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
| Type | Description | Occurrence |
|---|---|---|
| Laminar | Streamlined, parabolic velocity profile | Normal arteries, arterioles |
| Turbulent | Chaotic, eddies, higher energy loss | Pathologic (stenosis, fistula) |
| Plug flow | Flat velocity profile | Ascending 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:
| Location | Mean Pressure (mmHg) |
|---|---|
| Left ventricle (systole) | 100-120 |
| Aorta | 90-100 |
| Large arteries | 85-95 |
| Arterioles | 60-80 |
| Capillaries | 20-30 |
| Venules | 10-15 |
| Veins | 5-10 |
| Vena cava | 0-5 |
| Right atrium (CVP) | 0-6 |
| Right ventricle (systole) | 15-25 |
| Pulmonary artery | 10-20 |
| Pulmonary capillaries | 5-10 |
| Pulmonary veins | 5-10 |
| Left atrium | 5-12 |
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
| Measurement | Value | Difference |
|---|---|---|
| Central aortic SBP | 100-120 mmHg | Lower than peripheral |
| Brachial SBP | 5-10 mmHg higher than central | Pulse pressure amplification |
| Radial SBP | 10-15 mmHg higher than central | Further amplification |
| Femoral SBP | Similar to brachial | Minimal 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:
| Organ | Flow (mL/min) | % of CO | Resistance (R units) |
|---|---|---|---|
| Kidney | 1200 | 24% | Low |
| Brain | 750 | 15% | Low |
| Heart | 250 | 5% | Very low |
| Skeletal muscle | 1200 | 24% | Variable |
| Skin | 500 | 10% | Variable |
| Splanchnic | 1400 | 28% | Moderate |
| Other | 200 | 4% | 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 arteries | 10% |
| Small arteries | 15% |
| Arterioles | 50% |
| Capillaries | 15% |
| Venules | 5% |
| Veins | 5% |
Compliance
Compliance (C) = ΔV / ΔP
| Vessel | Compliance |
|---|---|
| Aorta | Low stretch but large volume |
| Muscular arteries | Low |
| Arterioles | Very low |
| Capillaries | Very low |
| Venules | Moderate |
| Veins | Very 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
| Vessel | Cross-Sectional Area (cm²) | Velocity (cm/s) |
|---|---|---|
| Aorta | 2-4 | 40-60 |
| Large arteries | 5-10 | 20-40 |
| Arterioles | 50-100 | 1-3 |
| Capillaries | 2500-5000 | 0.03-0.1 |
| Venules | 200-400 | 0.2-1 |
| Veins | 50-100 | 5-20 |
| Vena cava | 3-5 | 15-40 |
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
| Parameter | Normal Range | Measurement |
|---|---|---|
| Central venous pressure (CVP) | 0-6 mmHg | Central line |
| Pulmonary artery pressure | 15-30/5-10 mmHg | Swan-Ganz catheter |
| PCWP | 4-12 mmHg | Swan-Ganz (wedged) |
| Cardiac output | 4-8 L/min | Thermodilution |
| Cardiac index | 2.5-4.0 L/min/m² | CO / BSA |
| SVR | 800-1200 dyn·s·cm⁻⁵ | (MAP-CVP)/CO × 80 |
Shock States
| Type | Cardiac Output | SVR | Filling Pressures |
|---|---|---|---|
| Hypovolemic | ↓ | ↑ | ↓ |
| Cardiogenic | ↓ | ↑ | ↑ (PCWP) |
| Septic | ↑ (early), ↓ (late) | ↓ | Normal or ↓ |
| Obstructive | ↓ | ↑ | ↑ (CVP, PCWP) |
Hypertension Hemodynamics
| Type | Primary Abnormality | Hemodynamic Profile |
|---|---|---|
| Essential HTN | Increased SVR | CO normal or low, SVR high |
| Isolated systolic HTN | Arterial stiffness | Wide pulse pressure |
| Hyperdynamic circulation | Increased CO | CO high, SVR normal |
| Renal HTN | Increased volume | CO normal, SVR high |