Hemodynamics: Principles of Blood Flow

Complete tutorial on hemodynamics - blood flow, pressure, resistance, and the physical principles governing circulation. Includes Poiseuille law, Laplace law, flow types, and clinical applications.

Hemodynamics 4 min read
This content is for informational purposes only. Always consult a healthcare professional.

Hemodynamics is the study of blood flow through the cardiovascular system. The principles governing blood flow are based on the physics of fluid dynamics applied to the vascular system.

Fundamental Principles

Ohm Law Applied to Circulation

Flow (Q) = ΔP / R

Where:

  • Q = Blood flow (mL/min or L/min)
  • ΔP = Pressure difference across the system (mmHg)
  • R = Resistance to flow (mmHg·min/L)

Systemic circulation:

  • Cardiac output = (MAP - CVP) / SVR
  • Normal CO: 5 L/min
  • Normal SVR: 800-1200 dyn·s·cm⁻⁵

Poiseuille Law

The resistance to flow in a tube depends on:

R = (8ηL) / (πr⁴)

Where:

  • η = Blood viscosity
  • L = Tube length
  • r = Tube radius

Clinical importance:

  • Flow is proportional to r⁴ (doubling radius increases flow 16x)
  • Small changes in vessel diameter greatly affect flow
  • Vasoconstriction dramatically increases resistance

Types of Blood Flow

TypeDescriptionOccurrence
LaminarStreamlined, parabolic velocity profileNormal arteries, arterioles
TurbulentChaotic, eddies, higher energy lossPathologic (stenosis, fistula)
Plug flowFlat velocity profileAscending aorta (initial)

Reynolds Number (Re): Predicts turbulent flow

Re = (ρ × v × d) / η

Turbulence occurs when Re > 2000

Pressure in the Cardiovascular System

Pressure Gradient

Blood flows from high pressure to low pressure:

LocationMean Pressure (mmHg)
Left ventricle (systole)100-120
Aorta90-100
Large arteries85-95
Arterioles60-80
Capillaries20-30
Venules10-15
Veins5-10
Vena cava0-5
Right atrium (CVP)0-6
Right ventricle (systole)15-25
Pulmonary artery10-20
Pulmonary capillaries5-10
Pulmonary veins5-10
Left atrium5-12

Blood Pressure Components

Systolic blood pressure (SBP): Peak arterial pressure during systole (100-140 mmHg)

Diastolic blood pressure (DBP): Minimum arterial pressure during diastole (60-90 mmHg)

Pulse pressure (PP): SBP - DBP (30-50 mmHg)

Mean arterial pressure (MAP):

  • MAP = DBP + 1/3(PP)
  • MAP = (SBP + 2×DBP) / 3
  • Normal: 70-100 mmHg

Central vs. Peripheral Pressure

MeasurementValueDifference
Central aortic SBP100-120 mmHgLower than peripheral
Brachial SBP5-10 mmHg higher than centralPulse pressure amplification
Radial SBP10-15 mmHg higher than centralFurther amplification
Femoral SBPSimilar to brachialMinimal difference

Resistance

Types of Resistance

Total peripheral resistance (TPR):

  • Resistance of the entire systemic circulation
  • Normal: 800-1200 dyn·s·cm⁻⁵
  • Regulated primarily by arteriolar diameter

Organ-specific resistance:

OrganFlow (mL/min)% of COResistance (R units)
Kidney120024%Low
Brain75015%Low
Heart2505%Very low
Skeletal muscle120024%Variable
Skin50010%Variable
Splanchnic140028%Moderate
Other2004%Moderate

Resistance in Series vs. Parallel

Series resistance: Total resistance = R₁ + R₂ + R₃ + …

  • Example: Aorta → arterioles → capillaries

Parallel resistance: 1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + …

  • Example: Organ circulations in parallel

Vascular Resistance by Vessel Type

Vessel% of Total Resistance
Large arteries10%
Small arteries15%
Arterioles50%
Capillaries15%
Venules5%
Veins5%

Compliance

Compliance (C) = ΔV / ΔP

VesselCompliance
AortaLow stretch but large volume
Muscular arteriesLow
ArteriolesVery low
CapillariesVery low
VenulesModerate
VeinsVery high (20-30× arterial compliance)

Venous capacitance:

  • Contain 60-70% of total blood volume
  • Sympathetic venoconstriction mobilizes blood
  • Venous return determines cardiac output

The Windkessel Effect

The aorta and large elastic arteries serve as a pressure reservoir:

Systole:

  • Aorta expands, stores kinetic energy as potential energy
  • About 50% of stroke volume is stored temporarily

Diastole:

  • Aorta recoils, releases stored energy
  • Maintains forward blood flow during ventricular relaxation
  • Converts pulsatile flow to continuous flow

Clinical significance:

  • Arterial stiffness reduces Windkessel effect
  • Increased pulse pressure in elderly
  • Increased afterload on the left ventricle

Velocity of Blood Flow

VesselCross-Sectional Area (cm²)Velocity (cm/s)
Aorta2-440-60
Large arteries5-1020-40
Arterioles50-1001-3
Capillaries2500-50000.03-0.1
Venules200-4000.2-1
Veins50-1005-20
Vena cava3-515-40

Relationship: Velocity = Flow / Cross-sectional area

Shear Stress

Shear stress on the endothelium from flowing blood:

τ = 4ηv / r

Where:

  • τ = Shear stress (dyn/cm²)
  • η = Blood viscosity
  • v = Flow velocity
  • r = Vessel radius

Physiologic effects:

  • Endothelial NO release (high shear → vasodilation)
  • Endothelial gene expression
  • Atherosclerosis localization (low shear areas at branch points)

Clinical Hemodynamics

Cardiac Output Measurement

Fick method:

  • CO = VO₂ / (CaO₂ - CvO₂)
  • Requires O₂ consumption and blood gas measurements

Thermodilution:

  • Cold saline injected into right atrium
  • Temperature change detected in pulmonary artery
  • Most common clinical method

Hemodynamic Monitoring

ParameterNormal RangeMeasurement
Central venous pressure (CVP)0-6 mmHgCentral line
Pulmonary artery pressure15-30/5-10 mmHgSwan-Ganz catheter
PCWP4-12 mmHgSwan-Ganz (wedged)
Cardiac output4-8 L/minThermodilution
Cardiac index2.5-4.0 L/min/m²CO / BSA
SVR800-1200 dyn·s·cm⁻⁵(MAP-CVP)/CO × 80

Shock States

TypeCardiac OutputSVRFilling Pressures
Hypovolemic↓↑↓
Cardiogenic↓↑↑ (PCWP)
Septic↑ (early), ↓ (late)↓Normal or ↓
Obstructive↓↑↑ (CVP, PCWP)

Hypertension Hemodynamics

TypePrimary AbnormalityHemodynamic Profile
Essential HTNIncreased SVRCO normal or low, SVR high
Isolated systolic HTNArterial stiffnessWide pulse pressure
Hyperdynamic circulationIncreased COCO high, SVR normal
Renal HTNIncreased volumeCO normal, SVR high