Notes on Blood Pressure Compensation Mechanisms

Introduction

• Focus on compensation mechanisms for low blood pressure (hypotension).
• Discuss hormonal, neural, and chemical responses to low blood pressure.

Definition of Hypotension

• Hypotension: Low blood pressure, defined as a systolic blood pressure less than 100 mmHg.
• Severe cases can drop to around 80 mmHg or lower, such as in hypovolemic shock.

Detection of Blood Pressure Changes

• Baroreceptors: Specialized pressure receptors detect changes in blood pressure.
  • Located in two areas:
    • Aortic Sinus: Detects blood pressure changes in the aorta, sends signals via the Vagus nerve (CN X).
    • Carotid Sinus: Located at the bifurcation of the carotid artery, sends signals via the Glossopharyngeal nerve (CN IX).
• Information from baroreceptors is sent to the nucleus of the tractus solitarius in the medulla.

Response Mechanisms to Low Blood Pressure

1. Sympathetic Nervous System Activation:

   • Cardiac Accelerator Center: Stimulates heart rate and contractility.
   • Vasomotor Center: Increases total peripheral resistance via vasoconstriction.
   • Cardiac Inhibitory Center: Inhibited to prevent slowing of heart rate.

2. Key Equations:

   • Blood Pressure = Cardiac Output x Total Peripheral Resistance
   • Cardiac Output = Heart Rate x Stroke Volume

Mechanisms in Action

• Heart Rate Increase:

  • Activation of cardiac accelerator center leads to:
    • Increased heart rate via norepinephrine acting on beta-1 adrenergic receptors.
    • More calcium influx = more action potentials = increased heart rate.

• Increased Contractility:

  • Norepinephrine also increases contractility in myocardium via the same mechanism, leading to more powerful contractions and increased stroke volume.

• Vasoconstriction:

  • Norepinephrine binds to alpha-1 adrenergic receptors in blood vessels, causing contraction and increasing vascular resistance, raising blood pressure.

Hormonal Responses

• Renin-Angiotensin-Aldosterone System (RAAS):

  • JG Cells in Kidneys release renin in response to low blood pressure.
  • Renin converts angiotensinogen (from liver) to angiotensin I.
  • Angiotensin I is converted to angiotensin II in lungs via ACE (Angiotensin Converting Enzyme).
  • Angiotensin II:
    • Stimulates aldosterone release from adrenal cortex.
    • Increases thirst by acting on hypothalamus.
    • Promotes ADH (vasopressin) release.

• Aldosterone's Role:

  • Increases sodium reabsorption in kidneys, promoting water retention.
  • Increases blood volume and blood pressure.

• ADH's Role:

  • Increases water reabsorption in kidneys via aquaporin-2 channels.
  • Increases blood volume and pressure.

Excretion Control

• Low blood pressure leads to decreased urine output (Oliguria), conserving blood volume.

Influence of the Brain on Blood Pressure

• Limbic System and Hypothalamus can affect cardiovascular responses during stress or anxiety.
• Can increase heart rate and contractility through sympathetic nervous system activation.

Conclusion

• Overview of body mechanisms to counteract hypotension.
• Next topic: Compensation mechanisms for high blood pressure.