M04.03.08 Control of blood pressure

Blood pressure (BP) regulation is a critical aspect of cardiovascular physiology, involving a complex interplay of neural, hormonal, and renal mechanisms. Understanding these regulatory systems is essential for medical students to grasp how the body maintains hemodynamic stability and responds to pathological states.

Components of Blood Pressure Regulation

Blood pressure is determined by cardiac output (CO) and systemic vascular resistance (SVR): BP=CO×SVR

• Cardiac Output (CO): The volume of blood pumped by the heart per minute.
• Systemic Vascular Resistance (SVR): The resistance offered by the systemic blood vessels.

Key Regulatory Mechanisms

1. Neural Mechanisms
   • Autonomic Nervous System (ANS)
   • Baroreceptor Reflex
   • Chemoreceptor Reflex
2. Hormonal Mechanisms
   • Renin-Angiotensin-Aldosterone System (RAAS)
   • Antidiuretic Hormone (ADH)
   • Atrial Natriuretic Peptide (ANP)
3. Renal Mechanisms
   • Pressure Natriuresis
   • Renal Sympathetic Nerve Activity

Neural Mechanisms

Autonomic Nervous System (ANS)

The ANS plays a crucial role in short-term BP regulation:

• Sympathetic Nervous System (SNS): Increases heart rate, contractility, and vasoconstriction.
• Parasympathetic Nervous System (PNS): Decreases heart rate and promotes vasodilation.

Baroreceptor Reflex

Baroreceptors in the carotid sinus and aortic arch sense changes in BP:

• High BP: Increases baroreceptor firing → Inhibits SNS → Decreases BP.
• Low BP: Decreases baroreceptor firing → Activates SNS → Increases BP.

Chemoreceptor Reflex

Chemoreceptors in the carotid and aortic bodies respond to changes in blood pH, O2, and CO2 levels, indirectly affecting BP.

Hormonal Mechanisms

Renin-Angiotensin-Aldosterone System (RAAS)

The RAAS regulates BP by controlling blood volume and SVR:

1. Renin Release: Triggered by low BP, low Na+, or SNS activation.
2. Angiotensin II: Potent vasoconstrictor; stimulates aldosterone release.
3. Aldosterone: Increases Na+ and water reabsorption in the kidneys.

Antidiuretic Hormone (ADH)

ADH, released by the posterior pituitary, increases water reabsorption in the kidneys, thus increasing blood volume and BP.

Atrial Natriuretic Peptide (ANP)

ANP, released by atrial myocytes in response to high blood volume, promotes Na+ and water excretion, thus decreasing BP.

Renal Mechanisms

Pressure Natriuresis

Increased renal perfusion pressure leads to increased Na+ and water excretion, reducing blood volume and BP.

Renal Sympathetic Nerve Activity

Sympathetic activation decreases renal blood flow and glomerular filtration rate, promoting Na+ and water retention, thus increasing BP.

Table: Summary of Blood Pressure Regulatory Mechanisms

Side Notes: Key Points to Memorize

• Baroreceptor Reflex: Rapidly adjusts BP through ANS.
• RAAS: Key hormonal system controlling blood volume and SVR.
• ADH and ANP: Hormones with opposing effects on blood volume.
• Pressure Natriuresis: Kidney’s response to increased BP by excreting more Na+ and water.

Bibliography

1. Guyton, A. C., & Hall, J. E. (2016). Textbook of Medical Physiology (13th ed.). Elsevier.
2. Hall, J. E. (2015). Guyton and Hall Physiology Review (3rd ed.). Saunders.
3. Boron, W. F., & Boulpaep, E. L. (2017). Medical Physiology (3rd ed.). Elsevier.
4. Barrett, K. E., Barman, S. M., Boitano, S., & Brooks, H. L. (2012). Ganong’s Review of Medical Physiology (24th ed.). McGraw-Hill Education.
5. Golan, D. E., Tashjian, A. H., Armstrong, E. J., & Armstrong, A. W. (2017). Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy (4th ed.). Lippincott Williams & Wilkins.