Question

Describe how arterial compliance is responsible for ensuring constant blood flow from a pulsatile source (the heart).

Answer 1

Pressure differences in the form of cardiac output and vessel compliance create blood flow.

Machanism or Explanation with example:-

When considering physiologic blood flow, it is simplest to think of the blood flowing through pipes or cylinders, and from that basic understanding add in the complexities. Normal blood flow can be thought of as simple laminar flow in most instances (for exceptions, see pathology). A visualization of laminar flow can be seen in the figure below. Laminar flow is characterized by a gradient of flow lines representing different blood velocities at different locations in the tube. The reason for these differences in blood flow velocity is wall stress (a type of shear stress). When a fluid, in this case, blood, flows through a pipe, friction exists between the fluid and the wall of the tube. This friction decreases the velocity of the blood closest to the wall (hence the shorter lines on the diagram closer to the tube wall). Another factor within the realm of laminar flow is the Reynolds number. The Reynolds number is a value for a given fluid to model the conditions at which that fluid will remain in laminar flow. A variable affecting the Re number of a fluid, in this case, blood, is viscosity. In the case of blood, it is a product of its constituents: cells and protein. The Reynolds number considers the velocity of flow and external factors that might contribute to turbulent flow.

The following is the equation for the Reynolds number, (where Re is the Reynolds number, ρ is the density, V is velocity, D is the diameter of the cylinder, and μ is the viscosity:

Re= ρVD/μ

Turbulence is more likely to develop at a high Re number.

One of the most significant wrinkles in this simplified model is the principal of blood vessel compliance. Compliance is the amount of distention for a given amount of pressure. As such, when blood is pumped from the heart, the blood vessels do not act as complete rigid tubes. They expand and contract with the pressure changes due to their elastic nature.

Compliance can be modeled with the following equation, where C is the compliance, V is volume, and P is pressure:

C = ΔV/ΔP

Essentially, the greater the change in volume for a given pressure change, the greater the compliance. Physiologically, veins have greater compliance than arteries under normal conditions. This is because arteries are thicker and more muscular than veins (less distensible). The result is a high-pressure system within the arteries and a lower resistance (low-pressure system) in the veins.

Pressure differences in the form of cardiac output and vessel compliance create blood flow.

This governing principle is quantified by Ohm’s law of fluid flow which states the following where flow (Q) is equal to the pressure gradient (ΔP) divided by resistance (R):

Q = ΔP/R

Physiologically, this means that blood flow is equal to the change in pressure divided by the systemic resistance. In other words, to increase blood flow, one could either increase the pressure difference (e.g., increased cardiac force) or decrease the systemic vascular resistance (e.g., dilate blood vessels). Blood vessel resistance can be thought of as how difficult it is to pass blood through a given set of vessels. Intuitively, the size and shape of the blood vessel can alter the ease of blood flow. A helpful analogy for blood flow resistance is motor vehicle traffic. The cars represent the ease of blood flowing through a blood vessel. If we made the road narrower or add a toll booth, fewer cars can pass a given point for a set amount of time. This is analogous to making the blood vessel narrower (blood vessel diameter is a factor for resistance) which increases resistance.