💓

Hemodynamics Core Principles

Sep 28, 2025

Overview

This lecture explains the core principles of hemodynamics—the study of blood flow through the circulatory system. It covers definitions, key equations, types of flow, energy changes, vessel anatomy, and physiological factors affecting blood movement, all foundational for understanding Doppler ultrasound.

Introduction to Hemodynamics

  • Hemodynamics is the study of how blood moves through the circulatory system.
  • Understanding hemodynamics is essential for interpreting Doppler ultrasound.
  • Two main measurements:
    • Volume flow rate (also called flow rate or flow): how much blood passes a point per unit time (e.g., mL/s).
    • Velocity: how fast blood moves at a point, including both speed and direction (e.g., cm/s, toward or away from the transducer).

Basic Principles and Formulas

  • Volume flow rate (Q): Expressed as volume per time (e.g., mL/s or gallons/min).
  • Velocity: Expressed as distance per time (e.g., cm/s), always includes direction.
  • Blood flow is affected by:
    • Pressure differences (gradients)
    • Resistance within vessels
    • Vessel size (diameter/radius)
  • Viscosity: The resistance of a fluid to flow; describes fluid thickness and is measured in poise.
    • Water has low viscosity (flows easily); honey has high viscosity (flows slowly).
    • Blood is about five times more viscous than water.
    • Medical conditions can alter blood viscosity:
      • Anemia: Fewer red blood cells, blood is less viscous (thinner).
      • Polycythemia: More red blood cells, blood is more viscous (thicker).
    • Hematocrit: Percentage of blood made up of red blood cells; high in polycythemia, low in anemia.
  • Pressure: The driving force for blood flow; measured as force per unit area (e.g., Pascals, pounds per square inch).
    • Flow only occurs when there is a pressure difference (gradient) between two points.
    • The heart and gravity are the main sources of pressure in the circulatory system.
  • Volumetric flow rate formula:
    • Q = ΔP / R
      • Q: volume flow rate (mL/s)
      • ΔP: change in pressure (force/area)
      • R: resistance (poise)
    • Flow increases with greater pressure difference and decreases with higher resistance.

Resistance and Poiseuille’s Law

  • Resistance (R): Opposition to flow, influenced by:
    • Viscosity of the fluid (thicker fluid = more resistance)
    • Length of the vessel (longer vessel = more resistance)
    • Radius of the vessel (smaller radius = much more resistance)
  • Resistance formula:
    • R = (8 × length × viscosity) / (π × radius⁴)
      • Increasing viscosity or length increases resistance.
      • Increasing radius (even slightly) greatly decreases resistance (radius to the fourth power).
  • Poiseuille’s equation:
    • Combines flow rate and resistance:
      • Q = (ΔP × π × radius⁴) / (8 × length × viscosity)
      • Alternatively, can use diameter instead of radius (changes constants).
    • Vessel radius/diameter has the largest effect on flow rate.
  • Clinical relevance:
    • Small tubes (narrow vessels) have high resistance and low flow.
    • Large tubes (wide vessels) have low resistance and high flow.
    • The body can control blood flow to organs by changing vessel diameter (vasoconstriction/vasodilation).

Types of Blood Flow

  • Laminar flow: Streamlines are parallel and organized.
    • Plug flow: All layers move at the same speed (often at vessel entrance).
    • Parabolic flow: Center moves fastest, outer layers slower due to friction (most common in vessels).
    • Disturbed flow: Streamlines are parallel but not straight, often near branches or slight narrowings.
  • Turbulent flow: Chaotic, swirling movement with eddies and vortices.
    • Occurs after severe narrowing (stenosis) or at high velocities.
    • Forward flow persists, but with variable speeds and directions.
  • Sound and clinical findings:
    • Laminar flow is silent (like a calm river).
    • Turbulent flow is noisy (like rapids or a waterfall).
      • Bruit: Turbulent flow heard with a stethoscope.
      • Thrill: Turbulent flow felt as a vibration.
  • Reynolds number: Predicts flow type.
    • <1500: laminar flow
    • 1500–2000: indeterminate/disturbed

    • 2000: turbulent flow

Flow Patterns in Vessels

  • Pulsatile flow:
    • Caused by cardiac contractions.
    • Seen in arteries.
    • Flow velocity varies with each heartbeat (systole and diastole).
    • Spectral tracing shows alternating forward and backward movements.
  • Phasic flow:
    • Caused by respiration.
    • Seen in veins.
    • Flow velocity varies with breathing (inhalation/exhalation).
    • Spectral tracing shows changes in flow with respiratory cycle.
  • Steady flow:
    • Constant speed.
    • Seen in veins during breath-holding or in the portal vein.
    • Spectral tracing shows a consistent velocity.

