U01.07.010 Resistance, pressure, flow

Understanding hemodynamics is crucial for the USMLE Step 1 exam. This topic follows basic principles of physics and fluid dynamics, particularly Ohm’s law.

Fundamental Equations

Ohm’s Law Analogy: ΔP=Q×R\Delta P = Q \times RΔP=Q×R Similar to: ΔV=I×R\Delta V = I \times RΔV=I×R where:
- ΔP = Pressure difference (driving force)
- Q = Flow rate
- R = Resistance
Volumetric Flow Rate: Q=v×AQ = v \times AQ=v×A where:
- Q = Flow rate
- v = Flow velocity
- A = Cross-sectional area

Resistance and Flow

Factors Affecting Resistance

Resistance is determined by vessel characteristics: R=8η×Lπr4R = \frac{8 \eta \times L}{\pi r^4}R=πr48η×L

where:

η = Viscosity
L = Vessel length
r = Vessel radius (most significant factor due to the fourth power relationship)

Total Resistance in Circulatory Networks

Configuration	Equation	Example
Series Resistance	RT=R1+R2+R3	Arteries → Arterioles → Capillaries
Parallel Resistance	1RT=1R1+1R2+1R3\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}RT1=R11+R21+R31	Capillary beds in systemic circulation

Capillaries have the highest total cross-sectional area and the lowest flow velocity to facilitate gas and nutrient exchange.
The pressure gradient drives blood flow from high pressure (arteries) to low pressure (veins).

Key Physiological Concepts

Parameter	Effect on Flow and Resistance
Arterioles	Major contributors to total peripheral resistance (TPR)
Veins	Largest blood reservoir, capable of high compliance
Viscosity	Primarily determined by hematocrit

Clinical Correlations

Condition	Effect on Viscosity (η)	Example
Hyperproteinemia	↑ Viscosity	Multiple myeloma
Polycythemia	↑ Viscosity	Increased RBC count
Anemia	↓ Viscosity	Reduced RBC count

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