Flow

Fluids are substances that flow or "move" and take the shape of a container. Gases and liquids are fluids that can flow, and these materials can be characterized by density and viscosity. The units for density and viscosity are in poise or kg/m-s. Density is mass over volume, and the mass is a measure of the resistance of an object to acceleration. This resistance is known as "inertia". An example of inertia is the resistance of blood to the vessel wall, which can be seen in a laminar-type flow. Viscosity, on the other hand, is the resistance to flow offered by a fluid in "motion". So if there is more resistance or pressure, than fluid motion is altered thus flow speed is changed. A perfect example can be seen in blood. Blood has a higher viscosity than water, that is, blood is much "thicker" and 55 times that of water. In a pt diagnosed with polycythemia, for example, the blood viscosity will be about 0.10 poise d/t the increasing amount of RBC's in that person's blood system. If, however, the pt is diagnosed with anemia, then blood viscosity will decrease to about 0.02 poise.

In order for flow to occur, a pressure difference is required. If the pressure from Point A is equal to Point B, then no flow will occur. So a pressure difference must be required for flow to occur because generally fluids move from high-pressure to low-pressure. This can also be seen in the Bernoulli Effect, which is the drop off in pressure and increase in flow speed at stenosis (narrowing of lumen). The pressure difference can be generated by the heart, which can "pump" or increase the volumetric flow rate. The volumetric flow rate can be described as Q, which is proportional to the pressure difference (force/area) and inversely proportional to resistance (poise). Therefore, as pressure difference increases, then Q increases; and as flow resistance increases, then Q decreases.

Flow resistance in a long, straight tube depends on the fluid viscosity, tube length, and radius of the vessel. That is, if tube length and viscosity increases, then flow resistance increase thus Q decrease. If, however, radius increase, then flow resistance decrease thereby increasing Q. Using Poiseuille's equation to determine Q can be applied but only for steady flow in long straight tubes without stenosis. Therefore, the equation serves only to provide a rough approximation of the conditions in blood circulation. If, however, stenosis is present, then the equation is converted to the "average flow speed", which is used to determine the average velocity in a vessel with stenosis.

There are five different types of flow: Plug, Laminar, Parabolic, Disturbed, and Turbulence. In plug flow, blood moves in "one" unit thus having no difference in flow speed. They can be seen in the branches of vessels (i.e. RA's of mid ABD AO), and as the blood goes down the tube, the flow becomes laminar.

Laminar flow consists of layers with differences in flow speeds, with the center having the highest speed, and the layer closest to the vessel wall having the lowest (d/t inertia)speed. As a result, when using doppler, laminar flow will exhibit only one color (either red or blue, depending upon color map) with the center appearing bright. Laminar flow is the most common flow in our circulatory system, and is described as good flow without stenosis. They are broken down by two types: Parabolic and Disturbed.

Parabolic flow is a form of laminar flow, but with the addition of more layers and organized speed. However, parabolic flow occurs only in "long straight tubes" therefore the flow is not dominant within our blood system. A "laminar nonparabolic flow" is most commonly seen in blood circulation because the vessels generally are not long and straight. The other form of laminar flow is disturbed flow, which is described as a forward "curved" flow that occurs in the region of stenoses or at a bifurcation (i.e. DST ABD AO). In disturbed flow, particles of fluid still flow in the forward direction.

In the final category, turbulent flow or turbulence can be described as random or choatic (i.e. entropy). The reason for this is the presence of blood circles also known as "eddie's", which move at different speeds. The onset of turbulent flow can be predicted by using Reynold's number, which is proportional to flow speed, diameter, and density. That is, as flow speed, diameter, and density increases, then Reynold's number increases; And if the number increases at a critical rate (at about 2000), then suspect turbulent flow and presence of a clot. Turbulent flow can be seen during post-stenosis, whereas at stenosis, disturbed flow is seen. When using doppler, turbulent flow will exhibit mixed colors.