Evolution of blood flow from the heart to the periphery
Blood movement through the vascular bed is heterogeneous and changes with distance from the heart. During the cardiac cycle, pressure undergoes significant fluctuations, but they gradually attenuate. The prominent elasticity of the walls of the aorta and large major arteries plays the main role in this smoothing out.
Depending on the caliber of the vessel, the nature of the flow changes dramatically:
- Aorta and large arteries: blood flow has a pronounced pulsatile character; blood moves forward in separate pulses, synchronously with cardiac contractions.
- Arterioles and capillaries: pulsations are completely damped, and blood flow becomes absolutely continuous, which is critically important for adequate transcapillary exchange.
Two fluid flow regimes
Hemodynamics describes two fundamentally different regimes of blood movement through vessels: laminar and turbulent. The transition between them is directly related to changes in fluid velocity.
- Laminar flow. This is strictly ordered movement. Blood moves in regular layers that slide parallel to the direction of flow (the so-called axial flow). Layers do not intersect or mix with each other.
- Turbulent flow. Occurs with a substantial increase in movement velocity. Order is completely destroyed: intensive mixing of absolutely all fluid layers occurs, accompanied by the formation of numerous vortices.
Reynolds number ($NR$)
To predict the regime in which blood will flow in a specific vessel, a mathematical criterion is used — the Reynolds number ($NR$). It is calculated using the following formula:
$NR = (\rho \cdot D \cdot v) / \eta$
Where:
- $\rho$ (rho) — fluid density;
- $D$ — diameter of the blood vessel (tube);
- $v$ — average flow velocity;
- $\eta$ (eta) — fluid viscosity.
Evaluation of the obtained value makes it possible to precisely determine the regime:
- If $NR < 2000$, the flow is guaranteed to remain laminar.
- If $NR > 3000$, the flow transitions to turbulent.
Factors causing turbulence
Vortices in blood flow do not appear randomly. Turbulence occurs when certain physical and anatomical conditions are met. Factors that provoke the disruption of laminar flow include:
- Decrease in blood viscosity.
- Increase in flow velocity.
- Large blood vessel diameter.
- High fluid density.
- Presence of irregularities on the inner vascular walls.
A special role is played by the "shear thinning" phenomenon. The essence of this phenomenon is that with a significant increase in blood flow velocity, one fluid layer begins to actively shift relative to another. This physical interaction leads to an actual decrease in blood viscosity, which, according to the Reynolds formula, further increases the risk of developing turbulence.
Physiological significance of turbulence
Turbulent flow is not always a pathology; it has clear localization and conditions of occurrence in a healthy body.
Initially, vortex flows normally exist at the moment blood is ejected from the left ventricle into the aorta. Turbulence also naturally occurs at blood vessel branching points (bifurcations), where flow geometry is disrupted. In other arteries, flow may temporarily become turbulent with a sharp increase in blood flow velocity — for example, during intense physical exertion.
The main physiological consequence of turbulence is energetic. Such flow requires additional energy expenditure to propel blood through the vascular bed. As a result, this creates a significant additional workload on the heart's pumping function.
Briefly: answers to topical questions
What velocity distribution profile of the layers is characteristic of laminar blood flow?
Laminar blood flow is characterized by ordered movement: blood moves in layers parallel to the direction of flow, forming an axial flow. In microvessels, a parabolic flow profile is described: due to the transverse pressure gradient, formed elements shift from the periphery to the center. As a result, the movement velocity of cells through the capillary is higher than the velocity of whole blood; this is associated with the phenomenon of decreased blood viscosity in microvessels.
In which cardiovascular pathologies does turbulent blood flow occur?
In the provided sources, turbulent blood flow is described in the following cardiovascular pathologies:
- renal artery stenosis — blood flow turbulence is detected distal to the stenosis;
- acute mitral regurgitation — the retrograde jet from the left ventricle into the left atrium appears dark on cine-MRI due to turbulent flow.
The sources also indicate that vortices initially exist when blood is ejected from the ventricle into the aorta and at vessel branching points — bifurcations; in arteries, flow can become turbulent with an increase in blood flow velocity, for example during physical exertion.
To remember the factors of turbulence, look at the Reynolds formula: everything in the numerator (density, diameter, velocity) causes vortices when increased. And what is in the denominator (viscosity) preserves laminar flow when increased.
Class and exam questions
Why is blood flow continuous in capillaries and pulsatile in the aorta?
In the aorta, blood moves in separate pulses due to rhythmic cardiac outputs. As it moves toward the periphery, these pressure fluctuations are completely smoothed out due to the high elasticity of the walls of major vessels.
What is the "shear thinning" phenomenon?
This is the phenomenon of decreased blood viscosity with increased blood flow velocity. It occurs because at high velocity, fluid layers begin to intensively shear relative to each other.
Where can turbulent blood flow normally be found?
In a healthy body, vortices form when blood is ejected from the ventricle into the aorta, as well as at vessel bifurcations (branching points). In other arteries, they may appear during heavy physical exertion due to increased flow velocity.
What else to study on this topic
- Mechanisms of smoothing arterial pressure pulse waves
- The effect of vascular wall irregularities on hemodynamics
- Hemodynamics at blood vessel bifurcations
- Myocardial energy expenditure in overcoming turbulence
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