Hemodynamics of Blood Flow in Circulatory System

Hemodynamics of Blood Flow in the Circulatory System

• Hemodynamics is the study of blood flow.
• Blood flow is categorized as laminar or turbulent.
• Laminar flow is smooth and silent, like liquid in narrow tubes.
• In laminar flow, the outer layer of blood is stationary, velocity increases towards the center of the stream until it reaches maximum velocity.
• Laminar blood flow occurs if velocity is below a critical velocity.
• Turbulent flow is disturbed, forming eddies in various directions.
• Turbulent flow produces sounds (bruits/murmurs) detectable by a stethoscope.
• Turbulent flow can lead to endothelial damage and atherosclerosis.
• The probability of turbulence, also known as the Reynolds number, is directly related to blood density, velocity and vessel diameter, and inversely proportional to blood viscosity.
• The Reynolds number formula is Re = pDV/η, where Re is the number, p is the density of blood, D is the diameter of the tube, V is the velocity of flow, and η is the viscosity of the fluid.
• Flow is usually not turbulent if Reynolds number Re is less than 2000.
• Turbulent flow may occur(transitional flow) if Re is greater than 2000.
• Turbulence is almost always present if Re is greater than 3000.

Intended Learning Outcomes

• Differentiate laminar and turbulent blood flow.
• Identify transmural (distending) pressure and critical closing pressure of blood vessels.
• Recognize pressure-volume relationships in blood vessels.
• Explain velocity of blood flow in different blood vessels.
• Discuss Bernoulli's principle.
• Identify and explain Laplace law and its significance in heart, lungs, and capillaries.

Clinical Notes

• Examples of turbulent flow:

• Normally in artery branching and when blood is ejected from the aorta.

• Auscultation (listening to sounds with a stethoscope) causes blood flow sounds.

• Pathologically due to constrictions (e.g., atherosclerotic plaques), increased blood velocity (e.g., anemia) or cardiac valve abnormalities (stenosis or regurgitation)

• Transmural (distending) pressure of blood vessels and critical closing pressure*

• Blood vessels remain open because internal pressure (Pi) is greater than external pressure (Pt).

• Transmural pressure (ΔP) is the difference between internal and external pressures (Pi-Pt).

• A positive transmural pressure causes vessel expansion.

• A negative transmural pressure causes vessel collapse.

• Critical closing pressure is the pressure at which flow stops and vessel collapse occurs.

Pressure-Volume Relationship in Blood Vessels

• Vascular elasticity is the ability of a vessel to return to its normal shape after being stretched.
• Vascular compliance (distensibility) is the change in volume of a vessel for a given change in pressure.
• Compliance is inversely related to elasticity, with high elasticity meaning lower compliance.
• Veins have higher compliance than arteries.
• Arteries act as pressure reservoirs.
• Veins act as blood reservoirs.

Velocity of Flow

• Velocity is the distance traveled per unit of time, and flow is the volume of blood moved per unit of time.
• Velocity of flow is inversely related to the cross-sectional area of blood vessels.
• Capillary velocity is low to allow for nutrient and waste exchange.

Law of Laplace

• States that tension (T) in a hollow vessel (e.g., a blood vessel) is the product of transmural pressure (P) and radius (r) divided by wall thickness (w). The formula is T = Pr/w
• The radius of a capillary is small, reducing its wall tension so it can withstand high pressures without rupturing.
• Increased chamber radius causes increased myocardial workload in dilated hearts .

Bernoulli's Principle

• Total energy = kinetic energy + potential energy, is constant in a tube or blood vessel.
• A pressure drop in blood vessels results from resistance (energy lost as heat) or kinetic energy conversion.
• Narrowing of vessels increases kinetic energy and decreases potential energy, the reverse occurs when vessels widen.
• Narrowed blood vessels, such as in atherosclerosis, lead to increased blood flow velocity and decreased lateral pressure in the narrowed portion, encouraging the narrowing to persist.

Student Activity (True/False)

• Laminar flow has a sound: False
• Velocity in Aorta is greater than that of capillaries: True
• In atherosclerosis systolic pressure is decreased: False

References

• Multiple sources provided, referencing different authoritative publications on medical physiology.