Blood Pressure: Introductory Concepts & Terminology

1. Blood Pressure (BP) — Definition

• Formula:
  BP = Cardiac Output (CO) × Total Peripheral Resistance (TPR)

• Core Concept:
  Blood pressure is determined by how much blood the heart pumps and the resistance that blood encounters as it flows through the vessels.

2. Cardiac Output (CO)

• Definition:
  The volume of blood pumped by the heart per minute.

• Formula:
  CO = Heart Rate (HR) × Stroke Volume (SV)

• Units:
  Milliliters per minute (mL/min)

A. Heart Rate (HR)

• Definition:
  Number of heartbeats per minute.

• Regulated by:

  • Parasympathetic Nervous System: Decreases HR.

  • Sympathetic Nervous System & Hormones (Epinephrine, Thyroid Hormone): Increase HR.

  • Ions (Ca²⁺, K⁺, Na⁺): Imbalances can increase or decrease HR.

B. Stroke Volume (SV)

• Definition:
  The amount of blood pumped per heartbeat (mL/beat).

• Determined by Three Factors:

1. Preload

   • Definition: The stretch on the heart muscle before it contracts (related to End-Diastolic Volume, EDV).

   • Increased by: Increased blood volume → ↑ EDV → ↑ Preload → ↑ Stroke Volume

2. Contractility

   • Definition: The force of the heart’s contraction.

   • Increased by: Sympathetic activity (Epinephrine/Norepinephrine), hormones (Glucagon, T3/T4), and Calcium → ↑ Stroke Volume

3. Afterload

   • Definition: The resistance the heart must overcome to eject blood.

   • Increased by: Hypertension, atherosclerosis, or high peripheral resistance → ↓ Stroke Volume

3. Flow, Velocity, and Cross-Sectional Area

A. Flow (or Flow Rate)

• Definition:
  Volume of blood moving through a vessel per unit time.

• Units:
  Cubic centimeters per minute (cm³/min) or milliliters per minute (mL/min).

• Key Relationship:
  Flow ≈ Cardiac Output

B. Velocity of Blood Flow

• Definition:
  The speed of blood movement.

• Formula:
  Velocity (cm/min) = Flow (cm³/min) ÷ Cross-Sectional Area (cm²)

• Key Relationships:

  • Velocity increases with ↑ Flow (↑ Cardiac Output).

  • Velocity decreases with ↑ Cross-Sectional Area.

C. Cross-Sectional Area

• Definition:
  The total area of all blood vessel lumens at a specific point in the circulatory system.

• Key Insight:
  While individual capillaries are tiny, the total cross-sectional area of all capillaries combined is enormous.

• Order of Vessels (by increasing total cross-sectional area):
  Aorta → Arteries → Arterioles → Capillaries (largest total area) → Venules → Veins

• Physiological Importance:

  • Velocity is highest in the aorta (smallest total area).

  • Velocity is lowest in the capillaries (largest total area).

  • Slow capillary flow allows for efficient exchange of gases, nutrients, and wastes.

4. Total Peripheral Resistance (TPR)

• Definition:
  The total resistance to blood flow in the systemic circulation.

A. Flow–Resistance–Pressure Relationship

• Formula:
  Flow = ΔP / R, or rearranged as CO = BP / TPR

B. Poiseuille’s Equation (Factors Affecting Resistance)

• Formula:
  R = (8 × η × L) / (π × r⁴)

  where:

  • η (Viscosity): Thickness of the blood

  • L: Length of the vessel

  • r: Radius of the vessel

• Impact of Each Factor:

  • Viscosity (η): Directly proportional to resistance

    • ↑ Viscosity → ↑ Resistance (e.g., Polycythemia, Dehydration)

    • ↓ Viscosity → ↓ Resistance (e.g., Anemia)

  • Length (L): Directly proportional to resistance

    • ↑ Length → ↑ Resistance (seen in taller or heavier individuals)

  • Radius (r): Inversely proportional to r⁴ — the most critical factor

    • Vasoconstriction (↓ r): Greatly ↑ Resistance

    • Vasodilation (↑ r): Greatly ↓ Resistance

5. Types of Blood Flow

A. Laminar Flow

• Description:
  Smooth, streamlined flow in concentric layers.

• Characteristics:

  • Fastest in the center, slowest at the vessel wall.

  • Silent and low resistance.

  • Pressure and flow maintain a linear relationship.

B. Turbulent Flow

• Description:
  Chaotic, irregular flow.

• Causes:
  Atherosclerotic plaques, vessel narrowing, or physiological disturbances (e.g., blood striking heart valves).

• Characteristics:

  • Increases resistance.

  • Decreases flow efficiency.

  • Requires higher perfusion pressure.

  • Produces audible sounds (e.g., bruits in arteries, murmurs in the heart).

  • Pathological when caused by plaque or vessel damage.

6. Pressures: Systolic, Diastolic, and Mean Arterial Pressure (MAP)

A. Systolic Blood Pressure (~120 mmHg)

• Definition: Peak arterial pressure during ventricular contraction.

• Represents: The force generated by the heart.

B. Diastolic Blood Pressure (~80 mmHg)

• Definition: Minimum arterial pressure during ventricular relaxation.

• Represents: The baseline pressure maintained by arterial elasticity.

C. Pulse Pressure

• Definition: Difference between systolic and diastolic pressures.

• Formula: Pulse Pressure = SBP − DBP
  Example: 120 − 80 = 40 mmHg

D. Mean Arterial Pressure (MAP) (~93 mmHg)

• Definition: The average pressure driving blood through the tissues during one cardiac cycle.

• Formula: MAP = DBP + (⅓ × Pulse Pressure)
  Example: 80 + (⅓ × 40) = 93 mmHg

• Importance: Key determinant of tissue perfusion.

E. Perfusion Pressure

• Definition: The pressure gradient that drives blood flow through a tissue.

• Formula: Perfusion Pressure = MAP − Central Venous Pressure (CVP)

• Since CVP is low (~3–8 mmHg), perfusion pressure ≈ MAP in most cases.

7. Korotkoff Sounds

• Definition: The sounds heard through a stethoscope during manual blood pressure measurement.

• Procedure:

  1. Inflate the cuff above systolic pressure to occlude the brachial artery.

  2. Slowly deflate the cuff.

  3. The first tapping/swishing sound = Systolic BP.

  4. The point where sounds disappear = Diastolic BP.

Summary & Purpose

This foundational knowledge explains how the body regulates blood pressure and maintains perfusion.
Understanding the relationships between CO, TPR, flow, resistance, and MAP is essential for grasping the body’s compensatory mechanisms in conditions like hypertension, shock, or heart failure.