Energy and Losses in Circulation

  • Pressure gradients represent energy differences that drive blood flow.
  • Law of conservation of energy: Energy cannot be created or destroyed, only transformed (e.g., potential to kinetic).
  • Types of energy loss:
    • Viscous loss: Energy used to overcome fluid’s internal stickiness (higher in thicker blood).
    • Frictional loss: Energy lost as heat due to friction between blood and vessel walls.
    • Inertial loss: Energy lost when blood changes direction or speed, especially at stenoses or with pulsatile flow.

Stenosis and Bernoulli’s Principle

  • Stenosis: Narrowing of a vessel or valve, significantly affecting blood flow.
    • Five key effects:
      1. Blood changes direction entering and exiting the narrowing.
      2. Velocity increases through the stenosis to maintain flow rate.
      3. Turbulent flow occurs distal to the narrowing.
      4. Pressure decreases within the stenosis (pressure gradient forms).
      5. Loss of flow pulsatility before the narrowing.
  • Bernoulli’s principle: As velocity increases in a stenosis, pressure decreases (energy is conserved by converting pressure energy to kinetic energy).
    • After the stenosis, pressure increases and velocity decreases as flow returns to normal.

Hydrostatic Pressure

  • Hydrostatic pressure: Pressure exerted by a column of fluid due to gravity.
    • Formula: P = h × g × ρ
      • h: height of fluid column
      • g: gravity
      • ρ (rho): fluid density
    • In the body:
      • At heart level: hydrostatic pressure is zero.
      • Above the heart: negative hydrostatic pressure.
      • Below the heart: positive hydrostatic pressure, increasing with distance from the heart.
    • Clinical implications:
      • Upright patients have higher pressure in lower body (e.g., ankles).
      • Supine (lying down) patients have even pressure throughout.
      • Blood pressure should be measured at heart level for accuracy.
      • Measured pressure = true blood pressure + hydrostatic pressure.

Vessel Anatomy & Physiology

  • Vessel structure:
    • All vessels have three layers:
      • Tunica intima: innermost layer.
      • Tunica media: middle muscular layer (thicker in arteries for high pressure).
      • Tunica adventitia: outer connective tissue layer (contains vasa vasorum in large vessels).
  • Arteries:
    • Thick tunica media for handling high pressure and pulsatility.
    • Arterioles (small arteries) can constrict (vasoconstriction) or dilate (vasodilation) to regulate flow.
  • Veins:
    • Thinner walls, less muscle, but very flexible (high capacitance).
    • Contain valves to prevent backflow.
    • Store about two-thirds of total blood volume.
    • Can dilate to increase flow back to the heart.

Venous Return and Respiratory Effects

  • Venous return is aided by:
    • Valves: Prevent backflow, ensuring one-way movement toward the heart.
    • Calf muscle pump: Muscle contractions squeeze veins, pushing blood upward.
    • Respiratory changes: Breathing alters pressure in thoracic and abdominal cavities, affecting venous flow.
      • Inhalation: Diaphragm moves down, thoracic pressure decreases (increases return from arms/head), abdominal pressure increases (decreases return from legs).
      • Exhalation: Diaphragm moves up, thoracic pressure increases (decreases return from arms/head), abdominal pressure decreases (increases return from legs).
  • Pressure gradient: Venous system has low pressure (about 15 mmHg), right atrium even lower (about 8 mmHg), maintaining forward flow.

Key Terms & Definitions

  • Hemodynamics: Study of blood flow in the circulatory system.
  • Volume Flow Rate (Q): Amount of blood passing a point per unit time.
  • Velocity: Speed and direction of blood flow.
  • Viscosity: Fluid’s resistance to flow.
  • Resistance (R): Opposition to flow, affected by viscosity, length, and radius.
  • Laminar Flow: Orderly, parallel flow (plug, parabolic, disturbed).
  • Turbulent Flow: Chaotic, disorganized flow with eddies and vortices.
  • Stenosis: Narrowing of a vessel or valve.
  • Bernoulli’s Principle: Inverse relationship between pressure and velocity in a narrowing.
  • Hydrostatic Pressure: Pressure from a fluid column due to gravity.
  • Vasoconstriction/Vasodilation: Narrowing/widening of vessel diameter, affecting resistance and flow.

Action Items / Next Steps

  • Complete workbook activities and answer end-of-chapter open-ended questions to review and reinforce these concepts.
  • Focus on understanding how changes in pressure, resistance, vessel diameter, and physiological factors affect blood flow.
  • Be able to describe and identify types of flow, energy losses, and the effects of vessel anatomy and respiration on circulation.

Full transcript