What is Blood Pressure?   What is the Clinical Significance of High or Low Blood Pressure?

Definition:

• Blood pressure is the force exerted by blood on the walls of blood vessels.
• Highest when blood leaves the heart in the aorta.

Normal Ranges:

• Systolic pressure: ~120 mm Hg — during ventricular contraction (systole).
• Diastolic pressure: ~80 mm Hg — during ventricular relaxation (diastole).
• Mean arterial pressure (MAP) = Diastolic BP + 1/3 (Systolic BP – Diastolic BP) = 80 + 1/3 (120-80) = 80 +1/3 (40) = 80 + 13.333 = ~93 mm Hg — average pressure in arteries.

Pressure Gradient:

• Blood flows from high pressure regions (aorta, arteries) to low pressure regions (veins, right atrium).
• Pressure drops progressively through the Systemic Circuit:
  • Aorta: ~120 mm Hg (systolic), 80 mm Hg (diastolic), and 93mm Hg (MAP)
  • Capillaries: ~35 mm Hg
  • Venous system: ~18 mm Hg
  • Right atrium: ~2 mm Hg
• The pressure difference (e.g., 93mm Hg to 2 mm Hg) drives blood flow.
• The body maintains blood pressure within normal limits (~120/80 mm Hg).

Factors Influencing Blood Pressure:

• Cardiac output:
  • Higher heart rate or stroke volume increases pressure.
• Total Peripheral Resistance:
  • Increased resistance (e.g., narrowed arteries) raises blood pressure.
  • Blood Flow is inversely related to resistance, so when resistance increases (vasoconstriction), blood flow decreases in that area.  Likewise, when resistance decreases (vasodilation), blood flow increases in that area.

Importance of Maintaining Normal Blood Pressure

Why control blood pressure?

• High blood pressure (hypertension):
  • Damages blood vessel walls, causing thickening and stiffening (loss of elasticity).
  • Forces the heart to work harder, risking cardiac strain, heart deterioration and failure.
  • Often asymptomatic but can silently cause vascular damage, increasing risks of atherosclerosis, organ damage, stroke, kidney failure, and cardiovascular disease.
• Low blood pressure (hypotension):
  • Reduces tissue perfusion.
  • Leads to hypoxia, which can especially affect the brain causing dizziness, fainting, confusion, blurred vision, fatigue.
  • Risk of inadequate blood flow to vital organs, especially brain.
  • The body compensates by increasing HR via medulla to restore pressure.

Local (Autoregulation) & Systemic (Neural and Hormonal) Regulation of Blood Pressure:

1. Autoregulation (local control):
   • In tissues (e.g., arm muscles), local changes (like increased activity or hypoxia) trigger vasodilation—reducing resistance to increase blood flow.
   • Example: During exercise, muscles signal for vasodilation to meet oxygen demand.
2. Systemic Short-term Regulation: Neural Mechanisms
   • The medulla oblongata adjusts HR and vessel tone via the cardiac accelerator (sympathetic) and vagal (parasympathetic) centers.
   • High blood pressure: Triggers parasympathetic response, leading to vasodilation, slowed HR, and reduced cardiac output.
   • Low blood pressure: Triggers sympathetic response, increasing HR, contractility, and vasoconstriction to raise BP.
3. Systemic Long-term Regulation: Hormonal Mechanisms
   • Renin-Angiotensin-Aldosterone-ADH System (RAAAS):
     • Activated when blood pressure (BP) and/or blood volume (BV) drops.
     • Kidneys release renin, converting angiotensinogen into angiotensin I.
     • Angiotensin Converting Enzyme (ACE) produced in lungs: Converts angiotensin I to angiotensin II.
     • Angiotensin II:
       • Potent vasoconstrictor (raises BP).
       • Stimulates aldosterone secretion from the cortex adrenal glands—to retain salt and water, increasing blood volume and BP.
       • Stimulates posterior pituitary gland to release ADH (Antidiuretic hormone), which reabsorbs water in kidneys, further increasing blood volume and BP.
   • Response to High Blood Pressure:
     • Release of Atrial Natriuretic Peptide (ANP):
       • Released from atria when blood volume/pressure is high.
       • Promotes natriuresis (salt excretion) by the kidneys.
       • Within the nephron tubules, water follows salt to be excreted in the form of urine, lowering both blood volume and BP.
       • Inhibits renin, aldosterone, and vasoconstriction—promoting vasodilation, further lowering blood volume and BP.

Clinical Considerations:

• Hypertension:
  • Common; about 25% of Canadians affected.
  • Often undiagnosed; increases risk of cardiovascular complications.
  • Risk factors include obesity, inactivity, poor diet.
• Hypotension:
  • Can cause dizziness, fainting, organ hypoxia.
  • The body responds by increasing heart rate to compensate.

Summary:  

Maintaining blood pressure within a healthy range is essential for effective tissue perfusion and preventing vascular and cardiac damage. Regular monitoring and lifestyle adjustments are key to managing blood pressure health.

The body maintains blood pressure through rapid neural responses (parasympathetic and sympathetic) and long-term hormonal regulation via the RAAAS system and atrial natriuretic peptide. These systems work together to modulate HR, contractility, vessel diameter, and blood volume, ensuring tissues receive adequate blood flow under varying physiological conditions.