Hemodynamics: In-Depth Study Notes

Hemodynamics: A Comprehensive Study Guide

Basic Concepts of Blood Flow

  • Volume Flow Rate: The volume of blood moving during a particular time.
    • Units of Flow: \text{volume/time}
  • Velocity: Speed in a given direction.
    • Units of Velocity: Any distance divided by time (e.g., \text{cm/s}).

Forms of Blood Flow

There are three basic forms of blood flow:

  • Pulsatile Flow:
    • Characterized by variable velocity.
    • Result of cardiac contraction.
  • Phasic Flow:
    • Occurs when blood moves with variable velocity.
    • In response to respirations (e.g., in veins).
  • Steady Flow:
    • Occurs when a fluid moves at a constant speed or velocity.
    • Less common in healthy arterial circulation, but can be induced by pathologies like severe stenosis downstream.

Laminar vs. Turbulent Flow

Laminar Flow

  • Defined as relatively uniform, layered flow of a liquid through a vessel.
  • Types of Laminar Flow:
    • Plug Flow:
      • Occurs when all layers and blood cells travel at the same velocity.
      • Commonly seen at the entrance of large vessels.
    • Parabolic Flow:
      • Laminar flow with faster velocities in the center of the vessel lumen.
      • Slower velocities along the vessel wall due to friction.

Turbulent Flow

  • Defined as chaotic flow patterns in many different directions and at many speeds.
  • Thrill: Tissue vibration associated with turbulence.

Reynolds Number (R_e)

  • A unitless number indicating whether flow is laminar or turbulent.
  • Laminar Flow: \text{Reynolds number} < 1,500
  • Turbulent Flow: \text{Reynolds number} > 2,000
  • Between 1,500 and 2,000 is a transitional zone where flow can be unstable.

Energy in the Circulatory System

Energy Source

  • Cardiac Contraction: Provides the initial energy to the circulating blood.

Energy Gradient

  • Blood moves from locations of higher energy to locations of lower energy.
  • This gradient drives blood flow throughout the body.

Forms of Energy in Blood

  1. Kinetic Energy:
    • The energy an object has due to its motion.
    • Determined by an object's mass and speed at which it moves through the formula E_k = \frac{1}{2}mv^2.
  2. Pressure Energy:
    • A form of potential energy with the ability to do work.
    • Represents the stored energy required to keep vessels expanded.
  3. Gravitational Energy:
    • A form of potential energy stored in an object due to its height above Earth.
    • Significant in upright positions, influencing hydrostatic pressure.

Energy Loss in Circulation

Energy is continuously lost as blood flows through the circulatory system, primarily in three ways:

  1. Viscous Loss:
    • Related to the internal friction within a fluid (viscosity).
    • Determined by hematocrit, the percentage of red blood cells. Higher hematocrit means higher viscosity and greater viscous loss.
  2. Frictional Loss:
    • Occurs when flow energy is converted to heat as one object rubs against another, specifically between blood and the vessel walls.
  3. Inertial Loss:
    • Relates to the tendency of a fluid to resist changes in its velocity.
    • Energy is lost when the speed of fluid changes, regardless of whether it speeds up or slows down (e.g., at stenoses, bifurcations, or any change in vessel diameter).

Stenosis and Its Effects

Definition

  • Stenosis: The abnormal narrowing of a passage in the body (e.g., a blood vessel).

Effects of Stenosis on Blood Flow

Stenosis significantly alters blood flow dynamics:

  • Changes in Flow Direction: Blood flow patterns become disturbed.
  • Increased Velocities within the Stenotic Region: To maintain volume flow rate, blood speed must increase through the narrowed segment (Continuity Equation).
  • Downstream Turbulence: The sudden expansion of the vessel after the stenosis often leads to chaotic flow patterns.
  • Pressure Gradient Across Stenosis: A significant drop in pressure occurs distal to the stenosis due to energy loss and conversion of potential to kinetic energy.
  • Conversion of Pulsatile Flow to Steady Flow: In severe cases, the pulsatile nature of arterial flow can be damped, leading to more steady flow distal to the stenosis.

Bernoulli's Principle

  • Principle Statement: With steady flow, the sum of all forms of energy (kinetic, potential, gravitational) remains constant. Specifically, the sum of kinetic and pressure energy remains constant.
  • Relationship between Velocity and Pressure: When velocity increases, pressure decreases.
  • Simplified Bernoulli Equation (for calculating pressure drop across a stenosis): \Delta P = 4V^2
    • Where \Delta P is the pressure gradient (in mmHg) and V is the velocity (in m/s) at the point of interest (e.g., in the stenosis).

Related Principles

  • Flow \times Resistance = Pressure Gradient (Analogous to Ohm's Law)
  • Current \times Resistance = Voltage (Ohm's Law in electrical circuits)

Venous Hemodynamics

Vein Adaptation to Increased Flow

  • Veins are highly compliant vessels.
  • To accommodate more volume (increased flow), veins change shape:
    • From an