Blood pressure - mechanism 1 & 2, Blood Pressure - Control 3, 4, & 5, Blood Pressure - Clinical 6

Introduction to Blood Pressure Mechanics

• Blood pressure (BP) is a key aspect of the cardiovascular system, which consists of the heart (cardio) and blood vessels (vascular).

• The primary factors affecting BP are:

  • Heart: responsible for generating the pressure through its pumping action.

  • Blood Vessels: their resistance, particularly the diameter (radius), plays a crucial role in affecting blood flow and pressure.

Understanding Cardiac Output and Peripheral Resistance

• Cardiac Output (CO): the volume of blood pumped by the heart in one minute; calculated by:

  • CO = Stroke Volume × Heart Rate

• Peripheral Resistance: related to the resistance in blood vessels.

  • The most important factor in resistance is the radius of blood vessels:

    • Decreased radius = Vasoconstriction (increases resistance)

    • Increased radius = Vasodilation (decreases resistance)

Blood Pressure Equation

• Blood Pressure can be expressed as:

  • BP = CO × Peripheral Resistance

• Changes in CO and resistance greatly influence BP:

  • Increase in heart rate or stroke volume leads to increased CO and thus increased BP.

  • Increase in resistance (due to vasoconstriction) leads to increased BP.

Regulatory Mechanisms for Blood Pressure

• It's vital to maintain BP:

  • Low BP can lead to inadequate blood supply to tissues.

  • High BP can cause damage to blood vessels and organs.

• Short-term regulation primarily uses the nervous system, while long-term regulation involves hormonal responses and the urinary system.

Short-term Regulation of Blood Pressure

• The body responds to changes in BP quickly through:

  • Baroreceptors: pressure sensors located in arteries.

    • They detect stretch of arterial walls due to BP changes.

  • Signals from baroreceptors sent to the medulla oblongata to assess BP status.

    • A high rate of signals indicates high BP; low signals indicate low BP.

  • Sympathetic Nervous System activation leads to:

    • Increased heart rate and contractility (increased CO).

    • Vasoconstriction, increasing peripheral resistance.

  • Parasympathetic Nervous System activation reduces heart rate and thus lowers BP.

Long-term Regulation of Blood Pressure

• Involves hormonal control and kidney function:

• Renin-Angiotensin-Aldosterone System (RAAS):

  1. Kidneys release renin in response to low BP.

  2. Renin converts angiotensinogen (from the liver) into angiotensin I.

  3. Angiotensin I is converted to angiotensin II by Angiotensin Converting Enzyme (ACE) in the lungs.

  4. Angiotensin II acts to:

     • Stimulate thirst and fluid intake, increasing blood volume.

     • Cause vasoconstriction, increasing resistance.

     • Stimulate aldosterone release, promoting sodium and water retention by kidneys.

     • Stimulate Antidiuretic Hormone (ADH) to promote water reabsorption in kidneys.

Clinical Implications of Blood Pressure Regulation

• Hypertension can lead to adaptations in baroreceptors, making them less sensitive to high BP.

• Treatment of hypertension often involves multiple medications to:

  • Lower heart rate and contractility (reducing CO).

  • Cause vasodilation to lower peripheral resistance.

  • Use diuretics to manage blood volume.

• Caution is needed—rapidly lowering BP can trigger compensatory mechanisms, raising BP back to abnormal levels.

• Management of hypertension is gradual, as it mimics long-term physiological changes.