins, decreasing SVR and venous return.
• Parasympathetic Response:
  • Increases muscarinic receptor activity, decreasing heart rate by affecting pacemaker potential.

Summary

• MAP is regulated by adjusting SVR, heart rate, and stroke volume through the autonomic nervous system.
• Baroreceptors provide rapid feedback to maintain blood pressure homeostasis.